Potterton Kingfisher MF RS50 - overheating and clicking

24 May 2020
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United Kingdom
Hi all

I'm looking for some advice / views please.

I have an old (installed 2000) Potterton Kingfisher MF RS50 boiler that has developed an intermittent fault.

My system is gravity fed with a cold water tank and header tank in the loft and a single zone switch which - I'm told - is why I can have hot water without heating, but not heating without hot water. My hotwater tank in an upstairs bedroom cupboad is similarly very old with no built in thermostat, meaning - I'm told - that the thermostat dial on the boiler controls everything.

Since we moved in a couple of years ago the boiler has been fine, but lately has developed a worsening but quite intermittent problem. Sometimes, it will overheat (hissing, gurgling, kettling, whining sounds in the boiler and a rumbling / thunking in the pipes upstairs, and - on one occasion - the header tank overflowing out into the garden - I think this was due to a number of overheat events gradually filling it up until one finally sent it above the overflow level). When it does this we tend to panic and kill it at the control box and leave it off for a while. Chances are when we switch it on again, it will be fine again for a few days.

The other thing it does is produce a lot of clicks from within the boiler - in the top section of the boiler around the fan rather than at the bottom section around the gas valve underneath the flame unit. It sounds a bit like a solenoid sort of noise. It usually gives us a couple of clicks prior to firing up - I understand this is normal - but always when it stops firing it gives around 6-7 'click-click's, before going quiet until the next firing cycle. Occasionally, it gets into a real short-cycling loop where it will fire for a minute or so, shut off and give a constant series of clicks (including some where it sounds like it's almost about to fire up again but doesn't) before then firing again for another minute or so, etc.. I attach a link to a video of it doing this yesterday (please ignore the kids and banging door in the background):

One thing we have noticed is that all these problems appear to arise when we're just using the hot water loop, that is with the heating switched off at the clock or the hall thermostat not calling for radiator heat. If it starts down its problem routes, switching on the heating always seems to settle it down - you can sort of hear a flush of water (from the radiator loop?) into the boiler and everything seems to calm down - the kettling sounds in the boiler cease if it's doing that, and if it's in its short-cycling loop switching on the heating also resolves that and it goes back to its normal behaviour.

I have recently located the central heating pump and zone switch in the basement and both appear to be relatively newly replaced and in good working order.

We have tried turning down the thermostat on the front of the boiler as the water was certainly running too hot, but this doesn't seem to have fixed the issue (although the hot water is a bit cooler!).

Does anybody have any suggestions about what might be the problem? I have had a plumber around who wasn't sure and I'm waiting for him to call me back, but we discussed some scatter-gun possibilities including faulty PCB, overheat sensor or thermistor.

I'm keen to repair it if possible rather than replace, and I'm told you can still get all these parts.

Any thoughts / suggestions gratefully received :)

Many thanks.
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Sounds more like a pcb issue, but I've had flaky temperature controls too. Should be a dead simple fault find for a proper boiler repairer....
Overheating is generally caused by a lack of circulating water. Many things can cause this such as blocked heat exchanger, pump failure, narrowing of pipework. The clicking sounds like the gas solenoid opening and closing, you would need to use a multi meter to test whether the fault is the valve or PCB. Hope this helps.
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Many thanks for the replies to my query and apologies for taking time to respond. It turns out the fault was the PCB after all: had it replaced a couple of weeks ago and everything is now running fine again. I took apart the old one and there are definite black marks around the base of some components, so clearly a bit burnt out and not happy. So our 20 year old boiler marches on for the moment. Cheers!

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