CH Boiler Problem - Potterton C70

[DavidM]

(yes I know it's 35 years old and needs replacing - I'm just trying to nurse it through until spring!)

The boiler is overheating (radiators too hot, HW scalding and boiler creaking a bit). The room and HW cylinder thermostats are working ok, so I thought the problem was the boiler thermostat. Replaced it with a new one last week but problem still persists. If I turn the boiler thermostat down to zero it clicks off and the boiler shuts down, but putting it to any other setting from 1 to 6 the boiler just keeps going until the room/cylinder stat shuts it down. Turning the knob through it's range there's no feel/sound of it switching as I'm sure the old one used to do. I suppose the new thermostat could be faulty - any way of testing it?

I have recently flushed the system in anticipation of getting a new boiler, but then plans delayed so I refilled with new inhibitor. When draining the system I found both the drain tap on the lowest rad and the one on the boiler frozen/blocked, so I fitted a new one on the rad but didn't bother with the boiler one (at the time though at new boiler was imminent). Could there be an air lock or sludge in the boiler which is somehow affecting the thermostat but still allowing the boiler to produce (very) hot water?

Grateful for any advice. David (remove _notme from email address)

[Andrew Gabriel]

What sort is it? Does it have a phile on the end of a thin copper tube? If so, test it by dunking the phile in a mug of hot water with a separate thermometer. The temperature range from min to max should probably be something like

60C through to 80C or 85C.

[TonyB]

If I turn the boiler thermostat down

Could be faulty, it's not unheard of. I'd be looking for a dodgy connection to/from the stat in the first instance.

Having just tried mine ( different system altogether ) I realise that the boiler stat only clicks when the water temp is up. Otherwise the click is off the scale to the low end all the time, if that makes sense. You could try running the boiler for say 20 mins and then see if you can get the stat to click. However, that will only tell you that the stat is sensing temperature, not that it would be in a position to shut down the system.

At the other end of the circuit from the stat there must be some sort of valve or servo that actually turns the system off and it may be that end that's faulty.

TonyB

[Set Square]

Silly question: I suppose the little bulb on the end of the stat's capillary tube *is* correctly plugged into its hole in the heat exchanger - so that it senses the boiler temperature? Is it making good contact. [You used to be able to get some heat conducting paste to put in the hole to make sure that it contacts ok. Probably still can!]

[DavidM]

Thanks for replies so far. I've done some more checks:

1. Removed thermostat and checked it in a pan of hot water and with a meter - switches correctly between 60 and 80 C. The old thermostat I took out also works correctly grrrrrrrr - £33 needlessly spent!
2. Checked the electrical connections - the boiler gas valve is only connected to the bioler thermostat, no where else, so nothing can be bypassing the thermostat.
3. Checked that the thermostat bulb is pushed fully into the tube in the boiler.
4. The gas valve turns off if the boiler supply is swithed off or if the boiler thermostat is turned to the off position, so I'm pretty sure it's not just the gas valve sticking (the whole gas valve assembly was renewed last year anyway).

Any more ideas please? David (remove _notme from email address)

[Andrew Gabriel]

Would be useful to narrow down a thermal problem or an electrical problem. If you take the phile out of the tube and dunk it into a mug of nearly boiling water, does that cut off the gas valve? (Obviously, take care with mugs of water and mains around.)

[Set Square]

So the thermostat works when you test it in water. The output from the stat is the only thing connected to the gas valve - but when the boiler gets hot, the stat doesn't switch off when it should. Is that correct?

If so, it can *only* be that the stat is not sensing the temperature of the boiler for some reason. Try cleaning out its pocket with a suitable sized drill bit (operated by hand!) to make sure there's no gunge in there that's providing insulation and preventing proper contact. Then use some heat conducting paste.

It that doesn't work, it sounds like there could be some sludge inside the heat exchanger which is stopping the hot water from getting to the area of the thermostat pocket.

[TonyB]

Andrew's idea sounds like a good test, can you do that one? TonyB

[Ed Sirett]

The only other possibility is that the capiliary tube is kinked so that when the thermostat is in position it does not work correctly?

[DavidM]

Am continuing to investigate - thanks for advice so far. David

[DavidM]

Update: The thermostat tube did have a couple of rather tight bends, so straightened it out and rerouted it a bit (new hole drilled in control box). Also cleaned out the tube in the heat exchanger into which the bulb fits, and it now seems to be working ok. Radiators still get a bit hot, but given all the changes/replacements over the years it's probably overdue for rebalancing the whole setup, so that's the next job on the list. Sods law says that next week I'll get a sensible quote arrive for fitting a new boiler Many thanks for all the advice! David. (remove the _notme from email address)