Jaguar 23kw combi boiler overheat problem F1

6 Sep 2011
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United Kingdom
Recently the hot water has occasionally stopped during a longer shower. The boiler then shows an F1 error. If i stand in front of the boiler with the hot tap on i can hear the boiler jump into life and the clicking sound of it trying to light, but it does not fire up. If i turn it off, take off the cover, click in the reset button on the thermostat(?) then reassemble and turn back on, it all works great, but then after a week or two i get woken up by the missis screaming in the shower at 6.30am when its gone cold again and it almost gives me a heart attack.
The pressure is at 1.2 bar. Any ideas as to whats causing this?
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It sounds as if it may be overheating.

If its on hot water only then its probably a blocked plate heat exchanger.

Unfortunately these are very difficult to remove to chean/replace and often the gas valve needs to removed making it definately not a DIY job.

Tony Glazier
Thanks for the reply Tony, a blocked heat exchanger makes sense.
There's another thing though, the boiler fires up by itself and i dont think it should. Here's some background details:

Jaguar 23kw Combi boiler Installed Nov 2002
Water in the system looked murky when loosening the screw on the pump a little. The older of the two downstairs rads does not get hot at the top - possible crud in the system. Central heating seems to work fine - has been on for a couple of weeks on the timer.

The other problem:
The boiler fires ok when the domestic hot water comes on, and when DHW is turned off the burner stops. The issue is that randomly afterwards with the DHW still off, the boiler fires up for a couple of mins, then stops, then does it again a few times. I'm guessing this could be related to a sticky pin in the diverter valve, or would that mean that the burner would stay on way after DHW was turned off, until the DV pin finally clicked off the microswitch, rather than randomly coming on now and then afterwards? (The microswitch at the end of the DV worked fine when jumpered on the board).

I have on a couple of occasions had problems getting the boiler to fire after a thermostat reset, and it has shown up F2 (thermistor failure) and an F3 (Heat exchanger blockage) error.

A friend checked it over and suggested cleaning/flushing the system (£300) & changing the DV (£260). But i dont see how that would fix the overheat issue? He said there was a touch of water in the expansion tank too and that that wasn't good.

Am i being daft considering these fixes? Should i be looking at a whole new boiler? Or should i even consider the Valent £299/year cover with free part costs (unless they deem the boiler unfixable) ?

Thanks for all your help !
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The diverter valve does not need to be changed, cleaned maybe.

Cleaning the system often/should include cleaning the boiler too.

The boiler is totally repairable.

Many dont want to because it takes a little time and fitting a new boiler is much more profitable for them.


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