Central heating problem

Joined
15 Dec 2007
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United Kingdom
Hi all,

I've got a peculiar (well, to me at least) problem with my central heating, and I was wondering if anyone had any ideas as to what the problem might be.

I noticed it just this morning. When the heating is turned on (and the room temperature is below the thermostat temperature), the boiler's demand light turns on, and the boiler lights. However, about fifteen seconds later, the demand light goes off and the boiler just stops heating. However, I believe the pump is still going, as I can hear the water circulating through the radiators, although the radiators then cool back down as the water is not being heated. The thermostat and control unit/timer appear to be working fine (in that I can use either to turn the heating on/off as they should, but when the heating is turned on, the above behaviour is exhibited).

The boiler is a Potterton Lynx 2, and the central heating system is sealed. I'd noticed that the system pressure was very low a couple of weeks ago, and this morning (after discovering the problem described above), I worked out how to repressurise the system and did so, taking it up to a pressure of 0.5 bar when cold (incidentally, I don't have the boiler manual, so could someone confirm that this is the correct pressure please? I was basing that on the fact that the pressure gauge is coloured red below 0.5 bar and green above it...). I also reset the boiler by turning it off at the mains, waiting for a minute or two and then turning it back on. Neither of these things seems to have had any effect with regards the heating problems.

I know very little about plumbing and central heating systems generally, so please keep replies relatively simple! Many thanks in advance for your help!
 
No difference. I don't think it is due to the system pressure because it was working fine last week, despite the fact that the pressure gauge was pretty much at the bottom of the scale.
 
When the boiler has gone off as you described, turn on the hot water for 20 seconds and then off again, and see if the heating comes back on. how good is the hot water?
 
The heating doesn't come back on, but the hot water is very hot.
 
At least we know that valve, fan, aps and pcb work as should, pcb on dhw side that is
It is normal that the pump keeps running after the heating is turned off for up to 10 minutes though usually shorter.
On some models, the DEMAND light is actually not a demand indicator but a flame indicator, hence light off may mean there is no flame, but there is a demand.
How long does the pump run after flame goes out?
 
Ah, mystery solved. The boiler completely packed in last night (no hot water, no heating). I had a maintenance person come this morning, and the pcb is shot. I had started to suspect the control circuitry, so it's nice to know that an educated guess wasn't far from the mark! Looks like it'll be a new boiler then...

Thanks for the help!
 

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