Potterton Puma 80 - Intermittent CH

Hi, I have got a Puma 80 that has been giving me some problems the last few weeks. The issue is around the CH, I've had no problems drawing hot water on demand at any time. The problem with the heating is that it is intermittent. When it's on the timer the boiler fires up for a short while then turns itself off, even though the thermostat is set way above the current temperature. It comes on for about 1 minute then turns off for about 5-10, and runs through this cycle throughout the timer period. I switched it to constant to see if that made a difference and it worked. I just controlled it through setting the thermostat. It was working fine for about 2-3 weeks, firing everytime I turned the thermostat up past the ambient temperature, but now it's doing the same on constant as it was on the timer, so it only ever fires intermittently now regardless of the thermostat setting and temperature.

I've taken the front off to have a look and the pump appears to be working fine (HW is still drawing fine and the boiler fires everytime I use the HW). I did notice that the relay on the PCB is clicking off when the CH turns off each time, it's relay RL2. The PCB has actually been replaced in the last two years (it caught fire in the corner, as they do). So the electrics on that should be fine.. should be. Having said that the red LED is not lit.

Can anyone offer an explanation for the intermittent firing? I'm thinking it may be the CH sensor switch but I don't have a meter to measure resistance etc. Is it obvious to anyone?

Any help gratefully received, apologies for the length of post, I just wanted to get all the detail in. Thanks.

Sounds 2 me like the wax capsule in the diverter valve is getting stuck in hw mode,make sure theres no dripping taps etc,straight forward job 2 change wax capsule

Hi, thanks for the reply, appreciate it. Just wondered whether if it was getting stuck on HW should the boiler be firing or does it not necessarily fire unless you actually turn the tap on? I can't find any taps that are leaking.

The boiler will still fire for heating but will go off after short while cos wax capsule is not opening up heating circuit.its a common fault on a puma like the p.c.b burning out

Thanks. The last couple of days the heating has apparently been working in that the rads are hot, even though the boiler only fires for 30-60 seconds every 5 minutes or so. It still flicks itself off but the radiators are heating up, albeit gradually. Is this consistent with the valve not working?

Yes......wax capsule needs changing. Some heat is passing into the heating circuit but m ost is just going round the boiler....hence boiler reaches temp and cuts off

Not difficult if you just change the front section

Great, thanks. Is it a DIY job (for someone of average ability) or should I get a pro in?

Thanks

If you have the manual do it yourself..........drain boiler first and turn off cold mains......saves having water pour out onto the pcb......