On Saturday, 10 October 2020 09:10:06 UTC+1, 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.

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.

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.

If the pump switches off when water hits 80C, the stat's working.
A multimeter & a thermometer should get you further forward.

Likely a bad pump (often easy to repair), bad stat, muck cloging system. Less likely: zone stat problem.


NT