During a recent trip to the Alps the heating system in the van failed overnight when it was -15degC outside – we woke to find it was 1degC in the van, cold air being blown from the heating vents and the fresh water had frozen!
Below are some details about how the Truma blown air heating system works, and how to perform some basic diagnostics. This might get you out of trouble but remember that messing about with gas appliances is inherently dangerous, get something wrong and you may gas yourself, blow your van up, or worse! – only work on your system if you are confident and competent to do so…..
First up – an overview of how the system works:-
The Trumatic C3402 is a gas fired combined hot water and blown hot air heating system. (There are different variants, such as the more powerful C6002 and the newer Combi 4/6. All work on the same sort of principle, the main different being the newer systems are horizontal rather than vertical)
The boiler consists of a central gas burner/combustion chamber, combustion air is blown into the burner assembly by a small fan. The hot gasses pass over a set of heat exchanger fins which surround the combustion chamber before exiting via the flue pipe to the outside of the vehicle. The flue is actually coaxial (think a tube within a tube), where fresh combustion air drawn into the heater through the outer tube and exhaust gasses are expelled through the inner tube. This is a simple yet clever system that helps maximise heat energy extracted from the exhaust gas while also not taking combustion air from inside the vehicle (which would cause a draft somewhere as air would have to get in to the van to replace it)
Surrounding the exhaust gas heat exhanger fins are another set of heat exchanger fins, though these are sealed from the customer gasses. For blown air heating air from inside the van is blown over these fins by a larger fan to heat it up before entering a manifold at the bottom of the boiler where it is blown out through the four hot air distribution ducts.
Finally surrounding these heat exchanger fins is a 12 litre water jacket, and heat energy from the combustion chamber not extracted by the blown air heating ends up in this jacket, heating the hot water.
There are a couple of different style control panels used with this system, ours is the older style which looks like this:-
There are some switches to select between off/hot water only/hot water and heating & for selecting the hot water temperature, a thermostat knob for setting the room temperature and some lights to let you know what is going on.
The boiler control panel red light will flash if the battery voltage is too low to operate the unit (about 10.5V I believe), and light up solid red if the boiler has ‘locked out’ (shut itself down due to something being wrong)
In theory a lockout can be caused by a variety of things, such as:-
- No combustion airflow detected
- No flame detected
- Boiler overheat
In our case when we woke in the morning there was no red light on the controller and the combustion and room fans were still running, suggesting the boiler thought everything was fine – the flame had clearly gone out though!. I reset the system (turned it off and on again!), the boiler attempted to start but went to a lockout state (red light) shortly afterwards.
In a normal startup sequence (hot water demand only) you can usually hear the various parts of the heater operate this order:-
- Combustion fan starts (think a quiet-ish fan noise)
- Gas solenoid operates (loud click) and gas ignitor starts firing (fast sequence of quiet clicks, you have to be very close to the boiler to hear this)
- Gas lights (a muffled thump type noise from inside the boiler)
Note: if you have the control panel set to heating and hot water the room fan will run also. The same sequence of events above will still happen but the noise from the room fan makes it harder to hear what is going on!
First things to check are is the gas still working (checking the hob is the easiest) and battery voltage. Our hob was working fine and the battery was reading 11.9V so it all looked OK. Note: Butane gas will not vapourise below about 3degC so Propane is required for cold weather use, we were in the mountains in the winter so had two 11kg Propane cylinders on board.
Our system seemed to have developed multiple faults all at once, sometime the gas would light but the ignitor would then start firing again (even though the boiler was still lit) suggesting the boiler thought the flame had gone out. Occasionally it sounded like the gas solenoid hadn’t fired at all (suggesting either a defective solenoid or defective driver circuitry). Sometimes the system would run but then the room thermostat became unresponsive (the system would shut off when the van got up to temperature but then not re-start when it cooled down).
To me this pointed to a defective PCB (main control board) – as this was the only component I could think off that could cause so many of these symptoms. Either way it was time for a strip down and inspection to see what was going on, the unit had been rattling around in our old bus for the last 18 years after all and we’ve got no record of it ever being serviced!
Strip down and inspection
In our Hymer B544 the boiler is located underneath the wardrobe, emptying the wardrobe and lifting up the base allows you to see the unit as installed:-
Here you can see the exhaust/combustion air flue (black hose), hot air ducts (brown hoses), PCB (control board) housing (black box on front of boiler) and hot water outlet (red hose connected to right hand side of unit)
You can also see the 240V inlet (white unit in side of van behind boiler) and 240V distribution box (white box in bottom right hand corner of picture) – but these don’t have anything to do with the Truma system!
The small black box with the red knob on it (between the two left hand hot air ducts) is the frost protection valve. This is connected to the water system and opens if the temperature in the van drops below about 4degC, emptying the contents of the boiler out underneath the van. This is to prevent the boiler freezing with water in it causing the hot water cylinder to crack. This vale is held closed electrically so if you disconnect the habitation batteries or blow the fuse supplying 12V to the heater it will open and drain the boiler. Once it has activated it has to be manually reset by pulling the red knob upwards (if you need to keep it closed temporarily but the temperature in the van is too low to allow it to latch a clothes peg can be used to hold it closed, don’t forget to take it off once the van has warmed up though!).
Here is another view of the frost protection valve:-
Removing the shelves in the lower section of the wardrobe and unscrewing the wooden panel behind them allows much better access to the unit:-
You can now also see the electric ignitor unit (small blue box clipped to the right hand mounting leg under the boiler)
Removing two small screws in the top of the PCB housing box allows the cover to be removed for access to the PCB:-
I’ve unplugged the electrical connections in the above photo, the board is just clipped into the housing so can easily be removed for inspection now. There was no obvious problems on visual inspection (no burnt looking components, leaking capacitors or damaged solder joints, but I lack the electronic diagnostics skills to go much further than this!)
The room fan and housing unit can be removed by disconnecting the electrical connection to the fan (down the back of the unit near the fan) and undoing the 4x 9mm plastic nuts on the top of the unit. The whole assembly can them be lifted up and removed, leaving the boiler looking something like this:-
Here you can see the central can housing the burner/combustion chamber, the outer heat exchanger fins that the room fan blows air over and the hot water cylinder on the outside.
With the fan housing removed you can get down the back of the unit to disconnect the flue – warning: it can be difficult to get it reattached and you are supposed to use a new o-ring on the exhaust connection whenever the flue is re-attached so I don’t recommend disconnecting it unless you have to. I disconnected it to check for excessive soot etc. in the exhaust which would point to a combustion problem, but it was all clean in my case.
To get access to the burner assembly you now need to move the whole unit. First job is to disconnect the hot air ducts – unfortunately they are held in place with little plastic tabs top and bottom that have to be depressed to withdraw the duct, and the top tabs are almost impossible to access with the unit assembled (it doesn’t appear to have been designed for disassembly!). I opted to just remove the front two duct hoses (which involved breaking the top tabs on each one) and leaving the rear ducts connected. It meant I couldn’t remove the whole unit from the van but I could at least turn it on its side to work on it.
Here is a view of one of the disconnected hot air ducts, you can see the locating tab on the white collar on the end:-
You also need to disconnect the gas supply to the unit. In the above photo I have already disconnected it but you can see the union on the boiler where it attaches (in front of the red hose underneath the boiler) – don’t forget to turn the gas of before you disconnect it! (in fact – don’t forget to turn the gas off before you do any work on the system!)
The unit is bolted down on three legs, with these undone, the cold water feed to the unit disconnected (I disconnected it at the frost protection valve) and the electrical feed disconnected (mine was connected on a flying lead near the frost protection valve) the unit can be carefully lifted up and turned over for access to the underside:-
The burner assembly can now be seen held into the bottom of the unit by three 8mm nuts. To remove it you need to take the gas valve/solenoids off (held on by two screws, it has already been removed in the above photo but it would have been attached to the burner behind the water hose at the bottom of the photo). You also need to unscrew the combustion fan air feed ducting (black plastic housing in the middle of the photo) – there are three screws that attach this to the burner, two are obvious but one is hidden inside the ducting to get to it you need to remove the air flow sensor from the ducting (the little pcb you can see next to the ducting in the photo above). With this removed you will be able to see the screw inside the ducting, a feed a long screwdriver down through the mounting leg and into the ducting to remove it (see photo above – I left the screwdriver in to make it clear!).
The three 8mm nuts can now be removed and the whole burner/combustion chamber assembly slid out of the bottom of the unit (it may take a little persuasion as it’s likely been in there for at least ten years)
This is the whole assembly out of the boiler, with the electric ignitor unit still attached:-
The combustion chamber (large metal tube over the burner) can just be slid off. This is what my burner looked like underneath:-
There is a bit of soot buildup in the middle of the unit but generally it looks it decent condition. The probes are looking a bit worse for wear though. (the two close together are connected to the ignitor, it creates a high voltage spark between these electrodes to light the gas. The probe on its own is the flame sensor – the boiler uses this to know if the gas has successfully lit or not, or to tell if the flame has gone out during operation)
I believe you can buy new electrodes separately and they should be easy to fit once you have it all apart like this, but I found a good deal on a whole burner assembly so I decided to replace the whole thing.
Note – it looks like the boiler uses ‘flame rectification’ to detect if a flame is present, and the electrodes in these systems are very sensitive to dirt/corrosion on the electrode. It is entirely possible that just cleaning up the electrode with fine wire wool would have fixed the problem, but after getting this far into the guts of the unit I didn’t fancy trying it in case it didn’t fix it and I had to pull the whole lot apart again……..
Here is the shiny new unit:-
I also decided to replace the PCB as I found one for a good price. It’s actually from a later version of the C3402 and not completely identical, but looked close enough that I was willing to risk it! (all connectors are the same and most of the circuitry is identical, some of the components have been moved around a bit and the relays are from a different manufacturer. There is also a few additional components the original board didn’t have, which I think are to support some remote start functionality).
Here is a picture of the new board I found, in case you want to play spot the difference with the original further up this page!:-
With it all back together with the new parts everything seems to be working OK again – we’re back off to the mountains in a few weeks so I’ll update this if any of the faults reappear, or if it develops any new ones!
p.s. before doing any of this I did contact Truma technical service with details of the symptoms, but the best they could suggest was to get it booked in to a motorhome dealer for a full strip down and inspection. As you can see the strip down is reasonably labour intensive so while I didn’t bother to get a quote from anyone it’s fair to say it would have been expensive – especially when you are as tight as I am! – I opted to save the money and treat it to some new parts instead…
Hopefully this information has been useful – if you have any questions or comments please leave them below…….
UPDATE! – A reader has been in touch to share his experiences of a ‘blow back’ in the exhaust flue and the root cause of the problem, Many thanks to Marcus Tompkins for this information:-
“I had a “Blow Back” with my Trumatic C 3402 which blew the end off the external plastic cowl… I found the broken pieces and glued it back together for a temporary fix. … Boiler would not stay alight afterwards and “lock out” red led would come on….. it would light but only for 10 seconds max then shut down…. I replaced the flame detection electrode but still no fix…. turned out to be the flue pipe had sheared off where it connects via a clamp to the external cowl…… this caused the gas and air mix to be wrong therefore putting system into Lock down.
Hope this may help any other “have a go” repairers.”
UPDATE #2! – A reader is having some problems with a Truma roof cowl, if you have any experience with the roof mounted vents/cowls please take a look here and see if you can help!
UPDATE #3 – No hot water from your boiler? – click here to read how a Motor-Roam visitor diagnosed and fixed the problem……..
UPDATE #4 – Another reader (Paddy – see comments) was having problems with inconsistent starting of his boiler. When he stripped it down to investigate he found the ceramic holder of one of the electrodes had broken, allowing the electrode to move around inside the boiler. With this replaced his system is now working perfectly again. He has pointed out that the screws holding the electrodes tend to rust into the burner, so a good soak with penetrating oil beforehand is recommended! – thanks Paddy!
Here’s a picture of Paddy’s broken electrode:-
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