Boiler problem: Pump or Thermostat

[Jedzi]

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.

[Dave Liquorice]

Assuming a normal type CH pump can you get at the slotted cap in the center of the motor housing? If so remove it (there will be a bit of leakage) and insert a flat bladed screwdriver and turn the rotor easily manually and check it's spinning fast when powered.

Another possibility on a system that old is build up of sludge somewhere that is restricting the flow. Maybe in the heat exchanger.

[Harry Bloomfield, Esq.]

It could also be pipes which are silted up, causing poor flow - have you checked?

[Brian Howie]

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.

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.

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.

Brian

[Dave Plowman (News]

That isn't an engineer.

[Roger Mills]

Has the pump got isolator valves either side? If so, I'd be inclined to remove it and connect it to a 13A plug and test it with its 'wet' end dipped into a bowl of water. If it's ok, you should see a lot of water movement.

[tabbypurr]

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

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

NT