Potterton Combi 80 occasional overheat/lockout

[willhamlyn]

We have a combi 80 which has a persistant occasional lockout problem.

It started about a month ago when it started locking out on the HW side, this was traced to a sticky flowswitch which I've replaced together with the microswitch, (and the auto vent as that was leaking) that improved things on the HW side but I was still getting an occasional lockout when the CH was hot together with a lot of clicking on the main board.

That was a faulty relay so I replaced the main board, clicking fixed, less frequent lockouts

Now I still get an occasional lockout on the CH side accompanied by banging in the pipes and water from the vent, this occurs maybe once every 4 or 5 days although the banging pipes seems to occurs at pretty much the same time every day, late at night when the heating has been on for a few hours and sometimes after running a shower/bath in the morning.

I'm assuming there is some kind of overheat going on when the CH turns off so I've replaced the CH stat as it measured just below spec, no change.

The pump and fan seems to run fine and carries on running for a while when the heating goes off, the other two HW stat's measure within specs, diverter was only replaced about 2 months ago no heating of the CH pipes when HW is on, CH expansion vessel is up to pressure.

I'm at a a bit of a loss where to go from here as following the fault finding chart shows normal function, I've sat and watched the boiler when it turns off (that was fun) and the flame seems to stop right away.

Only thing I can think of is that the pump although working is running slow maybe and not dissipating all the heat when the CH is really hot? (it was replaced during a BG service last year)

Any thoughts? Engineer coming in the next couple of weeks for a service but wife and kids away next week so would be a good opportunity to sort it if I can.

Thanks!

 

[willhamlyn]

Nobody have any ideas then?

 

[absolute]

sounds like the a problem with the automatic bypass.

 

[adlplumbing]

check your rads hot top cold at the bottom then it may be sludge build up

 

[Andygasman2010]

Strange fault given all the things you've checked/changed already.

Last one of these I had the same fault it ended up being the diverter valve sticking in hot water position.

I used a clip-on temp probe on the flow pipe and watched it as I turned off the tap and even after the burner went off the temp still went up because it was still in hot water position and couldn't dissapate the heat.

Considering you've already changed the diverter valve you might want to check there isn't a dripping tap or possible leak on the mains hot pipe somewhere that's keeping it in hot water mode.

Run a bath or something for a good few minutes and rather than turn off the tap, turn off the cold main to the boiler and see if this resolves it.

 

[bengasman]

System is full of crap.

 

[misterdubya]

You need a new heat exchanger.

Mr. W.

 

[Agile]

Its an unusual situation on your model but all the evidence you have given points to inadequate heat transfer into the DHW.

I would do a thermal survey of the boiler to identify the problem.

Its unusual on that one but lime scaling on the DHW pipe could cause it. So could dirt in the primary side. You dont say where you are located so we dont know how hard your water is.

I would use chemical treatment on the HE but many others would just change it.

Tony

 

[willhamlyn]

Tell me about it, it's driving me nuts, it's been fine for the last 5 and then it's locked out twice today.

Thanks for all the replies .

Definitely no leaks or dripping taps and the HW pipe is stone cold when the CH is on. I've been in the garage all evening and noticed tonight that when the CH has just come on that the flame seems to sometimes go up into the HW height and cycle a bit sounds like the water is boiling or at least getting overly hot. (there is no red light so not in HW mode) although the CH stat was ok when I last checked it at 8 ohms I'm wondering if they ever play up intermittently when hot?

I have a spare somewhere so I guess changing it is the way to find out.

I'm in SE London and had considered it might be sludge although it's always had inhibitor in it so I've had X800 in it for the last couple of days.

Gas safe chappy is due next week sometime so I guess it may be up to him to sort.

Thanks again guys.

 

[Agile]

It sounds like the CH sensor may be intermittent or more likely the miniature spade connectors are corroded.

Tony

 

[willhamlyn]

it was indeed the sensor, it's been fine for about 3 weeks now. The old one was jumping around all over the place sometimes reading fine sometimes way out of range.

Boiler has never worked this well so hopefully all bugs are sorted now!

 

[Agile]

I am glad I had suggested that on the 24th Feb!

Tony