Have a strange problem with an S5002 in which from cold it will ignite and work perfectly for as long as it is left running but if turned off you have to wait until the unit has cooled right down before it is possible to relight it. The strange problem is that it is only doing this when it is in the van. Put on a Test rig on the bench and works perfectly !
Here goes then  Ok, apologies for the length of this post but wanted to try and explain everything that has been done so far to save people having to ask things that have already been covered.
A bit of background. Inherited the heater recently with a 7yr old LMC Van. Previous owners neglected just about every thing possible during the time they had it and advised that the heater had not worked for some time. Time for an overhaul and service.
Removed heater and started the strip , repair, rebuild of it. Electronic Ignition unit was toast as it had been fitted without the heatshield so that was the first part changed.
Removed Main valve assay and Jets and ultrasonically cleaned jets to ensue they were clear. I did expect the internals to be covered in the normal alloy corrosion etc but to be honest the internal surfaces of both heat exchangers were pretty good no soot and very little alloy powder but decided to clean anyway and same done with the flue manifold.
At this point all in all was pretty good so refitted heat exchangers to base plate along with flue manifold (new seals fitted) The internal ends of the flash over tube were a bit rusty and had started to misshape so fitted new tube and seals at the same time. Burners checked and both really clean actually with no debris, cobwebs or other crud inside. Checked the bottom aluminium covers for these and the internal mesh, 60 seconds in the bead blaster and both spotless. At this point before rebuilding I decided to also change the Magnetic Insert (valve) which is fitted in the base of the body of the Main Valve and also the Thermocouple as it seemed to make sense having gone this far so both were changed.
Whole unit then rebuilt connected to a temporary flue and test run on Propane on a test rig at 30mbar. Tested with manometer showing no pressure loss. On operating the unit normally the rear chamber (as you look at the heater) ignites as it has the spark electrode with this then flashing over to the front chamber within a second or two. Both burners left running for a few seconds to allow the thermocouple in the front chamber to warm and then releasing control and both main burners ignite. Clean Blue flame on both and all working as it should. This will work perfectly like this.
Now the problem starts ! Fit the unit back in the van lights perfectly as above and will stay lit as long as required but turn it off and attempt to relight it whilst the heat exchanges are still warm and it will ignite the rear chamber (small pilot burner) but will not flash over to the front so the thermocouple never gets heat and thus will not allow any gas to pass through the main valve to the main burners. Leave the unit cool for an hour and it will work perfectly ????
Tested pressure on Van and is fine showing 30mb and even swapped out the Regulator just to be sure problem still persists. Pipes are all clear and no obstructions or debris and every other appliance is working fine ie oven and hob. At this point I am really stumped as pretty much everything I think of can be ruled out by something that has been previously looked at or done. When the unit is running it burns with a nice even clean blue flame, not even the faintest flicker of yellow so if nothing else I know the combustion is clean. There are no restrictions around the air inlets and also checked the flue (full length) and roof cowl and again totally clear.
Removed the unit 50 miles later back on the test rig, another new flue seal and works perfectly ?? Double check and treble check everything again and tested intermittently over a period of 4 to 5 days and again back to the van. Fitted again, another new flue seal and still the same, as soon as it is warm it will not re light.
My thoughts, first was lack of gas pressure but tested at elbow below fire and showing as 30mbar which ruled that out plus the fact that once it is lit it burns perfectly and given the consumption on this if there was a lack of flow then I would have thought it would have been seen when in use. Second and really the only I can think of is the Main Gas valve is faulty but I cant explain how it will work perfectly on a test rig and not in the Van or at least not once it is warm. Thirdly Heat? Well can rule that out as when I have bench run it readings taken with a laser thermo at various places including the body of the Main valve show them to be much higher than when it is in the van.
Thankfully at this point the only thing I am glad of it that other than a slowly increasing bar bill I have not had to pay for any of the safety checks and certificates each time due to a friend who is Gas Safe registered but even he is now at a loss as to what could be causing this.
Even tried different gas, why? Well why not? What did I have to loose !! Logically the only thing I can see that could cause this would be the main gas valve but why if it will work in one situation and not the other? I dont relish the idea of spending another £100 to change it only to have the same problem but equally I dont fancy spending £500 to change the heater either when obviously there is a fault that can be rectified with this one?
Having had truma heaters in all our vans I have come across many problems but never anything like this one. They either work or they don’t and when they don’t it is normally a fairly simple process to diagnose why but having banged my head against this one now I am at a loss, there must be something that I am overlooking ??? Help !
Any suggestions would really be appreciated and if you have read this far then thanks for that too at least I didn’t type it all out in vain !
Ian
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Topic: Truma S5002 will not light when hot
