Vaillant boiler with frost stat

tcd212121

Hi,

I have what appears to be common problem - my boiler is coming on in cold weather and heating the house on full blast all night. I have a Vaillant ecotec plus 415 with ST799 controls and a Honeywell frost stat (no pipe stat). The boiler and stat are in the garage, and hence are not exposed to any warmth. Unsure of the sundial configuration.

My understanding is that my boiler already has an inbuilt frost protection cycle that will heat up to minimal temperatures and cut back out again when sufficient. I'm assuming the extra frost stat circuitry was left in place from the old boiler, although can't be sure. Is there any requirement to have this additional / external frost stat? If not, to fix the problem and avoid putting in a pipe stat, can I just break the frost stat circuit at the nearest boxed connection (without leaving any exposed wires of course)? Anything else I need to consider here?

Many thanks!

southcoastrgi

The boiler frost stat will protect the boiler it won't protect the pipework, fit a low limit pipe stat on the return pipe wired with the external frost stat which will cure your problem

tcd212121

Good point - thanks very much. After reading the online manual again, it looks like the internal frost stat measures the heating water outlet temp and not the return temp as I had originally assumed.

Which pipe stat do I need, a high or low temp model and what's the difference between their uses besides the obvious? I'm assuming it's just wired in series with the existing frost stat?

tcd212121

Low limit pipe stat - gotcha!