Boiler Cut Out

[shadowlight2010]

I have a Ravenheat Combi (came with the house). Although well kept and serviced has a problem when running a bath. After running hot water for a few minutes and then shutting it off the pilot is extinguished after about 60 secs with a definite mechanical noise as if something has sprung out.
i think its due to scale in the heat exchanger overheating the water and throwing the thermal cut out. Any ideas folks???

 

[chrishutt]

You don't say which Ravenheat model (presumably the 820/20?). As far as I know most Ravenheats use a Giannoni diverter valves. When running hot water, the diverter valve sends heated water around DHW heat exchanger circuit. When DHW demand stops and the burner cuts out, the diverter valve returns to CH position and the pump appears to continue briefly, so some hot water from the main heat exchanger is passed into the CH circuit to avoid overheating main heat exchanger. You should check whether this is happening - if the CH circuit was closed off (e.g. all TRVs shut, no by-pass) the excess heat could not dissipate.

 

[Agile]

Not exactly Chris, this is a four pipe heat exchanger I think!

The diverter diverts the water to run in a closed circuit loop while in DHW mode.

There are two possibilities. If the pump is ( wrongly ) not running during DHW usage the o/h stat may trip DURING DHW use. Even if it is and the HE is sludged or scaled up then the HE reaches an elevated temperature and causes the o/h stat to trip AFTER DHW use has ceased when CH is off.

Its messy and time consuming to clean out and in view of the age of the boiler many installers would recommend replacement.

Tony Glazier

 

[chrishutt]

Yes Tony, the 820/20 (if that's the model?) has a combined DHW/CH heat exchanger, so that in DHW mode the pump circulates primary water in closed circuit in contra-flow to DHW flow. As you say, failure of pump to operate could result in overheating, but the check I suggested would reveal pump failure because no hot water would be sent down CH pipework when diverter valve returns to CH position.

 

[Agile]

Sorry Chris, perhaps I did not fully comprehend what you were saying. However, I have never noticed that slight pump over run occur and don't remember John Wilson at Ravenheat mentioning it. That does not mean that it does not happen though.

Virtually all the ( several ) Ravenheats I have encountered with this common problem trip during DHW usage. Most seem to have been caused by debris within the primary part of the HE unless the pump operation was failed.

Tony Glazier

 

[chrishutt]

Tony, I was working on a Ravenheat 820/20 yesterday, when I noticed this slight flow into CH pipework when DHW flow stopped. The diverter valve looked new so I assumed this was normal operation. It makes sense to my mind to allow for some dissipation of residual heat in combined heat exchanger. Of course I can't be certain that it wasn't a peculiarity of that particular boiler. Even if pump stopped instantly with burner, the inertia of circulating water might be sufficient to create same effect, don't you think?

 

[shadowlight2010]

Thanks for all the info. All I can say is that the Central Heating works fine and does not cause any cut out even after prolomged use of the boiler. The DHW problem occurs a minute or so after running a bath . I switch on a HW tap when it cuts out and the water is almost boiling.. run it for a few secs to clear and then the pilot can be relit no problem. If the pump is not running on... is this a circuit board replacement? Is there a definitive way to check as 3 gas engineers so far have not managed to sort it out other than recommend a new boiler..

 

[Agile]

First check that the pump is turning when DHW is being taken but I expect it is.

It will be caused by scaling in the DHW tube OR debris in the primary side.

Messy and time consuming to clean.

Common problem so any competent engineer should be familiar. But some are lazy and dont want to charge the proper price for a five hour job.

Tony