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Truma Combi system


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Hi Everyone, I`m looking for help, on 2 occasions following refilling the water system from drain down the hot water pipe has blown a hole as it leaves the boiler outlet on first heat up. Could this be caused by insufficient air clearance thus leaving an air lock which in then turns to steam & overheats the pipe and blows at the first weak point?

My dealer has been very good but is at a loss as to the cause. Other than that the Truma Combi water/heating is brilliant.

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I’m not sure what “...the hot water pipe has blown a hole as it leaves the boiler outlet on first heat up” actually means.


I note that in this earlier posting




you mentioned having problems with ‘John Guest-type’ fittings, but I don’t know if that’s relevant in this case.


A Truma “Combi” boiler will have an elbow-fitting with integrated ‘aeration’ valve attached to its hot water outlet (image attached below)


In principle, if a Combi appliance is correctly filled with water so that the air within the heater’s water-tank is fully purged through a hot-water tap before the heater is started up, there’s no reason to anticipate that steam would be produced in the tank, form some sort of air-lock and then blow the elbow-fitting off the heater or blow a hole in the water-hose near to the heater’s outlet.


There is the possibility (I suppose) that the pressure within the hot-water system is reaching a point where the fittings/hose used cannot handle that pressure, and (as you’ve suggested) failure occurs at the hottest point which is at the water-tank’s hot-water outlet. However, assuming that your Swift’s water system includes a Truma safety/drain valve, that valve should release pressure within the system before it becomes critically high.


As the problem has only happened after you’ve refilled the water system, the first thing for you to do is to make absolutely certain that you’ve purged out all the air in the heater through a hot-water tap.


I’m also going to suggest that, before you initially heat water after refilling the heater, you switch off the water-pump and open one of the hot-water taps. That way, if excessive pressure within the water-tank has been the cause of the problem, this ploy should allow that pressure to disperse through the open tap.


Truth is I don’t know the answer and I don’t recall coming across this problem before. My Rapido’s Combi 4E doesn’t do this (though its safety/drain valve will occasionally ‘depressurise’ the system during hot-water heating) and - as your dealer can’t point to an obvious cause - the problem is likely to be rare.


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Hi Derek,

I thank you for your reply & suggestion, as you say I did have problems with some push fit fittings at first, my dealer replaced all of them that they could get at with a better product and that did cure the problem. I will be using the `van again over New Year so I will be extremely diligent in purging the system after filling so fingers crossed. My dealer is letting me have a length of pipe so that I can make running repairs if it happens again. Once again I thank you for your ideas, it`s good to have such a forum to seek advice from.

Regards Roger

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If it’s the water-hose that’s been failing (as your comment about your dealer giving you a piece of spare hose suggests to me) the problem may indeed be due to there being limited air-space around the end of the Combi that carries the water outlet and inlet - a possibility you asked about originally.


I don’t think there is anything in Combi installation instructions specifying a minimum-width gap between the heater’s water-outlet/inlet end and, say, a furniture panel. But if Swift has installed your heater so there’s little clearance at that end and there’s not a great deal of ventilation provided for the heater to ‘breathe’, this could potentially lead to problems.


Truma’s Combi installation instructions say


“...warming of the water and its resulting expansion may cause pressure of up to 4.5 bar to occur... The water lines for connecting to the boiler and the safety/ drain valve must be drinking water safe, pressure resistant (up to 4.5 bar) and hot water resistant up to +80 °C.”


I assume Swift has used ‘semi-rigid’ hose (example of one type in attached photo) and this SHOULD be sufficiently pressure and heat resistant to comply with Truma’s requirements, but who knows...


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Thanks again Derek, your info leads me to think that it is in fact a combination of air in the system and pressure build up. The hose used by Swift is a plain red (12 mm?) and is fitted to all the other `vans I have looked at of 2014/15 vintage. 9 cm is the nearest the boiler is to the bunk sides which should be more than enough space gap & the hose does not actually touch the boiler. The really annoying thing is, the fact that you cannot isolate the hot water line so that if problems occur you have to shut the whole system down, so no water at taps & no ability to flush the loo.

The fact that I have Club friends & colleagues also with Swift `vans who have not had these problems, I am beginning to wonder if I am doing something wrong & will go back to basics over New Year with crossed fingers.

Best regards



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It is strange...


I notice that “Combi” heaters with black casings (manufactured from March 2013 onwards) have a simple manually-operated 'over-lever’ type of safety/drain valve for UK versions, with the anti-frost “FrostControl” valve as an option.


However, as long as Swift fits either of those Truma valves, if excessive pressure develops in the system when water is heated the valve should reduce this to a safe level by letting bursts of water through it.


As air is compressible while water is not, even if air were somehow trapped in the boiler one might not expect that to cause a hose to fail.


The Combi could be isolated from the rest of the water system by putting an in-line shut-off valve in the heater’s cold-water supply - I did this on my Herald using a Whale-branded valve. Then, on a French campsite near Cherbourg with the heater full of hot water, the Whale valve fractured. Sometimes you just can’t win...

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