Moving Frost Stat in the Loft


29 Nov 2008
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United Kingdom
hi all,
there is a frost thermostat in my loft and it is most annoying. I believe it is badly positioned.

when it gets cold, as it has been of late, the froststat overrides the normal controls and fires up the heating. I guess this is what should happen to avoid the pipes freezing.
however, due to the stupid positioning of the froststat - away from the lagged pipes it is trying to protect, the loft space itself never heats up and the heating just stays on full, roaring away all the time.

this is most annoying at night as the noise and heating up of the flat often wakes me up.

rather than just moving the current frost thermostat (Honeywell, can't see a model number on it), I thought about getting one that fits on the pipe - seems more logical?

so, Honeywell have one (L641B1004) that I think will suit - how difficult will the wiring be?
have I missed anything?
the Honeywell info (page 3) seems to suggest I need a 2nd frost stat but I can't see why?

thanks in advance :cool:
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you need to get a pipe stat fitted as an interlock. this will then bring on the boiler say at 5degrees and switch it of when the pipe gets to say 30n degrees. staright forward job for a competent diyer .
cheers for the reply, but I think you've lost me :oops:

so I need a frost stat on the pipe to turn on the heating when it gets below a certain temp.
also need another normal? stat on the pipe to turn off the heating when they reach a certain temp.

is that right or have I misunderstood?
is there a specific way they need to be wired for this to work?
nah... you keep the frost stat in the loft. but you also fit a PIPE stat in line witht the frost stat and attatch than to the heating return pipework.
the pipe stat will operate to keep the exposed heating pipework in the loft warm. the pipe stat is the component doing the protection work. thwe frost stat simply tells it that conditions are right for its operation.
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I don't quite understand why the original frost stat in the loft would still be required?
doesn't it do the same job as the pipe stat?

wouldn't you want the pipe stat to just work whenever it was needed (ie. when the pipe gets too cold) regardless of what the original frost stat is doing?

in my mind it should just be a case of moving the frost stat from it's current (bad) position to be on the pipe it needs to protect?
is there a boiler or heating pipes in the loft?

the pipe stat turns the boler off when the pipes are warm... you dont want them warm all year round do you?
ahhhhhhh :idea:
I thought the pipe stat was a frost stat...

so from what you are saying it is a normal thermostat that acts as an override to the frost stat when the pipes reach a certain temp.

so the existing frost stat would detect that is was too cold and fire the boiler. this would heat up the pipes until the pipe stat detects that they have reached a safe temp and then overrides the frost stat and turns off the boiler.

is that right?

so would I wire it into some of the unused connections on the frost stat?
Simply keep the existing frost stat and wire, in series, a pipe stat strapped to the return pipe.

Now when the loft gets to the setting of the wall frost stat the heating is switched on but as soon as warm water has circulated back to the boiler the pipe stat will switch it off. The pipe stat will maintain the pipes at a low temperature until the main frost stat senses the ambient temperature in loft has risen.
I think I'm confusing myself lol.

frost stats, pipe stats and normal room thermostats are all essentially the same aren't they? they are just switches that are "on" when the detected temp is below the preset temp.

so yes, I understand now that it's easier to just wire in the extra pipe thermostat and have the two switches in series. then when either the ambient air temp or the pipe temp rises above the preset, the circuit is broken and the boiler will go out.

one further Q - is the return pipe from the CH? so the rads would warm up when the boiler fires up?
generally : stat will operate between say 10 and 30 degrees
2.frost stat will operate between 0 and 15 degrees
3. pipe stat will operate between 10 and 50 degrees.

the return pipe on the heating will be the last pipe to heat in the loft space.

post a pic it may be easier.

you are getting close, just need to brave the wiring. :LOL:
cheers for all the help guys, deffo understand this better now :)

for the pipe stat, I see that Honeywell have low/high limit versions.
low = 2 - 40c
high = 50 - 95c

which would be the correct one to get?
I guess it depends what it's fitted for.

Are there tanks in the loft ?

Maybe the frost stat is fitted to turn the CH on, so that heat will rise into the loft and help protect everything in the loft from freezing.

cold pipes
cold water tanks

maybe you should turn the stat down to 2°C so that it only works when near the limit.
well, yes there are tanks in the loft.

but since there is insulation down and all the pipes are lagged the ambient air temp in the loft doesn't really change much even if the flat below is warm.
that's the problem really - when it is cold the frost stat turns on the heating but never gets warm enough to turn it off.

the stat in the loft is turned right down currently, but still comes on when it's really cold (ie. this week) and makes the flat unbearable.

will go for the low pipe stat, then I can set it to heat to 30 or so, which should then prevent any freezing probs and stop the heating being on all night.

all good?

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