frost stat - how long should it run for?

Just checking my understanding of frost stats..

My customer has a boiler in a garage (12year old Vaillant/British Gas boiler on an unvented system), with an external frost stat wired in (not a pipe stat, but a 'room stat' from 0 to 10c).

If the ambient temperature drops below the set point (say 5c), could the boiler potentially be running continually (until the air temperature rises back up again)? The boiler wouldn't kick in (as the programmer wouldn't have kicked in). Zone valves to HW and CW would be closed. But reverse circulation could heat up the rads and cylinder, depending how long the boiler was running for.

Or would the boiler PCB know it's connected to a frost stat, and so to switch off when the boiler water return temp reaches 20c (or thereabouts)?

When I'm there (daytime) and warmer evenings, no reports of the boiler coming on. But I thought I'd recommend a new frost stat, just to be sure the current one (which is fairly old) hasn't lost its calibration and is coming on at too high a temperature.

My guess is that the installers left an obsolete form of frost protection in place, which assumes no zone valves and essentially ran the CH full time to keep dwelling above protection level.

These days, one normally uses the programmer(s) with a minium temperature set at, say 10°C – 15°C, to protect the house. This will also protect the boiler. If you want belt and braces then you can stick a frost-stat on the return to the boiler but its not necessary (in England) in my opinion. The boiler warranty may want one, I suppose; check the maker's instructions.

I'm not gas certified, or have any experience with gas boilers, so can't say how they like to behave. However on oil boilers with a frost stat you have a pipe stat wired in series with the frost stat to bring the CH on. When the frost senses say 5⁰c it sends a feed to the pipe stat, the pipe stat is set to cut power when it senses a temperature of between 10-20⁰c. The pipe stat is wired into the CH zone valve by-passing the room stat. By doing it this way you only bring the boiler on to take the chill off it and the system to prevent potential freezing. Without the pipe stat the frost stat will keep cycling the boiler until the temperature around the stat creeps above its set point. Obviously in a garage with large heat loss you may never get the temperature in the room above the stat set point.

Chuck said:

My guess is that the installers left an obsolete form of frost protection in place, which assumes no zone valves and essentially ran the CH full time to keep dwelling above protection level.

These days, one normally uses the programmer(s) with a minium temperature set at, say 10°C – 15°C, to protect the house. This will also protect the boiler. If you want belt and braces then you can stick a frost-stat on the return to the boiler but its not necessary (in England) in my opinion. The boiler warranty may want one, I suppose; check the maker's instru

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SJB060685 said:

I'm not gas certified, or have any experience with gas boilers, so can't say how they like to behave. However on oil boilers with a frost stat you have a pipe stat wired in series with the frost stat to bring the CH on. When the frost senses say 5⁰c it sends a feed to the pipe stat, the pipe stat is set to cut power when it senses a temperature of between 10-20⁰c. The pipe stat is wired into the CH zone valve by-passing the room stat. By doing it this way you only bring the boiler on to take the chill off it and the system to prevent potential freezing. Without the pipe stat the frost stat will keep cycling the boiler until the temperature around the stat creeps above its set point. Obviously in a garage with large heat loss you may never get the temperature in the room above the stat set point.

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This is how Grant (oil fired) suggest its done.

John.g said:

This is how Grant (oil fired) suggest its done.

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Yes John. As I said wired into to the CH valve if there is one but you still need the pipe stat otherwise you get cycling.

SJB060685 said:

Yes John. As I said wired into to the CH valve if there is one but you still need the pipe stat otherwise you get cycling.

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This is the one I meant to post, Grant suggest that the pipe stat is set to 25C, I think that also supply a non settable 2deg frost stat, formerly 4C I think, opens CH zone vale as you suggested.

Wiring depends on system but your frost stat should be powered through a pipe stat on return. Frost stat is only energised when the boiler return is below set point.
Have seen loads of frost stats that keep the heating running all night as no pipe stat.

Another issue is if the frost stat is mounted directly onto brick or stonework. Mount it onto a block of timber or ply.

Until the frost stat is satisfied then the boiler will cycle on the pipe stat between say 20/25C so frost stat location/setting still important.