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Truma C and the fan – Correct operational procedure


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There are two components in this puzzle so comments from anyone in the know would be gratefully received.


The first is the Truma Gasfernschalter (gas switch) which is pictured.


The second is the Trumatic C operating unit also pictured.


The Truma boiler was serviced about three years ago and works just fine.


Operational Procedure

When I purchased the van I was told that to start the boiler, if the gas was on, then I should first flick on the left hand rocker switch of the Gasfernschalter (which sets a fan running at the back of the van), turn on the boiler and then repeatedly push down the sprung loaded rocker switch on the right until the boiler lit.


However the Dethleffs manual that came with with the van makes no mention of this and indeed, if I ignore the vendors advice and just switch on the boiler it fires up quite happily.


I have a previous thread which might throw some light on the true purpose of the Gasfernschalter ( http://forums.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/Is-this-Truma-Gas-Valve-required-/46877/#M529557 ). You’ll note that the van is fitted with an oven.


So I have two questions


1. Is it normal for a Truma boiler to fire up automatically when turned on without having to manually hold open an electronic gas solenoid until it warms up? And does it normally also have to have an additional fan turned on to vent the van?


2. Might the Gasfernschalter be there specifically to ensure that the fan is operated when the oven is on? It is notable that the solenoid mentioned in the other posting was floating around next to the oven gas feed.


All thoughts welcome.





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Truma’s Gasfernschalter product is intended to remotely open and shut off a leisure-vehicle’s gas supply (see link below). It really has nothing to do with operating a Trumatic C-Series heater.




Your photos of your Dethleffs motorhome’s Truma heater’s control-panel (and the 2003 year of the vehicle) suggest that the heater is either a C-3402 or C-6602 gas-only model and the Operating Instructions for this appliance are here:




A Trumatic C-3402/6002 has two fans - a small one and a larger one. The small fan runs continuously when the heater’s gas-burner is operating, but the larger fan will only run when blown-air heating has been selected and the fan-speed (and the noise it makes) will vary in proportion to the heater’s output.


Switching on the heater should cause it to light automatically (green light illuminated on control-panel) and, provided no ‘fault’ occurs, that light should stay illuminated until the heater is switched off. If a ‘fault’ occurs a red light will illuminate on the control-panel and the heater will shut down.


I (diplomatically) suggest you forget everything you were told and not theorise until you have digested the information in the documents in the links above. After that, come back if you’ve got further questions.

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BruceM - 2017-09-27 5:13 PM...................................then I should first flick on the left hand rocker switch of the Gasfernschalter (which sets a fan running at the back of the van), ..............................


I can't add to the solution of your problem, but your comment above caught my eye. From the instructions Derek has found, it seems there is no associated fan to start running when the gas control valve is energised, or actuated to turn on the gas. The only time a fan should run is when the Trumatic heater is brought into use, which you say it does.


However, you can clearly hear this fan (but you don't say where it is, other than the rear of the vehicle).


I'm just wondering if someone has been indulging in a bit of DIY, and added a fan - possibly (pure conjecture) to force vent a smelly gas locker? Since it seems all is working as it should vis-a-vis both valve and heater, I'd be inclined to track down that fan and try to work out its purpose, because the way it is being operated seems wrong to me. With a fan that operates when the gas supply is initiated, I don't think I'd make any assumptions until I was confident that using it is safe!

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Many thanks for the links and suggestions. The Trauma instructions tally with those provided in the Dethleffs user manual.


It’s got me wondering wondering if the Truma uses a balanced flue much like a modern household boiler or whether it relies on air from the habitation unit to function, much like the other gas appliances in the van (hob, oven, fridge). I’ll see if I can track down a service manual schematic to work that one out.


Re the fan that gets turned on by the Gasfernschalter; the van has a kitchen at the back end of the habitation unit and the fan is behind that and blows air out through a vent in the back of the van. There’s a narrow void behind the kitchen appliances unit and it’s from there that the air is extracted. This area is also naturally vented with some permanently open vents.

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Downloadable documents relating to Truma products can be accessed from here:




Installation Instructions for C3402/6002 heaters are here:




The diagrams at the start of the Installation Instructions document and Paragraphs 2 and 3 on Page 6 will show that C-Series heaters have a combined air-intake/gas-fumes-exhaust ‘double-duct’ comprising two concentric flexible tubes. This duct’s inner end is connected to the heater, and the duct’s outer end is connected to a ‘cowl’ on the leisure-vehicle’s bodywork or (less commonly) to a roof-mounted chimney. The double-duct’s inner tube is made of stainless steel and carries to the outside air exhaust gasses expelled from the heater by its gas-burner fan. The double-duct’s outer tube carries air sucked in from outside the vehicle by the heater’s gas-burner fan. Efficiency benefits of this arrangement are that the air that will be entering the heater’s gas-burner chamber will have been warmed by the inner ‘exhaust tube', while the ‘exhaust tube’ will be cooled by the incoming air. The cooling function is undoubtedly more important than the warming one, as the exhaust duct of an equivalent to a C-Series appliance produced by a Truma competitor (Atwood’s “Confort 3” heater) could get hot enough to scorch cloth - not good when the exhaust duct was in a wardrobe!


It should be clear from Truma’s Installation/Operating instructions for the Gasfernschalter shut-off valve what this product’s design-objective is - and it’s not to switch on and off a kitchen-area extractor fan. Of course there’s nothing to prevent the Gasfernschalter control-switch being exploited to operate a kitchen-area extractor fan, and it would make some sense if the Gasfernschalter valve only switched on and off the gas supply to the kitchen gas-fuelled cooking appliances rather than the vehicle’s main gas supply. (You theorised this might have been the case in the earlier forum thread you mentioned above and I suspect you were correct.) Then, whenever the gas supply to the oven was switched on by the Gasfernschalter control-switch, the extractor fan would start running and would continue running until the Gasfernschalter control-switch was used to shut off the gas supply to the oven.


As the Gasfernschalter valve in your motorhome clearly does not switch the oven's gas supply on and off (as the valve is ‘loose’, not connected into the gas pipework) and - from what you’ve said - has no apparent effect on the the Truma heater that can be ‘fired up’ satisfactorily without fiddling about with the Gasfernschalter control-switch, I suggest you just use the Gasfernschalter control-switch to switch the fan on and off whenever you want that to happen.


(Me, I’d want to remove the Gasfernschalter two-button control-switch if I could, replacing it with a single-button version to operate the extractor fan. If you ever need to have work done on the vehicle’s habitation electrics, or sell the motorhome, it would make sense to first get rid of this sort of irritating complication.)



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Hi Derek, thank you for a rather brilliant and comprehensive response.


Regarding the mysterious Gasfernschalter, I’ve come across a picture of a similar aged vehicle with a Dethleffs branded oven fitted and which also has the Gasfernschalter. When I have time I’ll open the kitchen unit up and trace the wiring however I’m now 80% certain that it’s there as a fail safe mechanism to ensure the extractor fan is on when the oven is operated. I suspect the solenoid on the current one failed at some point and was replaced with a manual tap. For piece of mind, given that it appears to be a design feature and future owners may forget, I’ll probably put in a replacement Gasfernschalter and replace the manual tap with the solenoid.


Many thanks all for the excellent responses.

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I’ve just been checking the cabling with the multimeter and am now 100% certain that the Gasfernschalter is specifically for activating the oven gas extractor fan and fail safe oven gas solenoid. The extractor fan incidentally turns out to be situated directly behind the back of the oven.


Following testing I have also confirmed that the solenoid itself is faulty.


In an ideal world I’d locate a replacement solenoid without all the extra gubbins and just wire it in to the existing circuitry however it’s a Truma branded product so possibly made by them. I doubt that there’s an OEM equivalent out there. So in the meantime I’ll look to see if there is an alternative replacement.


The mystery of the Gasfernschalter is though finally solved.


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I seem to remember reading that there were problems with heat build-up around some ovens.


My guess is that the fan is there to remove excess heat when using the oven, which would explain why it seems to have been wired to the shut-off valve on the oven gas supply. As installed, this would ensure that anyone wanting to use the oven had first to operate the gas shut-off valve, which would coincidentally actuate the vent fan. When the valve failed (presumably closed!), it seems the previous owner, or their dealer's workshop, merely took the valve out of the gas supply pipe, leaving the fan to operate as before.


There is an incongruous looking in-line connection in the gas supply pipe just visible in one of the photos in your other thread, which I'd guess marks the original location of the shut-off valve.


From your post above regarding the similar installation on another similar van, it seems the installation was originally by Dethleffs, and that operating the fan in conjunction with oven use was judged essential - hence the "fail-safe" nature of the installation.


Until you can establish otherwise, therefore, I think you would be wise to ensure that fan is kept running whenever the oven is in use. I suspect Dethleffs will have had good reason to complicate the oven installation in this way. Even the Germans don't throw money at solving non-existant problems! :-)

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The casing of gas-ovens of the type fitted to leisure vehicles can certainly get pretty hot and require some means of dissipating that heat. This is usually done by having a ventilation grille in the vehicle’s bodywork behind/above the oven, but augmenting the grille’s ventilation capability with a fan makes good sense. I’ve never heard of this use of a Truma Gasfernschalter before and perhaps Dethleffs are unique in this approach.


How does the gas-oven light? if it has electric ignition, perhaps this is also only operative when the Gasfernschalter valve is open and the ventilation fan is running.

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The oven is lit manually (match etc) so no great sophistication. Given the age of the van I’m surprised the solenoid valve had failed. It must have been a maximum of nine years old when it was removed and I’d not have expected it to be the sort of part that wore out easily especially given it was only activated for the oven. Do you think under use might cause a part like this to seize?
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