Purpose of extra thermostat near boiler in outbuilding

[Owen]

We have just bought a house with the central heating boiler in an outbuilding. The settings go up to 10 degrees C - and it is currently set at 1-0 degrees. I have noticed that the boiler comes on even when timed not to do so, and I presume that this thermostat overrides the normal controls and is intended to maintain a minimum temperature - perhaps to aveert freezing. (Certanly the boiler turns off if I reduce the thermostat to 5 degrees.)

Is that right please and what temperature shoud it be set to - during the past snowy week the hearing seems to be on most of the time?

 

[canalman]

Where is this frost-stat in relation to the boiler?
A frost-stat needs to 'SEE' HEAT FROM THE SYSTEM in order not to call continuously whenever the ambient temperature falls near zero. Best place is near the return pipe to the boiler, then it sees heat once warmer water has got round the system, and shuts off. Its purpose is to warm the system water up only enough to stop freezing, not to warm up the whole locality! (Canalman)

 

[petej]

We fit an "air" frost stat" and a "pipe stat " on the return and wire them in series so that they have both to be calling for heat for the boiler to fire. I seem to remember that 3 to 5 degrees C is correct for the air stat.
This system stops the boiler from running for two long.
Pete