Marine HVAC Systems: Ship Air Conditioning and Ventilation

A ship’s HVAC system is four separate installations sharing one name: accommodation air conditioning designed to ISO 7547:2022, machinery space ventilation designed to ISO 8861:1998, cargo space ventilation governed by the IMSBC and IMDG Codes, and galley and sanitary exhaust governed by SOLAS chapter II-2. Each has its own design standard, its own equipment chain and its own regulatory driver. The statutory floor for crew spaces is MLC 2006 Standard A3.1 paragraph 7, which requires air conditioning on every ship except those regularly engaged in trade where temperate climatic conditions do not require it. The refrigerant that makes accommodation cooling possible is being withdrawn from the market on a fixed schedule under the Kigali Amendment, in force since 1 January 2019.

The four HVAC duty areas

Marine HVAC is not a single system. It is four interacting systems on the same hull, and the most common error in a newbuilding HVAC brief is to treat them as one.

The systems differ in what failure means. A degraded accommodation plant is a labour-rights breach under a convention a port state officer can inspect. A degraded engine room ventilation system derates the main engine. A wrong cargo hold ventilation decision produces a cargo claim. A neglected galley duct is an ignition path. Only the first is about comfort at all.

The marine engine room ventilation and uptakes article covers the machinery space calculation in full, and marine cargo hold ventilation covers the hold decision in operational detail. This article covers what is distinct to the accommodation plant, the refrigerant regime that now constrains it, and the points where the four systems meet SOLAS.

Accommodation air conditioning: design basis under ISO 7547:2022

ISO 7547:2022 fixes the design conditions every accommodation plant is sized against. It is the third edition, published April 2022, and its Foreword records that it cancels and replaces ISO 7547:2002 and also ISO 8862:1987, ISO 8863:1987, ISO 8864:1987 and ISO 9099:1987, incorporating the technical corrigendum ISO 7547:2002/Cor.1:2008. That merger is why the current edition carries normative annexes for machinery control rooms, the wheelhouse, dry provision store rooms and hot-air heating of wheelhouse windows, subjects that used to sit in four separate standards.

Any specification still citing ISO 7547:2002 is working from a withdrawn edition. The point is not pedantry about a year: values move between editions of this standard. The solar excess temperatures for light vertical and light horizontal surfaces are transposed between the 1985 and 2022 editions, so a figure carried forward from an old design file can be wrong in the specific way that survives review.

Design conditions

Clause 4.2 sets the summer design conditions: outdoor air at 35 degrees Celsius and 70 percent relative humidity, indoor air at 25 degrees Celsius and 55 percent relative humidity, and engine room air at 45 degrees Celsius. Clause 4.3 sets the winter design: outdoor minus 20 degrees Celsius, indoor 22 degrees Celsius, with an explicit note that the document does not specify winter humidification requirements.

The standard states outdoor humidity as a relative humidity, not as a wet bulb temperature. That distinction is worth holding onto, because a design point quoted as a dry bulb and wet bulb pair has usually been converted by someone, and conversions go wrong. At sea level pressure, 35 degrees Celsius at 70 percent relative humidity corresponds to a wet bulb near 30.2 degrees Celsius. A frequently repeated pairing of 35 degrees dry bulb with 28 degrees wet bulb is a different and milder point: it corresponds to about 59 percent relative humidity, not the 70 percent the standard specifies, and sizing to it undersizes the plant.

Cooling capacity is sized against the summer condition and heating capacity against the winter condition, so a single plant must satisfy two unrelated worst cases. An owner on a fixed trade often specifies above the standard rather than at it, and that is a commercial decision rather than a compliance one.

Outdoor air and the occupancy basis

Clause 4.4 sets the outdoor air supply at not less than 0,008 cubic metres per second per person, which is about 28.8 cubic metres per hour. There is no separate cabin rate and public space rate. What varies is the occupancy the space is counted at, and clause 4.5 fixes that: cabins at their design number, saloons at one person per 2 square metres, mess and dining rooms at one per 1,5 square metres, recreation rooms at one per 5 square metres, the captain’s and chief engineer’s day rooms at four, other private day rooms at three, the hospital at beds plus two, gymnasiums and games rooms at four, first-aid rooms at two, offices at two, and the machinery control room at three.

Reading the rate as 8 cubic metres per hour rather than 0,008 cubic metres per second undersizes the outdoor air by a factor of 3.6. It is an easy slip and it produces a plant that meets its temperature set point while the carbon dioxide concentration climbs in a closed cabin.

The cooling load calculation

Clause 5 sums four components: 5.2 heat transmission through the structure, 5.3 solar heat gain, 5.4 heat gain from persons, and 5.5 heat gain from lighting and other sources. The airflow and air balance calculation is clause 6, and it is separate from the load calculation.

Clause 5.3 handles solar gain by fixed excess temperatures above the 35 degree Celsius outdoor condition rather than by irradiance tables indexed to latitude and orientation. The excess temperatures are 12 kelvin for a light vertical surface, 29 for a dark vertical surface, 16 for a light horizontal surface and 32 for a dark horizontal surface, with clear glass heat gain taken at 350 watts per square metre. Paint color is therefore a design input, not a cosmetic choice: a dark vertical surface carries more than twice the excess temperature of a light one.

Clause 5.2.3 Table 2 gives total heat transfer coefficients in watts per square metre kelvin: 0,9 for a weather deck not exposed to sun and for the ship side; 0,8 for a deck or bulkhead against an engine room or cargo space; 0,7 against a boiler room; 0,6 for a deck against open air or a sun-exposed weather deck; and 6,5, 3,5 and 2,5 for single, double and triple glazing. Clause 5.2.4 adds surface film coefficients of 80 watts per square metre kelvin for an outer surface in a 20 metre per second wind and 8 for an inside surface at 0,5 metres per second, with a steel correction factor of 1,2 for plane insulation and 1,45 for corrugated.

On a tropical passage the ventilation load, transmission through sun-exposed decks and solar gain through wheelhouse glazing are the components that dominate. The wheelhouse is a special case with its own normative annex, and it is treated as a control station under SOLAS chapter II-2.

Psychrometrics: sensible heat, latent heat and dew point

The total cooling load splits into two parts that behave differently, and coil selection follows the split rather than the total. Sensible heat changes air temperature at constant moisture content. Latent heat removes moisture at constant temperature.

The sensible heat ratio is sensible cooling divided by total cooling. Accommodation spaces run high; galleys and laundries run much lower, because their load is mostly water vapour. A coil chosen for a high sensible heat ratio runs a warm, dry surface, and a warm surface removes no moisture at all if it never falls below the entering air dew point. Put that coil on a laundry and the space holds its set point temperature while the humidity climbs. The complaint that follows is a cabin that reads correct on the thermostat and feels wet, and adjusting the thermostat cannot fix it.

The engineering responses are a separate coil for the high latent zone, colder chilled water to that zone alone, or a run-around coil that pre-dries the incoming outdoor air before it reaches the main coil.

Dew point governs condensation. Supply air leaving a cooling coil at 13 degrees Celsius dry bulb and 95 percent relative humidity has a dew point near 12.2 degrees Celsius. Chilled water is colder than that, so any uninsulated chilled water pipe in a conditioned space will run wet continuously. The consequence is not a drip in a tray. It is rust staining through the deckhead, biological growth in the void behind the lining, and structural corrosion behind bulkhead panels that nobody sees until a refit opens them. Closed cell foam insulation is sized so the pipe’s outer surface stays above the highest dew point the space will see, and in a tropical accommodation block that dew point is set by the outdoor air, not by the conditioned room.

The chilled water plant and its terminals

Accommodation cooling on a commercial vessel with substantial accommodation is normally a chilled water system. A refrigeration machine in the engine room or a dedicated machinery space produces chilled water, which is circulated to air handling units serving public spaces and to fan coil units serving individual cabins. The refrigeration cycle itself is the vapour compression cycle described at marine refrigeration and cargo cooling : a compressor raises refrigerant vapour to high pressure, a seawater-cooled condenser liquefies it, an expansion valve drops the pressure, and the liquid evaporates in the chilled water heat exchanger.

The reason chilled water dominates is containment rather than thermodynamics. One refrigerant charge stays confined to one machinery space, and any number of terminals can be served from it through water pipework that carries no refrigerant at all. That matters more now than it did, because charge limits, leak detection and machinery space ventilation all follow from the refrigerant’s safety classification, and confining the charge confines the problem. Direct expansion, with refrigerant in the terminal coil, suits smaller vessels with short pipe runs where a water loop is not justified.

An air handling unit contains, in flow order, a mixing plenum blending outdoor and return air, filters, a cooling coil, a heating coil, a supply fan and a sound attenuator before the duct. Separate units serve the bridge, the machinery control room and the hospital, which allows independent set points and prevents cross-contamination. A fan coil unit is a small terminal box that recirculates room air over a water coil under thermostat control and adds no outdoor air of its own. The usual arrangement pairs the two: the air handling unit delivers tempered outdoor air to the space, and the fan coil unit trims its temperature.

Heating and humidity

In the North Atlantic, Baltic and Arctic the heating load governs. Auxiliary boiler hot water is the traditional medium, circulated to air handling unit heating coils and to fin-tube convectors under cabin windows, which counteract the down-draught off cold glass. On ships with surplus waste heat the demand is often met from engine jacket water or exhaust gas recovery without running the boiler for HVAC at all, which is covered at waste heat recovery system . Reversible heat pumps are gaining ground on short-sea vessels and battery-electric ferries , where the boiler is undersized for pure heating duty.

Humidity is harder to hold in the cold than in the heat. Cold outdoor air carries very little moisture, so when it is heated to cabin temperature its relative humidity collapses without added moisture, and a steam humidifier in the air handling unit supply section is the standard answer. ISO 7547:2022 clause 4.3 does not specify winter humidification requirements, so where humidification is fitted the control target comes from the owner’s specification or from class rules rather than from the standard.

Refrigerants and the HFC phase-down

The refrigerant in a marine chiller is now a supply-chain risk as much as an engineering choice. The Kigali Amendment to the Montreal Protocol, adopted at the Twenty-Eighth Meeting of the Parties in Kigali between 10 and 15 October 2016 and in force since 1 January 2019, added hydrofluorocarbons to the controlled substances and set a phase-down of production and consumption.

The Protocol places no obligation on a shipowner, a ship or a flag. It controls production and consumption by Party, where consumption is production plus imports minus exports. A ship in international trade is never a consumer under the Protocol. What reaches the ship is second order and entirely real: national implementing law on who may buy and handle the gas, quota-driven price, and falling availability at bunker and repair ports.

The schedules

For non-Article 5 Parties, Article 2J(1) caps consumption at a percentage of a baseline that is the average of 2011, 2012 and 2013 consumption of Annex F substances plus 15 percent of the Annex C Group I hydrochlorofluorocarbon baseline, all in carbon dioxide equivalents:

Article 2J(3) mirrors this for production. Article 2J(2) and 2J(4) provide an alternative track starting in 2020 with a 25 percent hydrochlorofluorocarbon add-on, applying only where the Parties so decide.

Article 5 Parties are split into two groups. Group 1, under Article 5 paragraph 8 qua (a), freezes at 100 percent from 2024 to 2028, then steps to 90 percent for 2029 to 2034, 70 percent for 2035 to 2039, 50 percent for 2040 to 2044, and 20 percent from 2045. Its baseline is the average of 2020, 2021 and 2022 plus 65 percent of the hydrochlorofluorocarbon baseline. Group 2, under paragraph 8 qua (b), freezes at 100 percent from 2028 to 2031, then steps to 90 percent for 2032 to 2036, 80 percent for 2037 to 2041, 70 percent for 2042 to 2046, and 15 percent from 2047, on a baseline of the 2024 to 2026 average plus 65 percent of the hydrochlorofluorocarbon baseline. Paragraph 8 qua (g) preserves a high ambient temperature exemption on criteria decided by the Parties.

Two points are commonly garbled. The 2019 step for non-Article 5 Parties is a reduction to 90 percent, not a freeze at the baseline. And no group reaches 20 percent in 2047: Group 1 reaches 20 percent in 2045 and Group 2 reaches 15 percent in 2047. Kigali is also a phase-down to a residual level rather than a phase-out, which is why the schedules end at a percentage rather than at zero.

The Amendment had 174 parties as of 10 August 2026.

Global warming potential, and the assessment report that produced it

A 100-year global warming potential is unusable without the IPCC assessment report it comes from. Regulation (EU) 2024/573 recital 8 states that the global warming potential of hydrofluorocarbons is calculated on the Fourth Assessment Report and that for other fluorinated greenhouse gases the Sixth Assessment Report is used, and Annex I is structurally split into two column blocks to carry the distinction.

Quoting R-1234ze(E) at approximately 7 mixes the two bases inside one sentence: that figure belongs to the Fourth Assessment Report era, and the Regulation puts the substance on the Sixth.

Blends are computed, not looked up. R-410A, R-404A, R-407C and R-452B appear nowhere in the Regulation’s annexes. Annex VI sets the method: a weighted average derived from the sum of the weight fractions of the individual substances multiplied by their global warming potential, including substances that are not fluorinated greenhouse gases, with a weight tolerance of plus or minus 1 percent. Compositions come from ANSI/ASHRAE Standard 34: R-410A is R-32 and R-125 at 50/50, R-404A is R-125, R-143a and R-134a at 44/52/4, R-407C is R-32, R-125 and R-134a at 23/25/52, and R-452B is R-32, R-125 and R-1234yf at 67/7/26. Applied to the Annex I values that gives R-404A near 3,922, R-410A near 2,088 and R-407C near 1,774.

Lower-GWP options

R-32 is used in split and direct expansion equipment and has a pressure profile close to R-410A. R-452B is engineered as a near drop-in for R-410A and computes to roughly 697 on the Regulation’s own component values. Both are A2L. R-1234ze(E) is a hydrofluoroolefin used in large centrifugal chillers and is not a drop-in, because its lower operating pressure requires redesigned compressor and heat exchanger geometry.

Ammonia (R-717) has zero global warming potential and high efficiency, and it is toxic. NIOSH gives an immediately dangerous to life or health concentration of 300 parts per million, and the ACGIH threshold limit value is 25 parts per million as an eight-hour time-weighted average with a 35 parts per million short-term exposure limit. That toxicity, not its thermodynamics, is what confines ammonia to refrigeration machinery spaces remote from accommodation, usually working through a secondary glycol loop. Carbon dioxide (R-744) runs transcritical at high pressure and requires purpose-built equipment.

A2L is a safety subclass created by ANSI/ASHRAE Addendum ak to Standard 34-2007 for class 2 refrigerants with a maximum burning velocity of 10 centimetres per second or less when tested at 23 degrees Celsius and 101.3 kilopascals. The designation and classification system itself is ISO 817:2024, the fourth edition, which withdrew ISO 817:2014 and its 2017 amendment. Fitting an A2L changes the installation rather than the operation: charge limits per space, leak detection, machinery space ventilation, electrical equipment selection and hot work procedure during service all follow from the classification. The binding marine requirements come from the classification society and the flag administration, so the applicable class rule part and edition must be identified before a retrofit is specified rather than assumed from land-based practice.

What the EU F-Gas Regulation does and does not require of a ship

Regulation (EU) 2024/573, adopted 7 February 2024 and in force from 11 March 2024, repealed and replaced Regulation (EU) No 517/2014. It is widely misdescribed in shipping, so the scope question is worth settling before the duties.

The containment regime does not currently reach ships. Article 5(2), which lists the equipment subject to leak checks, prefixes every entry with the word stationary: refrigeration equipment, air-conditioning equipment, heat pumps, fire protection equipment, organic Rankine cycles and electrical switchgear. Article 5(3) lists the mobile equipment that is in scope and enumerates refrigerated trucks and trailers, refrigerated light-duty vehicles, intermodal containers including reefers, train wagons, heavy duty vehicles, vans, non-road mobile machinery used in agriculture, mining and construction, trains, metros, trams and aircraft. Ships and vessels appear in neither list. Article 6 is expressly limited to stationary equipment, and Article 7 record-keeping keys off Article 5(1).

Article 35(5)(b) closes the question. It requires the Commission, by 1 January 2030, to evaluate international developments relevant for the shipping sector and the potential expansion of the scope of containment requirements to fluorinated greenhouse gases contained in refrigeration and air-conditioning equipment of ships. A provision requiring assessment of a potential expansion to ships is only coherent if ships are outside the scope now.

What does apply

Article 4 carries no equipment list at all, so its duties reach an operator within EU jurisdiction. Paragraph 1 prohibits intentional release where not technically necessary. Paragraph 3 requires operators and manufacturers of equipment containing fluorinated greenhouse gases, and undertakings in possession of such equipment during its transport or storage, to take all necessary precautions to prevent unintentional release and all technically and economically feasible measures to minimise leakage. Paragraph 5 requires a detected leak to be repaired without undue delay. This is a general anti-emission duty. It produces no logbook, no leak-check interval and no certificate.

Article 11 and Annex IV prohibit placing listed equipment on the EU market from fixed dates. For split air conditioning and heat pumps, Annex IV point 9 runs: single split systems containing less than 3 kilograms of Annex I gases with global warming potential of 750 or more from 1 January 2025; split air-to-water up to 12 kilowatts at 150 or more from 1 January 2027; split air-to-air up to 12 kilowatts at 150 or more from 1 January 2029; split systems up to 12 kilowatts containing any fluorinated gas from 1 January 2035; split systems above 12 kilowatts at 750 or more from 1 January 2029; and split systems above 12 kilowatts at 150 or more from 1 January 2033.

Article 13 is what a superintendent feels first. Paragraph 3 prohibits using gases of global warming potential 2,500 or more for maintenance or servicing of refrigeration equipment with a charge of 40 tonnes of carbon dioxide equivalent or more, and from 1 January 2025 extends that prohibition to any refrigeration equipment regardless of charge size, with reclaimed and recycled gas permitted on conditions until 1 January 2030. Paragraph 4 applies the same 2,500 threshold to air conditioning equipment and heat pumps from 1 January 2026, with reclaimed and recycled gas allowed until 1 January 2032. Paragraph 5 drops the threshold to 750 for servicing stationary refrigeration equipment from 1 January 2032, chillers excluded. An R-404A top-up in an EU yard is therefore no longer available, whatever the ship’s flag.

The quota system and the Article 10 certification requirements complete the picture: quota drives price and availability at EU ports, and certification governs who may lawfully do the work. Certification is a Member State competence, not a flag state one.

Where ships genuinely are in scope: reefer containers

Article 5(3)(b) names refrigeration units of intermodal containers, including reefers. Article 5(5) defers the leak-check and record-keeping obligations for that equipment until 12 March 2027, so the duty starts then. The frequency table in Article 5(6) then applies: at least every 12 months below 50 tonnes of carbon dioxide equivalent, every 6 months from 50 to below 500 tonnes, and every 3 months at 500 tonnes and above, each interval doubling where a leakage detection system is fitted.

Where the record-keeping duty does apply, it is Article 7, triggered at 5 tonnes of carbon dioxide equivalent through Article 5(1), and the records are kept for at least 5 years. The threshold is expressed in carbon dioxide equivalent rather than in kilograms for a reason: 5 tonnes of carbon dioxide equivalent is about 1.3 kilograms of R-404A, 3.5 kilograms of R-134a and 7.4 kilograms of R-32. A kilogram threshold would mean a different thing for every refrigerant. A separate leakage detection system becomes mandatory only at 500 tonnes of carbon dioxide equivalent under Article 6(1), and only for stationary equipment.

More detail sits at marine reefer container systems . The separate ozone-depleting substances regime, including the record book required under MARPOL Annex VI, is covered at MARPOL Annex VI regulation 12 , and the assessment-report question is treated at AR5 GWP100 in maritime use .

MLC 2006: the crew accommodation floor

The Maritime Labour Convention, 2006 , adopted 23 February 2006 at the 94th (Maritime) Session of the International Labour Conference and in force since 20 August 2013, is the instrument that makes accommodation HVAC a legal obligation rather than a specification item. It revises the 37 instruments listed in its Article X. Regulation 3.1(1) states the headline duty: flag Members ensure that ships provide and maintain decent accommodation and recreational facilities for seafarers working or living on board.

The whole of the ventilation, air conditioning and heating requirement is Standard A3.1 paragraph 7. Paragraph 5 is a pointer that requires compliance with paragraphs 6 to 17 and contains no heating requirement of its own. Paragraph 8 is lighting. Paragraph 9 is sleeping rooms: individual rooms, berth dimensions of at least 198 by 80 centimetres, floor areas by tonnage band and locker volumes.

Paragraph 7 reads across four limbs:

• 7(a) sleeping rooms and mess rooms shall be adequately ventilated.
• 7(b) ships, except those regularly engaged in trade where temperate climatic conditions do not require this, shall be equipped with air conditioning for seafarer accommodation, for any separate radio room and for any centralized machinery control room.
• 7(c) all sanitary spaces shall have ventilation to the open air, independently of any other part of the accommodation.
• 7(d) adequate heat through an appropriate heating system shall be provided, except in ships exclusively on voyages in tropical climates.

Two things follow that are routinely stated backwards. Air conditioning is the default and temperate trade is the exemption, not a hot-climate trigger that switches the requirement on. And the navigating bridge is not among the spaces listed in 7(b): the three named spaces are seafarer accommodation, any separate radio room and any centralized machinery control room. A Member may exempt ships below 200 gross tonnage from 7(b) under paragraph 20.

There is no duct-cleaning requirement anywhere in Standard A3.1. The nearest text is Guideline B3.1.2 paragraph 2(b), which is Part B and therefore not mandatory, and which says only that air conditioning systems should facilitate easy cleaning and disinfection to prevent or control the spread of disease. That is a design attribute rather than a maintenance interval. Guideline B3.1.2 also sets no air change rate, no temperature and no humidity figure; the numbers come from ISO 7547:2022, class rules and flag legislation.

The shipboard duty that does exist is paragraph 18: frequent inspections on board by or under the authority of the master, that accommodation is clean, decently habitable and in good repair, with the results of each inspection recorded and available for review. That record is what a port state control officer asks to see.

Hospital accommodation is paragraph 12: ships carrying 15 or more seafarers and engaged in a voyage of more than three days shall provide separate hospital accommodation used exclusively for medical purposes, relaxable by the competent authority for coastal trade. It does not come from SOLAS chapter III, and there is no 100-person threshold. SOLAS III/4 is the evaluation, testing and approval of life-saving appliances and arrangements.

Which ships, and which rules

Regulation 3.1(2) is load-bearing for anyone reading this on an older vessel. The Code requirements relating to ship construction and equipment apply only to ships constructed on or after the date the Convention comes into force for the Member concerned, with keel laying or a similar stage of construction as the test. For ships constructed before that date, the construction and equipment requirements of the Accommodation of Crews Convention (Revised), 1949 (No. 92) and the Accommodation of Crews (Supplementary Provisions) Convention, 1970 (No. 133) continue to apply to the extent that they were applicable, prior to that date, under the law or practice of that Member. The cut-off is per flag, not one global date, and 20 August 2013 is only the date for Members for whom the Convention entered into force then.

Requirements that are not about construction and equipment apply to every ship in scope regardless of build date. Keeping the accommodation habitable and the plant working is an operational duty, not a construction one.

Where a Member cannot implement Part A as written, Article VI(3) and (4) permit substantially equivalent measures against a two-limb test, and any such measure is recorded in Part I of the Declaration of Maritime Labour Compliance. Standard A3.1(19) separately allows fairly applied variations for seafarers of differing religious and social practices.

Enforcement

Flag administrations inspect accommodation on registration, re-registration and substantial alteration under Standard A3.1(3), and the Maritime Labour Certificate runs for not more than five years with a mandatory intermediate inspection between the second and third anniversary dates.

MLC port state control is a freestanding regime under Regulation 5.2.1 and Standard A5.2.1, exercised through the same memorandum of understanding machinery as other port state control. It is wrong to say that Paris MoU and Tokyo MoU inspections cover only SOLAS. The certificate and the Declaration of Maritime Labour Compliance are prima facie evidence and inspection is normally limited to reviewing them, with a more detailed inspection triggered by clear grounds for believing conditions do not conform, or by a seafarer complaint. Accommodation is item 8 of the Appendix A5-I list. Whether a deficiency leads to detention is the officer’s assessment of whether it is a serious or repeated breach, and that judgment is not something the Convention makes for them. Further detail sits at port state control and marine crew accommodation and welfare .

The 2022 amendments, approved by the Conference on 6 June 2022, replaced Standard A3.1 paragraph 17 to require that recreational facilities, amenities and services include social connectivity, and inserted a new Guideline B3.1.11 paragraph 8 recommending that shipowners provide internet access with charges, if any, reasonable in amount.

Noise from the ventilation plant

MLC 2006 Standard A3.1(6)(h) routes noise and vibration to Regulation 4.3 and sets no figure. The figures are in the Code on Noise Levels on Board Ships, adopted by resolution MSC.337(91) on 30 November 2012 and effective 1 July 2014, made mandatory by SOLAS regulation II-1/3-12 introduced by resolution MSC.338(91).

The Code applies to new ships of 1,600 gross tonnage and above, with SOLAS II-1/3-12 catching ships whose building contract is placed on or after 1 July 2014, or whose keel is laid on or after 1 January 2015, or which are delivered on or after 1 July 2018. Eleven categories are excluded, including high-speed craft, fishing vessels, mobile offshore drilling units, pleasure yachts not engaged in trade and dredgers.

Paragraph 4.2 gives the limits in dB(A), in two columns for 1,600 to 10,000 gross tonnage and for 10,000 gross tonnage and above:

The cabin limit is tonnage-dependent, so a single figure quoted for all ships is wrong for half of them. This matters directly to HVAC design, because the terminal unit and the duct are often the only continuous noise source in a cabin at anchor.

Machinery space ventilation

Engine room ventilation is not comfort engineering. The duty is to deliver enough air to sustain combustion at rated power while holding the space temperature to a value that does not derate machinery or endanger the watch.

ISO 8861:1998, second edition, confirmed in 2022, sets the calculation. Clause 4 fixes the outside ambient air temperature at 35 degrees Celsius and limits the temperature rise from the air intake to the air passing from the engine room up to the casing entrance to 12,5 kelvin. Clause 5.1 requires the total airflow to be at least the larger of the combustion airflow plus the heat evacuation airflow, or 1,5 times the combustion airflow alone, which is the combustion demand plus 50 percent.

Combustion airflow uses 0,002 3 kilograms per kilowatt second for two-stroke engines and 0,002 0 for four-stroke, at an air density of 1,13 kilograms per cubic metre taken at 35 degrees Celsius, 70 percent relative humidity and 101,3 kilopascals. Boiler combustion air uses 15,7 kilograms of air per kilogram of fuel. Heat evacuation airflow divides total heat emission by the product of density, a specific heat of 1,01 kilojoules per kilogram kelvin and the 12,5 kelvin rise, then deducts 40 percent of the engine combustion airflow and the boiler airflow. Calculations assume simultaneous maximum rating under normal sea conditions, and manufacturers’ data takes precedence over the standard’s guidance values.

ISO 8861:1998 sizes the system by combustion demand and heat evacuation. It sets no air change rate for the engine room, so an air-changes-per-hour figure attributed to it is not from the standard.

The HVAC intersection is separation. SOLAS II-2 regulation 9.7.2.1 provides that ventilation systems for machinery spaces of category A, vehicle spaces, ro-ro spaces, galleys, special category spaces and cargo spaces shall, in general, be separated from each other and from systems serving other spaces. The qualifier is in the regulation: cargo ships below 4,000 gross tonnage and passenger ships carrying not more than 36 passengers may share a ventilation unit through separate ducts with an automatic fire damper in the galley duct near the unit. Full treatment is at marine engine room ventilation and uptakes and engine room environmental management .

Ventilation, fire and smoke control under SOLAS II-2

Ductwork is a continuous path between spaces, so a running ventilation system distributes smoke faster than fire spreads through structure. SOLAS chapter II-2 addresses this in regulation 5, not in regulation 8.

Regulation 5.2.1.1 requires the main inlets and outlets of all ventilation systems to be closable from outside the spaces ventilated, with the means of closing easily accessible, prominently and permanently marked, and indicating whether the shut-off is open or closed. Regulation 5.2.1.2 requires power ventilation of accommodation, service, cargo, control station and machinery spaces to be stoppable from an easily accessible position outside the space served, sited so it is not readily cut off by a fire in the space. Regulation 5.2.1.3 adds, for passenger ships carrying more than 36 passengers, that controls be grouped so all fans may be stopped from either of two separate positions as far apart as practicable.

The machinery space rule is regulation 5.2.2. Paragraph 5.2.2.1 requires means of control for opening and closure of skylights, closure of openings in funnels that normally allow exhaust ventilation, and closure of ventilator dampers. Paragraph 5.2.2.2 requires means of control for stopping ventilating fans, with the controls for machinery space power ventilation grouped so as to be operable from two positions, one of which is outside the space, and entirely separate from the means provided for other spaces. Paragraph 5.2.2.4 requires those controls to be outside the space they serve.

Regulation 8 covers a different set of subjects and is often miscited: 8.2 is protection of control stations outside machinery spaces, 8.3 is release of smoke from machinery spaces, 8.4 is draught stops, and 8.5 is smoke extraction from atriums on passenger ships, which must exhaust the atrium volume in 10 minutes or less.

The carbon dioxide interlock

Before a fixed carbon dioxide system discharges into a machinery space, ventilation must stop and dampers must close, or the flooding concentration is diluted below its design value. That duty comes from SOLAS II-2 regulation 5.2.2, not from the FSS Code: FSS Code chapter 5 contains no ventilation or damper provision at all. Its only pre-discharge timing requirement is paragraph 2.1.3.2, an audible alarm sounding in no case less than 20 seconds before release.

The FSS Code sets the quantity and the discharge rate, and they are two separate rules that must not be merged into one figure. Chapter 5 paragraph 2.2.1.2 sets the machinery space quantity at the larger of 40 percent of the gross volume of the largest machinery space protected, excluding the part of the casing above the level at which the casing area is 40 percent or less of the horizontal area of the space, or 35 percent of the gross volume including the casing. Paragraph 2.2.1.1 sets 30 percent for cargo spaces, and paragraph 2.2.1.3 permits a reduction to 35 and 30 percent for cargo ships below 2,000 gross tonnage where two or more machinery spaces are treated as one. Paragraph 2.2.1.5 separately requires that 85 percent of the gas be dischargeable within 2 minutes for machinery spaces. Free carbon dioxide volume is calculated at 0,56 cubic metres per kilogram under 2.2.1.4.

Fire dampers at A-class penetrations close on thermal actuation or on remote release; smoke dampers at main vertical zone boundaries close on a smoke detection signal. The two are not interchangeable, and the HVAC zone layout must follow the main vertical zone layout, because an air handling unit serving both sides of a boundary without dampers defeats the boundary. Detail sits at SOLAS chapter II-2 , marine fire detection and fixed fire fighting systems and the FSS Code .

Cargo space ventilation

Cargo hold ventilation is a moisture decision, and getting the direction wrong causes the damage it was meant to prevent.

Ship’s sweat forms when a vessel loaded in a warm, humid port sails into colder water: the hold steel falls below the dew point of the trapped air, and moisture condenses on the structure and drips onto the cargo. Cargo sweat is the reverse: a cool cargo loaded in a cold port is carried into a warm, humid climate, warm outside air is admitted, and moisture condenses on the cold cargo surface. Ventilating cures the first and worsens the second.

Two distinct rules govern the decision, and they use different measurements. The dew point rule, as INTERCARGO states it, is to ventilate if the dew point of the air inside the hold is higher than the dew point of the air outside, and not to ventilate if the dew point inside is lower than outside. The three degree rule compares the average cargo temperature at the time of loading with the outside air dry bulb temperature: ventilate if the outside air is at least 3 degrees Celsius cooler than the average cargo temperature at loading, and do not ventilate if it is less than 3 degrees cooler, or warmer.

The three degree rule is not a dew point margin. It compares dry bulb against cargo temperature, and it exists precisely because hold dew points often cannot be measured at sea. Merging the two into a single rule with a 3 degree dew point margin describes neither.

There is no general ventilation rate for a cargo hold. SOLAS II-2 regulation 19.3.4.1 requires at least 6 air changes per hour based on an empty cargo space for the specified dangerous goods that call for it, such as ferrosilicon UN 1408 and aluminium silicon powder uncoated UN 1398, with regulation 19.3.4.2 covering fan construction and wire mesh guards not exceeding 13 by 13 millimetres. For other cargoes a specific capacity is not defined: the individual IMSBC Code Appendix 1 schedule states the regime for that cargo and always overrides a general figure.

IMSBC Code subsection 3.5.6 is the provision that catches people out: for self-heating cargoes, mechanical ventilation is applied only in special circumstances, and in no case shall ventilation be directed into the body of the cargo. Subsections 3.5.1 and 9.3.2.1.3 require mechanical ventilation for cargoes liable to emit flammable gases forming an explosive atmosphere. Atmosphere measurement is a separate duty: SOLAS VI/3 requires an appropriate oxygen analysis and gas detection instrument for bulk cargo liable to emit toxic or flammable gas or cause oxygen depletion, with instructions for its use. Packaged dangerous goods are governed by the IMDG Code, which SOLAS chapter VII regulation VII/3 makes mandatory; the ventilation of cargo spaces carrying them is a SOLAS II-2/19 requirement recorded in the Document of Compliance.

Related detail sits at cargo liquefaction , IMDG Class 2 gases and cargo hold preparation standards .

Galley and sanitary exhaust

Galley extract is the heaviest exhaust duty on the ship and the one with the clearest fire consequence. Cooking generates heat, water vapour, grease aerosol and, where gas burners are fitted, combustion products. The design method is capture rather than volume: the hood is sized and positioned so the thermal plume off the cooking surface is captured before it escapes into the room, and the volume flow follows from that. Supply air is introduced at low velocity so it does not disturb the plume, and the galley is held slightly negative relative to adjoining spaces so odor does not migrate into the messroom or the alleyway.

The governing provision is SOLAS II-2 regulation 9.7.5, not 9.6. Regulation 9.7.5.1 applies to passenger ships carrying more than 36 passengers and requires that the duct be insulated to A-60 standard where it passes through accommodation, service spaces or control stations, with a grease trap readily removable for cleaning, an automatically and remotely operated fire damper at the lower end at the junction between the duct and the hood, a remotely operated fire damper at the upper end near the outlet, arrangements operable from within the galley to shut off the exhaust and supply fans, and access for cleaning. Regulation 9.7.5.2 covers cargo ships and passenger ships carrying not more than 36 passengers. Regulation 9.7.5.1.2 provides that these dampers need not pass the fire test in resolution A.754(18) or appendix 2 to the 2010 FTP Code, but shall be constructed of steel and capable of stopping the draught, and that the A-class requirement applies only to the part of the duct outside the galley.

Fixed extinguishing over the range is regulation 10.6.4, and it is specific to deep-fat cooking equipment. It requires an automatic or manual fire-extinguishing system tested to an international standard acceptable to the Organization, a primary and backup thermostat with an alarm on failure of either, arrangements for automatically shutting off the electrical power on activation, an alarm in the galley, and clearly labelled manual controls. The regulation carries no gross tonnage threshold of its own, and regulation 1.2.2.3 applies it to ships constructed before 1 July 2002 for new installations. The referenced test standard is ISO 15371:2024, the fourth edition, titled fire-extinguishing systems for protection of galley cooking equipment: the words deep-fat belong to the SOLAS regulation and were dropped from the standard’s title at the 2009 edition.

Sanitary spaces are held at slight negative pressure relative to corridors and cabins so odor cannot migrate. The flow path is one-way: fresh air enters the cabin at the terminal, migrates to the sanitary space through the door undercut, and leaves through a dedicated extract that runs continuously. MLC Standard A3.1(7)(c) requires that ventilation to be to the open air and independent of any other part of the accommodation. Galley equipment detail sits at marine galley equipment and provisions .

Controls, energy and carbon intensity

Shipboard HVAC control is layered. Room thermostats and humidity sensors feed zone controllers, which command terminal valve positions, fan speed through variable frequency drives, and chilled or hot water valve positions. Above them a building management system aggregates data, sets global modes for sea, port and emergency operation, and logs consumption. On large passenger ships that function is integrated into the ship’s automation system; on cargo ships a standalone controller is more common.

Three levers move HVAC energy, and they work because of how the load is composed.

Variable speed drives exploit the fan and pump affinity laws. Shaft power varies approximately with the cube of speed, so a fan at half speed draws roughly one eighth of full speed power before drive and motor losses. A 30 kilowatt air handling unit fan at 50 percent speed draws under 4 kilowatts. The saving is only realized where the control actually reduces speed, so a drive left at full frequency saves nothing at all.

Demand controlled ventilation uses carbon dioxide sensing to cut outdoor air supply when occupancy is below design. Outdoor air is the expensive air, because it arrives at the outdoor design condition and has to be brought to supply condition, so reducing its fraction reduces the load directly. The floor is the ISO 7547:2022 clause 4.4 rate applied at the clause 4.5 occupancy, and demand control trims toward that floor rather than below it.

Seawater free cooling routes the chilled water circuit through a plate heat exchanger against the seawater cooling system, rejecting the accommodation load directly to the sea and stopping the compressor. It becomes available when the sea is cold enough to produce the required chilled water temperature across a realistic heat exchanger approach, which in practice means winter operation in temperate and high latitude trades. Detail on the interface sits at marine sea water cooling systems .

State the free cooling saving as compressor electrical input, not as cooling duty. A chiller described as a 200 kilowatt machine is rated at 200 kilowatts of cooling; its electrical input is that duty divided by the coefficient of performance, so roughly 67 kilowatts at a coefficient of 3, 50 at 4 and 40 at 5. Net of the added circulating pump power the saving is smaller again. A claim that a 200 kilowatt chiller saves 180 kilowatts is mixing duty with input, because 180 kilowatts exceeds the machine’s own electrical draw at any realisable coefficient of performance.

Exhaust air heat recovery adds a fourth lever where the outdoor air fraction is high. A plate heat exchanger between exhaust and supply recovers sensible energy already spent; an enthalpy wheel recovers latent energy as well, which is what matters in a humid tropical trade where dehumidification dominates.

How HVAC reaches the carbon instruments

Through generator fuel, and through nothing else. MARPOL Annex VI regulation 28 applies to ships of 5,000 gross tonnage and above in the listed categories, and resolution MEPC.352(78) paragraph 4.1 computes the total mass of carbon dioxide from all the fuel oil consumed. Every kilowatt the HVAC plant draws is generated by an auxiliary engine burning fuel, and that fuel enters the calculation. See what is CII and CII corrective action plan .

The capacity denominator differs by ship type, which matters on a passenger vessel. MEPC.352(78) paragraph 2.5 names the deadweight-based indicator AER and the gross tonnage-based indicator cgDIST, and paragraph 4.2 assigns them: deadweight for bulk carriers, tankers, container ships, gas carriers, LNG carriers, general cargo ships, refrigerated cargo carriers and combination carriers; gross tonnage for cruise passenger ships, ro-ro cargo ships, ro-ro cargo ships carrying vehicles and ro-ro passenger ships. Describing the CII as carbon dioxide per deadweight-mile for all ships is wrong for the four types where HVAC load is heaviest.

To convert electrical load to emissions, use the specific fuel oil consumption and the MARPOL carbon conversion factor rather than a single remembered number. Resolution MEPC.364(79) gives 3.114 tonnes of carbon dioxide per tonne of fuel for heavy fuel oil and 3.206 for diesel or gas oil. At an auxiliary specific fuel oil consumption of 190 to 220 grams per kilowatt hour that is roughly 592 to 685 grams of carbon dioxide per kilowatt hour on heavy fuel oil, and 609 to 705 on marine gas oil. A figure near 200 grams per kilowatt hour is the fuel consumption, not the emission.

Refrigerant leakage does not enter any of these instruments. Regulation (EU) 2023/1805 Article 3 defines greenhouse gas emissions for FuelEU Maritime as carbon dioxide, methane and nitrous oxide, computed from the fuels and energy used on board. MEPC.352(78) paragraph 4.1 counts carbon dioxide from fuel oil. Directive 2003/87/EC Annex I, as amended by Directive (EU) 2023/959, covers carbon dioxide for maritime transport and, from 1 January 2026, methane and nitrous oxide. Leaked refrigerant has a real climate effect and is regulated through the fluorinated gas regime, but it does not appear in a CII score, a FuelEU balance or an ETS surrender.

The IMO Net-Zero Framework, a set of draft amendments to MARPOL Annex VI approved at MEPC 83 in April 2025, has not been adopted. MEPC’s second extraordinary session adjourned consideration of adoption on 17 October 2025, and MEPC 84 in the spring of 2026 did not adopt it. Any HVAC business case built on a carbon price under that framework is building on a measure that does not yet exist.

Maintenance, records and survey

Reliability on a commercial vessel is a spares problem before it is an engineering problem. A chilled water pump impeller on six weeks’ delivery keeps a crew in tropical heat for six weeks. The minimum list for a chilled water accommodation plant is a full compressor overhaul gasket set, expansion valve orifices or cartridges in the fitted sizes, pressure relief valve elements, chilled water pump shaft seals and impeller, filter stock for the fitted grade, and a reference cylinder of the fitted refrigerant.

The refrigerant cylinder is the item whose risk profile has changed. Under the Kigali phase-down and the EU quota system, availability of high global warming potential refrigerant at repair ports is falling, and Regulation (EU) 2024/573 Article 13(3) has barred EU servicing with gas of global warming potential 2,500 or more since 1 January 2025. A ship that cannot recharge after a major leak faces a diversion, not a delay. Spares policy generally is covered at marine spare parts and maintenance management .

Filters change on differential pressure, not on the calendar. A filter loads at the rate the air dictates, and a ship working grain or bulk cargo in a dusty port loads filters in weeks that would last months at sea. The trigger is the manufacturer’s stated terminal pressure drop across the bank. A calendar interval is wrong in both directions: it discards serviceable filters on a clean trade and starves the accommodation on a dirty one. On a variable speed fan the symptom of a loaded filter is rising fan speed at constant airflow; on a fixed speed fan it is falling airflow and under-ventilation complaints, which is the harder failure to notice.

Galley duct cleaning is the maintenance item with a fire consequence. SOLAS II-2 regulation 9.7.5 requires the grease trap to be readily removable for cleaning and requires cleaning access to the duct, but sets no interval; the interval comes from class rules and flag guidance, and the cleaning record is what an inspector asks for. Duct entry for cleaning is confined space work where the geometry demands it, and is governed by the ship’s permit system described at marine confined space entry and tank inspection .

Refrigeration plant maintenance centres on charge integrity and condenser cleanliness. Leak detection and charge logging by weight, condenser seawater flow and fouling, compressor oil condition and vibration on bearing-intensive machines are the recurring checks. Condenser fouling raises condensing temperature and therefore compressor power, and it moves discharge pressure toward the high-pressure cutout, so a machine that trips on high pressure in warm water is usually telling you about its tubes rather than its controls.

Comfort complaints cluster around four causes: thermostat set point or sensor offset, an outdoor air ratio that is wrong for the condition, chilled water temperature too high to meet the load, and draught from a mis-aimed diffuser. The diagnostic order is to log temperature and humidity at the complaint location, compare against the air handling unit supply condition, and confirm the terminal valve is modulating. A cabin that is cold by day and warm by night on one setting is describing solar gain, not plant capacity, and the fix is shading or diffuser aim rather than a lower chilled water temperature that penalizes every cabin on the circuit.

Limitations

The design values in this article are drawn from ISO 7547:2022 for accommodation, ISO 8861:1998 for machinery spaces, and the instrument texts named at each claim. Application to a specific vessel requires review of the following.

Parts of ISO 7547:2022 are not reproduced here. The published preview of the standard covers clauses 1 to 5.3. Clause 5.4 on heat gain from persons, clause 5.5 on lighting and other sources, clause 6 on airflow and air balance, and the normative annexes for machinery control rooms, the wheelhouse, dry provision store rooms and wheelhouse window heating are not quoted, because a licensed copy is required to state them accurately. Per-person heat gains, lighting gains per square metre and any occupied-zone air velocity limit should be taken from a purchased copy of the standard rather than from secondary sources, since figures in this family have changed between editions.

Flag state and class supplements. Flag implementing legislation for MLC 2006 may set higher outdoor air rates or more prescriptive equipment requirements than the ISO or ILO minimums, and each classification society publishes HVAC rules that add specificity for vessels seeking particular notations. The applicable class rule part, chapter and edition year governs, and this article does not substitute for it. That is especially true for flammable refrigerants: no generally applicable marine class requirement for A2L refrigerants in machinery spaces is stated here, because the requirements are society-specific.

Regional scope. Regulation (EU) 2024/573, Regulation (EU) 2023/1805 and Directive 2003/87/EC are EU instruments and are named as regional overlays on the IMO and ILO baseline. Their reach depends on where a ship trades, where equipment is placed on the market and where servicing is performed, not on flag alone.

Refrigerant transition timelines. The Kigali schedules and the EU quota calendar are set by instruments that can be amended, and Article 31(1) of Regulation (EU) 2024/573 empowers the Commission to amend the global warming potential annexes in the light of new IPCC assessment reports. Figures should be checked against the current consolidated text before they are used in a compliance calculation.

Energy figures. Plant ratings, load shares and payback periods vary by vessel, trade and hotel load profile to a degree that makes generic figures misleading, so this article gives calculation methods rather than typical values. The carbon dioxide per kilowatt hour range is derived from stated specific fuel oil consumption and published carbon conversion factors, and it moves with both.

Cargo-specific requirements. The IMSBC Code, the IMDG Code and the International Grain Code each carry cargo-specific ventilation requirements that override the general principles described here. The individual cargo schedule governs.

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