Installing boiler thermostat question

[David Hearn]

Received my new thermostat today and planning on fitting it this evening.

Couple of quick questions...

Firstly, the old thermostat had marked on it 16A 250V~ and T107oC. I assume that means that it can switch 16A at 250V AC and switches at 107oC. The new one (the part suggested by Baxi Spares and KeepTheHeatOn.com) has lots of figures on it (with what appear to be first 3 chars of part number) but the one for this part says 2(2)A 250V~ 90oC. Can I assume that this switches 2A @ 250V AC and switches at 90oC? Seeing as Baxi said this was the part, I'm tending to believe them that 2A switching is okay - and I guess that it just means that the water reaches 90oC rather than 107oC now? Does this sound okay?

Secondly, I drilled out the old siezed probe and the hole is now nice and clear. I used an 8mm drill and it fitted fine. 7mm was too small and 8mm was tight, but not too tight. This new probe is just under 8mm in diameter - so it should fit. My only question is whether I just insert it or whether I need to put anything like heat transfer stuff. From memory, when a similar probe was removed from a similar Baxi boiler it was dry without anything on it - so I'm assuming that's right - but I just wanted to make sure!

Thanks again.

D

[BillR]

Should be ok and its unlikely to have to switch much current anyway.

The boiler thermostats I've replaced were just push fit into the hole. Any heat transfer compound that doesn't set hard is likely to run out in a short time...

[Roger Mills]

The thermostat switches the gas valve on your boiler** - which is very unlikely to require more than 2 amps - so the one you have should be fine. I don't think that you can deduce anything about the switching temperature from the hieroglyphics in the part number - the old one certainly didn't switch at a water temperature of 107 degC, by which time it would be well and truly boiling! If it's a boiler stat, and if it fits, it should do the job. I assume that the end with the electrical contacts has a knob which you can twiddle to set the switching temperature?

It's a long time since I fitted one of these, but I seem to remember that you are supposed to use some conducting paste to make sure that the bulb makes good thermal contact with the pocket in the heat exchanger.

** The term "boiler" is somewhat of a misnomer - since if it *boils* the water it is malfunctioning! A flow temperature of around 80-85 degC is what you need for the central heating.

Roger

[David Hearn]

Well, I replaced the thermostat and it is working fine now. Noticed that its short cycling far more than it used to. Its not a room thermostat problem as it was constantly calling for heat, its that the new thermostat is switching off when it reaches temperature and then switching on again about 1 minute later. Still, not bothered too much with it.

Anyway - I noticed when I inserted the thermostat probe that there were small droplets of rusty water clinging to the sides of the hole. Either its condensation or its leaking slightly. Seeing as the old one siezed in there, I guess it was doing it before, and just was kept hot so water never had a chance to sit there. I'm planning on removing the probe every so often and cleaning the hole so this new probe doesn't seize as well. Sound sensible?

Thanks

D

[Ed Sirett]

I'd make sure the water is moving around well in the system. How long do you intend keeping the old boiler going? If you do have a small leak in the sensor pocket on the main heat exchanger this is all reason to change it next summer.

[David Hearn]

Well, everything appears to be working fine - rads are getting nice and hot and the house is warm, so I doubt there's any problem in the water flowing around.

How long to keep it going? Well, the truth is as long as possible. I have no plans on replacing the boiler for another few years - though that doesn't mean it won't give up the ghost before then!

Obviously, if I had the money to do it, I'd get it changed ASAP - however our situation is that we just don't have the money to spent on it - at least, not unless it died and wasn't repairable. I guess that's the joys of a mortgage in Guildford and a baby on the way... ;)

D