Staying warm when it's cold out or cooling off in the blazing summer sun can be tricky. To help, Adam Fiander gave us this guide to boat heating and air-conditioning
Being able to control the internal temperature of your boat not only improves the comfort and enjoyment of you and your guests, it can also extend the boating season and the life of critical electronic equipment and soft furnishings by keeping them warm and dry. Many modern systems can even be controlled from a smartphone app so you can set it to warm up or cool down before you arrive on board.
Can one system take care of boat heating & cooling?
Reverse cycle air-conditioning units can generate both cool air and warm air for year-round comfort but the surrounding sea-water temperature has to be sufficiently warm for the heat feature to work effectively. As a result, most cruising boats are fitted with separate heating and/or air-conditioning systems so those based in colder climates can prioritise heating while those in hotter countries can prioritise air-conditioning.
A good all-round option for boats wanting to cover both is a self-contained A/C unit and a diesel air heater as they are both relatively compact, easy to install and cost effective to run.
Combined heating and cooling systems are best suited to larger vessels as the larger, noisier parts can be kept in the engineroom and you can add a diesel water heater into the circuit for when the outside water temperature is too cold or the generators are switched off.
How does an air-conditioning unit work?
Air-conditioning changes the pressure of a refrigerant to move heat energy from one area to another. In cooling mode, a compressor pressurises the refrigerant into a hot gaseous state which passes to condenser coils (twin-walled copper pipes, with hot gas passing in one direction and cooling sea water in the other).
The heat is transferred to the sea water and ejected back into the sea. The refrigerant then flows to the evaporator, where it evaporates into a low-pressure gas, absorbing heat from a cabin.
In heating mode, the direction is reversed; the evaporator becomes the condenser and vice-versa. Heat is now extracted from the sea water and directed back into the cabins.
A typical water based system heats up water in the engineroom then circulates it around the boat to radiators and/or fan assisted heat exchangers
What’s the difference between systems?
Reverse-cycle ‘self-contained’ units have all the major components contained on one chassis. They can be conveniently concealed under beds, seats or behind cupboards, making them ideal for retrofitting to small or medium sized boats up to 50ft.
Split-gas systems are split between two units installed in different locations connected by copper tubing. The condensing unit, comprising the compressor and electrical components, is normally located in the engineroom, while the evaporator unit is installed in the living area. Two air handlers can connect to one condensing unit to cool multiple cabins or a single large area. This is ideal for craft up to 80ft.
For boats of more than 80ft, chilled water systems use an insulated closed loop system with the main chiller unit located in the engineroom or machinery space. The system is plumbed around the boat to individual air handlers and controls in each of the key guest areas.
The internal workings of an Eberspächer Airtronic D2 blown air heating unit
What’s the difference between an air heater and a water heater?
An air heating unit (or blown-air unit) is a compact device containing an internal burner with an electric fan that blows warmed air into cabins via concealed ducting and outlet vents. These units draw small amounts of diesel from the boat’s main tank and are suitable for boats from 15ft to 35ft. They provide rapid heat, with minimal start-up time at relatively low cost.
Water based heaters are larger plumbed-in units that use an internal burner to heat up water which is then pumped around the boat in pipes similar to a domestic central heating system. Heat can then be dispersed into the cabins through convectors such as radiators and towel rails, or through air vents connected to hot water matrix units and an electric fan. Water-based systems take a little longer to generate heat than blown-air units but they can also be used to heat water for washing-up and showers, without having to run the boat’s engine(s).
Modern systems allow you to set the temperature on board before you arrive at your boat
How are heating and cooling systems measured and rated?
The output of heaters is normally rated in kW while the output of cooling systems is measured in BTU/hr (British Thermal Units), even though both are measurements of thermal energy – 1kW is equivalent to 3412.14 BTUs.
When referring to an air-conditioning unit, the BTU/hr figure is the amount of heat it can extract per hour from a given space.
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What size do I need for my boat?
This comes down to a number of factors, such as the type and size of boat, the amount of glazing, the number of cabins, the total internal volume, the boat’s thermal insulation properties, and the intended cruising area of operation. A good installer will survey your boat for all these things, do the calculations and make an informed recommendation based on his findings.
For a basic diesel heating system, using a single blown-air or water heater, a good rule of thumb is: 2kW for sub 30ft boats; 4kW for 30-40ft boats; 5-6kW for 40-50ft boats and 8kW+ for boats over 50ft. A similar rule of thumb applies to the ratio of outlets to kW, with one outlet recommended per kW of heating capacity.
Blown air heaters are small enough to be installed in a variety of convenient locations
When it comes to A/C units, self-contained systems are normally best for boats with up to three main internal spaces. Sleeping cabins usually require 7-12,000 BTU while saloons usually need 12,000+ BTU. As a rough guide you need somewhere between 0.1kW – 0.2kW (300-600 BTU) per m3.
How much power do boat heaters and A/C units use?
For diesel air and water heaters power draw will always be highest at start-up. Most modern diesel air heaters from 2-6kW will use between 15W-100W but you need to add fan-blower consumption if natural convectors (radiators/towel rails) are not being used. Once core temperature is reached, they should run at 50% or less power.
The 50% power consumption rule applies to A/C units as well, which normally draw between 2A-8A when running a self-contained system of 6,000 BTU-20,000 BTU. Units with stop/start compressors have a high start load requirement that also needs to be considered.
Chilled water systems are often “variable speed” meaning there is no start load and units wind up from 2A to 30A or more, depending on the amount of cooling needed.
Do I need a generator to run them?
For smaller blown-air and water heaters, standard 12V and 24V leisure batteries are sufficient. Most air-conditioning systems will require a generator due to the high start-up loads, although with advances in solar panels and high-density lithium batteries this is no longer a prerequisite.
A Frigomar air-conditioning unit showing the condenser tubes that chill the air
What do they cost to buy/install?
For a theoretical installation of a 4kW diesel air-heater with three vent outlets and remote control to a 30ft motorboat, expect to pay in the region of £1,400 to £2,100 plus VAT for the unit itself, and around £950 plus VAT for labour.
On a 45ft boat with a 10kW plumbed-in water heater with four blown-air matrices, expect to pay between £4,500 and £6,000 plus VAT for the materials and £2,250 to £2,500 plus VAT for installation. For a top-end brand, this would likely include a programmable timer and touchscreen controls for each cabin.
For A/C systems, the price variables are that much wider but for a boat in the 50ft region, expect to pay in the region of £35,000 plus VAT for four self-contained A/C units with 44,000 BTU combined and a new generator, plus around £6,850 plus VAT for the survey, installation and commissioning.
A chilled-water system for an 80ft boat with 260,000 BTU and generator could be in the region of £100,000 plus VAT, with the survey, installation and commissioning estimated at around £12,000 plus VAT.
Fitting a heating system is best left to professionals due to the safety implications of getting it wrong
Can I retrofit a heater kit myself?
While it is possible to install a basic heating system yourself, we wouldn’t recommend it due to the inherent risks and dangers involved. Jobs such as drilling a hole in the fuel tank for the pick-up or in the side of the hull for the exhaust outlet are best left to accredited professionals as the consequences of getting it wrong could be catastrophic.
Do I also need to fit fire and carbon monoxide alarms?
Even though blown-air and water heaters are sealed units, our advice is always to fit a carbon monoxide alarm as the gas itself is odourless, invisible and lethally toxic. They are not expensive and could save your life.
What’s the difference between a marine kit and a motorhome kit?
Although individual blown-air heater units for motorhomes and small boats are similar (identical in some cases), the installation kits for boats are different to vehicles for safety reasons and the two are not interchangeable. Boat kits from reputable manufacturers are made to marine-safe ISO standards and conform to latest regulations. Be extra cautious if buying online.
Simple blown air heating systems are quick to fire up and relatively cheap to fit and run
How do I go about getting my boat heater serviced?
Thankfully, most blown-air units do not require regular annual servicing. Occasional-use boaters should consider an inspection and service every 3 to 4 years, or 1-2 years for more frequent boaters and liveaboards.
Signs telling you it’s time for a service include failure to start easily and/or white diesel vapour or grey smoke from the exhaust. Burner units may need occasional cleaning in the case of poor-quality diesel.
Maintenance and fault diagnosis for self-contained air-conditioning and water-chiller systems should be carried out by authorised installers or suppliers of the relevant brand. They will have the appropriate software and experience to diagnose and resolve any issues.
Why is one of my cabins slow to warm up?
For blown-air heaters, one of the likely causes is incorrect duct pipe balancing. This happens when the outlet ducts in the problem cabin are a smaller diameter to those in the other cabins. This imbalance causes the warm air to escape into cabins with larger ducts, leaving insufficient pressure to reach the problem cabin.
With a water heater circuit, the pump and heater tend to be located towards the back of the boat and sometimes the hot water tries to find a shortcut, through a matrix blower, radiator or calorifier, rather than travelling all the way round the boat. It’s also worth checking screwdriver-type isolating valves are set correctly to balance the flow.
Who makes the best boat heater systems?
Webasto, Eberspächer and MV Heating are among the best known names for air and water based heating systems, all of whom we spoke to when researching this article. For air-conditioning systems, Dometic (via Fischer Panda), Webasto, Frigomar, Nauticool and Climma (Veco) are among the best.
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