Which Water Heater Should I Buy?

Most water heater listings sell on two numbers: fuel efficiency and litre capacity. Both matter, but neither is where the decision starts. The right heater follows from how the vehicle is used, what is already fitted to it, and how hot water sits alongside heating the living space. With those three things settled, the specifications finally start to mean something. This guide works through them in order, so that by the end the shortlist is obvious rather than overwhelming.

The first question is whether you need heating as well as hot water

A combi heater warms the living space and the water from a single appliance. For winter touring, cold shoulder seasons or full-time living, that is one install, one fuel feed and one flue instead of two, and it frees up space that a separate heater would take. For a summer-only van in warm weather, a combi is more appliance than the trips call for, and a water-only heater costs less and fits into a smaller space. Anyone who already runs a separate space heater needs only a water heater, and adding a combi alongside it would pay twice for heating already covered. Settling this first removes half the catalogue at a stroke.

The fuel that suits you is usually the one already on the vehicle

Fuel type is where most of the marketing noise sits, and where an honest steer saves the most money. Each option has a real cost that the product page tends to leave out.

Electric is the simplest to fit. There is no combustion, so no flue to cut and no gas work. On 230V it needs a mains hook-up or a generator to run. On 12V it will run from the leisure battery, but heating water with electricity draws heavily on that battery, far harder than lighting or a fridge does. A 12V electric heater rewards a van with solar and good battery capacity, and it frustrates a small single-battery setup used off-grid for several days. The freedom it advertises is real only if the power behind it is there.

Gas (LPG) heats water quickly and runs anywhere, independent of the electrical system. The price of that independence is carrying and refilling bottles, and fitting a flue for the exhaust. It makes obvious sense for a van already running gas for cooking, since the supply is there to tap into.

Diesel draws from the vehicle's own fuel tank, so there is no gas bottle to carry and no separate fuel to buy. It suits builds that are going gas-free by choice, and heavy winter or full-time use where refilling gas bottles often becomes a chore. In this range diesel comes as part of a combi unit rather than as a standalone water heater, so it is worth considering alongside the heating decision above.

The plain version of all this: the cheapest and least disruptive heater is almost always the one that runs on a fuel the vehicle already carries. A gas van wants a gas heater, a big-battery solar van can take electric, and a gas-free build points to diesel. Fighting that usually means paying to add a fuel system you did not otherwise need.

A stored tank and on-demand hot water suit different needs

Storage heaters hold a set volume of water, heat it, and keep it warm. That is enough for a shower or two and the washing up, followed by a wait while the tank reheats. The behaviour is predictable and undemanding, and it forgives a modest water system.

Instant, or tankless, heaters warm the water as it flows, so the hot supply does not run out. That suits a busy horsebox or a larger van where several people wash one after another. The catch rarely mentioned on the box is that an instant gas heater needs a certain rate of water flow to fire and to hold its flame. A weak pump or low pressure can leave it cutting out mid-shower. An instant heater rewards a van with a strong, well-set-up water system and can disappoint one without, so it is a poor first choice for a small camper running a low-output pump.

Tank size follows the number of people washing, not the size of the van

Capacity is about how many people draw hot water close together, not the length of the vehicle. A 3 to 6 litre tank handles washing up and hand washing comfortably, and stretches to one careful shower. Around 10 litres is the usual choice for relaxed showering and for a couple travelling together. Larger tanks take longer to reach temperature and add weight that is carried on every mile. Buying more capacity than the trips actually use means heating water that goes down the drain unused, so the honest aim is enough, not most.

The fitting work belongs in the price

The number on the listing is only part of what a heater costs. Gas and diesel heaters burn fuel, so both need an exhaust flue cut through the floor or a wall, and gas work should be carried out by a competent, suitably qualified fitter. Electric heaters need neither, which is a large part of why they appeal to first-time self-builders doing the work themselves. Mounting position carries its own trade too. An underfloor heater frees up interior space but sits under the vehicle, exposed to road spray and weather. An onboard unit takes up a cupboard or part of a bed base, but stays protected and within easy reach for servicing. A heater that looks cheaper on the page can cost more once the install and any professional help are counted in.

Where each range fits

With those decisions made, the ranges sort themselves. The Electric Water Heaters and Pundmann twin rod models suit gas-free builds and hook-up touring, and earn their keep off-grid only where the battery and solar can feed them. The iMass Instantaneous gas heater is the pick for unlimited hot water in a horsebox or larger van with a strong water system, and the wrong pick for a small camper with a weak pump. The dual fuel Truma Ultrastore and Whale Expanse heaters cover touring that switches between hook-up and off-grid, with the Whale offering a choice of underfloor or onboard mounting. Where heating the living space is part of the job, the Truma Combi range does both from one unit in gas or diesel, and the Pundmann Air Combi models do the same on electric alone.

The best heater is not the one with the highest efficiency figure on the box. It is the one that matches how the vehicle tours, the fuel it already carries, and the water system feeding it. Anyone still unsure after all that is welcome to describe their setup to us and we will point them to the right shortlist.