Air Heaters On Boats

Choosing the right air heater for your boat is important. There are a number of things you should consider, including brand and heater size. There are also installation considerations like fuel source, heater location and, especially, the exhaust system.

Heater Brand

First and foremost, you should decide which brand of heater you would like for your boat. There are two main categories in boat heating. The top of the line are Wallas air heaters. These are made in Finland and designed from the ground up as a marine air heater. Wallas are really the ultimate in terms of quality, quietness and style.

Wallas Spartan diesel air heater

As an alternative to Wallas, there are also marinised caravan/RV heaters that can be set up for use on a boat. These are still high quality heaters by Eberspacher (Germany) or Belief (China) with two year warranties, quality parts, and professional, timely service.

eberspacher d2

The main deciding factors between Wallas and marinised RV heaters are as follows:

  • Price: Generally, a Wallas 22GB or 30GB diesel heater full install kit will cost $1000 to $1500 more than a marinised RV heater. See our full range of possible marine diesel heaters for more information.
  • Exhaust: Wallas heaters have coaxial exhaust and combustion air, which makes the exhaust pipes cool to the touch. They are designed for marine applications with their own hull fittings, deck lead throughs, etc. For the RV heaters, we assemble marine exhaust kits using either Webasto stainless inline mufflers or exhaust pipe and hull lead throughs. These heaters have very hot exhaust. We supply lagging, but  the installer must still be careful with the exhaust pipe installation. We strongly recommend that the original RV exhaust is not used in marine applications.
  • Exhaust exit point: Wallas heaters have the exhaust coming out of the top, whereas RV heaters have it coming out of the bottom or are side mounted. In most situations, the top exiting Wallas exhaust is easier to accommodate and install.
  • Carbon monoxide: The Wallas coaxial exhaust reduces the risk of carbon monoxide leaks as the exhaust is running inside the combustion inlet air pipe and any exhaust leaks are re-burnt by the heater. Marinised RV exhausts are single layer. They are high quality and reliable, but should be inspected regularly for cracks to ensure carbon monoxide cannot leak into the boat.
  • Boat drying: Wallas heaters have two inlets and two outlets. The idea is to connect one inlet to the cabin to recirculate warm air and one to the outside to bring in new, cooler air. This cool air forces warmer air out of the cabin via vents. The warm air takes moisture with it, reducing air moisture and condensation in the boat. Marinised RV heaters have only one inlet and outlet. The inlet air can be split to mix recirculated and outside fresh air, but it is more complex to install. Extra outlets on RV heaters are easier as we have Y junctions for the ducts available.
  • Aesthetics: Although looks can be subjective, the aircraft grade aluminium and stainless steel of the Wallas heaters looks fantastic. The plastic on the RV heaters just can’t compete. 

Choosing a Heater Size

Choosing the right size heater for your boat can be tricky. Factors to consider include:

  • Size of boat/area to be heated: The size of the heated area is more relevant than the size of the entire boat. A huge boat can have a small cabin or vice versa. Maybe only the sleeping quarters will need to be heated, or possibly the whole boat.
  • Number of outlets: Longer ducting and extra outlets reduces efficiency, so larger, more complex systems need bigger heaters to drive them.
  • Boat construction: Steel and aluminium boats lose heat much faster than wood or fibreglass boats.
  • Where you sail your boat: Clearly, if you are just dabbling around Pittwater in NSW, your heating needs differ to someone who is based in Tassie and heading south to explore. 

Fuel Supplies for Boat Heaters

Most boats have diesel on board, so the best choice is to tap into the existing supply. All diesel heaters have tiny fuel pumps and cannot pull diesel through large diameter fuel lines. The supplied fuel lines have 1.5mm ID and should be used. The simplest method of getting a fuel supply is to put a new intake into your existing tank. If access to the top of the tank is possible, this is a good option. If access isn’t easy, another option is to install a connection into the remote fuel filter on your main engine or generator. Most remote filters have a spare outlet that can be drilled and tapped for a 3/16 nipple. We can provide fittings for connecting into your fuel supply. 

If you don’t have diesel on board, or it’s too far away or too difficult to utilise, you also have the option to use an ancillary fuel tank. We have a range of suitable tanks that can be installed in engine rooms, fuel lockers or cupboards. Given the low fuel consumption of diesel heaters, even a 10L tank can offer hours of operation.

Heater Location

Whilst the Wallas heaters are marine specific, and the marinised RV heaters have a marine exhaust system, neither is designed to be splashed or immersed in salt water. We definitely don’t recommend installing heaters into bilges, old, damp engine rooms or any area that may be subject to splashing or excess humidity. Ideal locations include cupboards, dry machinery spaces, spare lockers, etc.

When choosing the heater location, access for the exhaust exit point and the recirculated and fresh air inlet sources should be considered. Access for the hot outlet air, ducting and vents is also important. We have various ducting and venting options available. Different sized heaters have different diameter ducting, so before you confirm the heater size, ensure the proper ducting will fit.

Choosing the location is also dependant on the heater type. Wallas heaters have the exhaust coming out of the top, which is easier for a marine installation. Marinised RV heaters have either bottom or side exiting exhausts, so space needs to be allowed to bring these up to the highest point. All heaters with hull or stern exiting exhausts must have a gooseneck in the exhaust – the exhaust must go up to a high point above the exit point before it goes down to the heater – to prevent water splashing back inside the heater. Hull or stern exhausts should always be as high as possible above the water line.

The table below gives some guide on heater sizes, but needs to be considered in conjunction with the factors noted above when thinking about a heater size.

Spring or Autumn Use

Winter Use

Approx Boat Size
Heater Options

Multiple 3kW or 4kW heaters from above list, depending on zones and need to have differential control of temperatures, eg. cabin versus sleeping areas.

Further Reading:

Eberspacher Marine Hydronic Installation Guide

Wallas Marine Heater Installation Guide


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