With Truma heaters now starting to corrode and fail after 30 years, and replacements being expensive, many owners are turning to diesel heaters - because they are cheap.
So the first rule is - think carefully - if you have the money - it is always best replace the Truma with another Truma - Carver are the same as Truma, and there are plenty of s/h ones about. There are also UK guys who can repair them. And of course there is the old originality argument - it's a classic van and original is best, but I am not a total purist, I respect the right of anybody to do what they like to their van, but it is no secret that I am a Truma and gas fan and don't like DHs in a Hymer, but I have owned 2, so it's not just based on sentiment.
You have to understand how the original Truma heater works in your Hymer - it's not a heater - it's a system. The Tuma fire both radiates, like a radiator, and it also blows hot air around the ducts in the van - so essentially it is the boiler of a central heating system. Inthe back of the Truma is a fan, and this fan draws air from the front of the fire, over the heat exchanger (that big aluminium box) and then into the ducts that run around the van. The result is that after warm up, the fire is quiet and background and all areas of the van are nicely heated - quiet and efficient.
What you need to remember is that it is that ducting system that is so important in this respect.
So when you install a DH - you have 2 choices - connect it to the ducts, or forget the ducts and just have the a single outlet from the DH blowing into the van.
And remember that a DH is a blown air heater only - it doesn't also radiate like the Truma used to do.
So you have to think carefully where and how you install it.
Next things you have to think about are its connections. A DH needs a fuel pipe supplying diesel, an exhaust pipe system to the outside, and a solid 12v electrical connection. Plus as discussed, it either needs interfacing to the warm air ducts, or it needs either it's own set up hot air distribution ducts, or it needs to be positioned in such a place as to direct its single hot air outlet into the van, in a comfortable way - ie where it will heat the whole van efficiently. And once you decide this, all the connections I just mentioned have to be plumbed in to that point.
The best scenario fr a DH install in a Hymer is to install it so that it blows into the existing Hymer hot air ducts - but even this is not straightforward. A DH has a heat outlet temperature sensor and if this gets too hot it will shut down the heater. A DH is a powerful beast, so the heat has to be directed into the ducts quickly and efficiently. if there is any back pressure - ie the ducts can't carry away the heat fast enough, then you can get problems, especially if you are using a big DH - it might just put out more heat than the ducts can handle, so will overheat and shut down. See below about sizing the heater to the van.
The next ting you need to know about is that a DH is a completely different beast to any other heater you have ever known. Almost every heater you have ever known either in a van or a house, gives heat quite quickly - you turn on a gas fire or a fan heater and you feel the heat almost immediately. But a DH isn't like that. Diesel fuel is not like gas, it doesn't combust easily. If you throw a match into a bowl of diesel, the match goes out! So diesel has to be cajoled and provoked to finally combust. The way it does this is to use a glow plug - similar to the ones in your engine to start on a cold morning. The DH has a combustion chamber into which the fuel is injected. This is done by a pulsed pump - a DH doesn't use an injector like a normal engine - that would be too expensive to manufacture. A DH uses a pump that squirts a small quantity of diesel into the combustion chamber every second or so. This is the familiar tick tick sound that a DH pump makes. The hotter you want the heater, the faster the pulses. But this method takes time. The DH starts by switching on the fan and the glow plug, and this needs 12v - the plug glows red hot and after a few pulses the diesel starts to burn. But it takes a while, about a minute. The pump runs slowly at first, giving the fuel chance to burn, and then increases, eventually to the level that is preset by the control panel. It is very similar in basic content to a jet engine - it needs a spin to get it going, and then it becomes a self feeding cycle - the more fuel you inject, the faster it spins!
So what you need to understand about this process is that it takes time! And once going, if you want to switch it off, the whole process has to happen in reverse. When you switch off a diesel heater, it has to go through a cool down cycle - the fuel is stopped, but the thing is still hot, so the fan has to be run until the temperature sensor reports the temperature low enough so that the fan can stop.
So here is the big disadvantage of DHs - they take time to wind up, and time to wind down. This means that if they are being switched on and off by a room thermostat, the whole process has to be repeated. It is nothing like a gas fire where the burner ignites immediately in response to the thermostat and gives heat immediately.
And in cold weather in a van, you can experience big differences in ambient temperature as it goes through this cycle - a common complaint from DH users is that the ambient temperature goes up and down dramatically because of this long warm up and cool down cycle.
But there is a way around this - if you can set the DH up so that it just runs constantly, putting out a constant stream of heat that is just the right amount to keep the van at the temperature you want, then there is no reason for it to shut down and warm up again. But this can be more difficult than it sounds.
The reason for this is that DHs are powerful beasts. They put out a lot of heat. The Truma in your van was either a 3002 in a 5 series van, or a 5002 in a 6 series van. The 3 and 5 refer to kilowatts. DHs come in different sizes, and the bigger ones are hardly more expensive than the samller ones, so people tend to go for bigger is better. But it's not - an 8kw DH, on its lowest setting will make a 5 series van like a sauna, and it won't go any lower - if it gets too hot all you can do is switch it off, let the van cool, and then switch it on again, and believe me, I've been there, it is not relaxing. You are trying to watch TV and the heater is going from zero to hero every 30 minutes and the temperature in the van is varying by 10 or 15 degrees - ie cycling between cold and hot. It is not pleasant - I have been there!
So the trick here is not to over-specify. Most Hymers only need a small DH - 3kw max. So that on it's lowest setting it gives out constant heat, but not too much. And then you can fine control the heat by opening windows and rooflights a crack. You want a DH that on its lowest setting will not overheat the van. You need to prevent the DH from having to go through its warm up cool down cycle - that means that it should always be running, with its lowest setting not to hot. Increasing heat from low is easy - you just press a button and the pump speed increases, and the extra heat comes quite quickly.
And this is the main point of this article - a correctly planned and installed DH can work very well. But it has to be that - planned and installed. It is not like a fan heater you can buy from Currys and just plug it in. You need good solid DIY skills - you need to cut holes in the floor, tap into your van's diesel tank, and make good 12v connections. And then you have to be willing to fine tune.
If you don't have these skills, or access to them, then you should consider just replacing the Truma - or even fixing it. Truma fires are very fixable - the spares are still available although the dealer expertise is starting to retire. But I have always said that anybody with the skill to install a DH also has the skill to fix a Truma.
Miscellaneous things to remember are .... a DH needs 12v, and plenty of it. It will draw 10 amps for about 5 to 10 minutes - this is for the glow plug. Once the fire is running it only needs 12v for the fan - less than 1 amp. But that 10amps is quite a draw, and if your battery is either tired or getting low that 10amp draw can pull down the voltage of the battery - and if the voltage drops below a preset threshold, the DH itself will detect low voltage and then shut down. So you need a robust battery system and good cabling to minimise this risk. One of the reasons Hymer used the Truma is that if there is no battery power needed at all, you can still get heat from a Truma - but it won't blow hot air because there is no power for the fan, but it will radiate - it needs no 12v power to run in radiate mode, and turn up high it will heat the whole van. A Truma will heat a van with no battery power at all.
Another thing to think about is pump noise - the pump makes a tick tick sound, and depending on where it is located this noise can be annoying. There are silent pumps available, but these need fitting and are often as expensive as the heater itself.
Next thing to consider is running cost - you are using a road taxed fuel for heating - at 2024 prices £1.60 a litre. For occasional holiday use it's not an issue, but for full timing through a cold winter it is expensive - they don't sip so little fuel that you don't notice it - that is a myth. A DH in cold weather, used 24/7 in a full timing or winter holiday situation will use a lot of fuel. You can mitigate this by using kerosene, heating oil, which is cheaper, but then you need a separate tank and this has to be located somewhere in your van, and has to be filled, so you have to either fit a big tank, or carry a container. And don't forget that kerosene is smelly!
So in conclusion - DHs are not bad! But it's horses for courses. They are pretty much the only choice for van most van and truck conversions, but installing one to replace a Truma in a Hymer is not a simple operation.
You have to take out this ....
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