potterton kingfisher boiling intermittently

[mikedams]

I have a potterton kingfisher MF RSL60 boiler piped with gravity feed hot water and pumped central heating. 99% of the time it runs perfectly, but every month or two (mostly in winter) it will boil over. The boiling seems to happen when the central heating has been running or a while and then the pump turns off. Its difficult to know for sure though because its quite erratic and intermittent.

Someimes we are in the house and hear it. If we are out it will have stopped boiling by itself for some reason (without lockout) by the time we get home, so the only way I know it boiled is because I push a bit of tissue paper up the header tank overflow pipe. We get home and find the paper has been pushed out of the pipe.

When it boils the overheat sensor doesn't lock out for some reason. If I disconnect a wire from the overheat sensor it does lock out - so that appears to work properly.

I've tried changing the temperature sensor and the whole main circuit board but still have the problem. Its difficult to solve because it can't be reproduced on demand. I have to wait a few months to see if it happens again.

Can anyone help me with this problem. I'm starting to think the only thing left is to replace it with another boiler (not potterton).

 

[twgas]

It would appear your problem is more likely to be pumping over, than boiling, which is more often a system fault, I would suspect a build up of sludge where the cold feed joins the system, pump set too high, no by-pass etc.

 

[mikedams]

When the problem occurs there is definitely overheating because hot water out of the taps is then much hotter than normal. Also you can hear it boiling if you're in the house at the time.

Another thing that might help is that if I'm in the house when it happens all I have to do to stop it boiling is force the central heating pump to start running again. i.e. its boiling over with the central heating pump off - only gravity feed for hot water still in effect.

 

[jackthom]

When you find it boiling has the gas burner stopped firing or is it still going without the pump?

 

[mikedams]

Still going without the pump

 

[D_Hailsham]

If the pump has stopped but the boiler continues running, the fault is in the HW circuit or the boiler.

Do you have zone valve in the cylinder circuit controlled by a cylinder thermostat to limit the hot water temperature?
Do you have a bypass (sometimes the towel rad) in the HW circuit.
Will the boiler go out if you turn the boiler's temperature control down?

 

[mikedams]

I dont think there's a zone valve - not sure exactly where it would be situated or look like though. There's no cylinder thermostat. We have a towel rad but I dont know where it joins the pipework exactly so dont know if its connected as a bypass or not.

The boiler did go off when I turned the control down - I last tried that a long time ago - havent tried it again more recently.

 

[D_Hailsham]

I dont think there's a zone valve - not sure exactly where it would be situated or look like though. There's no cylinder thermostat.

Valve looks like this:

View media item 11762
It would be connected in the return from cylinder to boiler. If you do not have a cylinder thermostat, you won't have a zone valve.

 

[mikedams]

We don't have one of those.

 

[gassafeman]

This is a semi-gravity system IIRC. No m/vlvs.

The boiler will fire with no pump if you select h/w on the programmer. There is no boiler interlock on gravity hot water.

Check your header tank firstly - it sould be 1/3rd approx. full.
Or, you may have an air-lock in the h/w primary or return. Especially if they are not supported correctly.

Or, the cold feed is blocked. Unusual on Gravity, but Ive seen it a couple of times. The cold feed should enter the 28 mm central heating systems h/water return pipe.

Hope this helps.

Paul.

 

[mikedams]

Thanks very much for your help so far. I hope you'll stay with me a bit longer.

My header tank is quite full. I did that deliberately so that I could get overflow on boiling to tell me when it happens.

When I had the boilder fitted (Oct 2003) they did a power flush so I dont think there will be any blockages anywhere. Do you agree?

I don't find I need to bleed any radiators ever - does that dispense with the airlock idea?

This boiler appears to have two returns - one from the hot tank and one from central heating.

Do any of your suggestions fit better for a problem that happens quite infrequently? Do you think going to the expense and trouble of converting to a zone valve or fully pumped might help?

 

[gassafeman]

When I had the boilder fitted (Oct 2003) they did a powerflush

I now suspect a blockage. Powerflush or not.

That, or your gas valve is sticking (rare) but not a job unless your an RGI.

Paul.

 

[nicknox]

Intermittent boiling on a Kingfisher MF is normally a faulty pcb.

 

[mikedams]

Intermittent boiling on a Kingfisher MF is normally a faulty pcb.

I replaced the pcb and still have the same symptoms.

 

[mikedams]

When I had the boilder fitted (Oct 2003) they did a powerflush

I now suspect a blockage. Powerflush or not.

That, or your gas valve is sticking (rare) but not a job unless your an RGI.

Paul.

Would another flush be of any benefit if the first one apparently left this problem behind?