Froststat and 3 way valve

23 Apr 2009
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United Kingdom
We have Vaillant boiler (not combi) in Garage with a Honeywell Froststat. All working with heating on more or less all night at present as so cold. Want to get a pipestat fitted but plumber to busy at present. Just want to clarify operation of froststat please.

So heating on all day via programmer in airing cupboard - supposed to go off at 9.00 pm but staying on due to temperature of garage - all good.

Programmer turns heating back on at 8.00am until 9.00am. At 9.00am, programmer turns heating off and turns hot water on for 30 minutes. At 9.30 both heating and hot water are off on the programmer - presumably the 3 way valve stays in the hot water position. Frost stat does not seem to kick heating back on even though still freezing in garage.

Does this seem correct please.
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As you have figured, the idea is that the frost stat is wired in such a way that it overrides the programmer and will still turn the heating 'on' even if the heating is set to 'off' at the programmer.

So there are two questions, you say that "it is still freezing in the garage" yet the heating doesn't remain on. At this point, is the frost thermostat calling for heat or not? It may feel cold in there, but frost stats are usually set just high enough to stop pipes freezing something like 4 or 5 degrees and it may be just above that.

Also, are you sure that there isn't a pipe stat already fitted somewhere? It would be a poor installation that just had a frost stat on its own. If there were a pipe stat somewhere it would explain why the heating didn't keep running when the heating went of at 9:30 which would be because the pipes were still warm and therefore the pipe stat would prohibit it.
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Sorry cant do a pic at present but Froststat is by the boiler. The electrics come down the wall from the airing cupboard with the programer into the froststat and out of the froststat into the boiler.

Definitely no pipestat. The froststat I think is a throwback to an older boiler. Last night the heating was on all night but this morning when the programer turned the water off (heating was already off) the froststat did not kick the heating back on. The pipes around the boiler were still warm / hot which must be the Vaillants own frost protection.

Thanks for all comments.

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