Inherited system another anomaly!

[doitmyself]

Checked and found both are operating as frost stats. If either are switched on the boiler comes on but not the pump! All seems a bit odd as the two stats are literally a foot apart. So much for my theory that the pipe stat was an overtemp device.

The frost stat (which measures air temperature) and pipe stat (measures water temp and fitted to return pipe) should be wired in series. The boiler lights when the air temperature drops below the frost stat setting (5-10C). The pipe stat turns the boiler off when the water returning to the boiler reaches the set temperature (25C).

If there is no pipe stat, the boiler temperature will rise until the boiler stat operates - may be 80C. This would heat the house unnecessarily and be wasteful.

 

[Bogart]

Well I reckon the pipe stat is stuffed as it does nothing. If I turn on the boiler by raising the frost stat up, so the boiler kicks in, the pipe stat can be set anywhere between the minimum 10˚C and the max 40˚C with no result. By my reckoning from what "doitmyself" said the stat must be shorted, or boiler would not fire. Finally is not 25˚C rather high, why heat the whole setup to this figure as a frost protection? Anyway my setup does not switch on the pump so seems to defeating itself!

 

[doitmyself]

Here is a wiring diagram showing how the frost and pipe stats should be connected. Use it to check your wiring. (Ignore terminal numbers - assumes Honeywell method - valve wire colours are the thing to follow.)

Don't forget that 25C is the temperature of the water in the pipe, not the air temperature. When the boiler is running for heating the water could be as much as 80C.

The frost/pipe stats should turn the pump on so the water circulates.

Could it be a DIY installation of the pipe/frost stat?

 

[Bogart]

Hi
Sounds very much like a DIY job, and a poor one at that. Having delved in found pipe stat wire in the terminal but covered in insulation tape, do not ask why, I have no idea either. System now seems to be working ok and pump now does run. Have copied your diagram for future reference. I never knew about the stat switching the boiler off, one learns something new everyday, many thanks.