On 10/10/2020 09:10, Jedzi wrote:

I have an ancient Potterton Kingfisher CF55, about 35 years old. It has generally been as good as gold, but we will replace it in about 18 months.


I've got one of these same age maybe. The only thing that's failed about
20 years ago was the pilot light monitor thermocouple. I also fitted a
RF snubber across the thermostat contacts which were arcing and
interfering with my radio.

For 6 months we've noticed that the boiler case gets very hot. 4 weeks ago, the boiler started short cycling, ie it comes on for about 5 or 10 minutes initially and then switches off for about 15 seconds, then on for 40, off for 15 etc. It's the same with heating & hot water.

I've had a heating engineer round and he thinks it's the pump or the boiler thermostat. He said that the pump looks & sounds like it's working, but he can't be sure.

https://cdn.aws.toolstation.com/imag.../800/71846.jpg

The pump usually has a screw in the middle , you can see if the impeller
is rotating if you take it out. They can sometimes stick if out of use
for a while. Your engineer will have checked this though. The CH
radiators not getting hot might be a giveaway.

https://www.youtube.com/watch?v=WspRGg40Ekw

The other possibility is that the water level is low. The header tank is
supposed to keep it topped up, but if it isn't and the water level is
low, the thermostat might start to act up. I had that too, but noticed
the upstairs radiators weren't heating. Try bleeding the radiators too.


I can either replace the thermostat or the pump. While he thinks the pump is slightly more likely, he advised trying the thermostat first on the grounds that it will cost about 25% of the pump replacement, so it's a better bet.

My gut feel is that it's the pump, as the thermostat seems to be doing what it should, ie cutting out when it gets too hot. I think the heating engineer's advice makes sense, but if it is more likely it's the pump - I don't want to waste £60 on the thermostat.


Brian
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Brian