Frost Stat Queries

rautospoon · Post by **rautospoon** » Thu Jan 09, 2025 10:36 am

I had a Vaillant ecoTech Plus Open Vent boiler fitted in my loft about 6 months together with replacement Honeywell Frost (set at 3C) and Pipe (set at 20C) stats.
These replaced a 30 year old boiler (Ideal Classic NF40) and identical frost and pipe stats, again in the loft to my bungalow.
My queries are :
1. What could be considered a "normal" time period between the boiler "kicking in" (when the air temperature falls to 3C) and it stopping (when the temperature in the return pipe reaches 20C) and vica versa ?
2. Does the frost stat temperature (3C) or the pipe stat temperature (20C) have priority ?
3. If the CH is "on" and has reached temperature on my Drayton room stat and has turned off, will the boiler still "kick in" if the temperature in the loft falls to 3C ?
4. As the pipework around the boiler is well insulated why are a frost and pipe stat actually needed ?!

(My previous boiler gave off much more "casing heat" than the new one so this would have an an effect on the frost stat)

I ask as the new boiler is coming "on and off" a lot more (in the current cold weather) than my previous one.
Many thanks.

Someone-Else · Post by **Someone-Else** » Thu Jan 09, 2025 12:08 pm

I am NOT a plumber or heating engineer, but I would say you have answered your own question

rautospoon wrote: ↑Thu Jan 09, 2025 10:36 amthe new boiler is coming "on and off" a lot more (in the current cold weather)

Sorry if you already know:
A frost stat is usually installed to protect the device (Boiler in this case) from freezing. If it gets below the set temperature, it will switch the boiler on to heat things up, thereby stopping any water from freezing and so expanding, splitting any pipes.
Heating cable (Which is wrapped around water pipes*) has a frost stat which switches on to prevent water in the pipe on which it is installed from freezing.
Frost stats are also used to switch off pumps if it is too cold.

All the above are examples of where a frost stat is installed where the frost stat itself overrides things in order to prevent damage due to frost.

*Not all water pipes have a heating cable, only pipes that if frozen will cause damage.

rautospoon · Post by **rautospoon** » Thu Jan 09, 2025 12:27 pm

Thanks for the info, but yes I am very well aware of the background to the "need for" and "reasons behind" the installation of frost stats.
What I am looking for (so far as can be practically provided) are specific answers (from experieced CH engineers on here) to my four questions.

dewaltdisney · Post by **dewaltdisney** » Thu Jan 09, 2025 12:44 pm

A frost stat is there for installations where the boiler could freeze. This is normally in situations where the house is not occupied and the system is not running. In normal use, the latent heat in the system would be sufficient never for the system water to go below zero in the seven or so hours off at night. In sub zero situations, if your boiler was off all night, the frost stat would kick in if the system water fell below its setting. In reality there is a natural gravity circulation from the warmer water in the radiators and pipes rising as your boiler is in the loft. The only pipe freezing you need to worry about is the condensate pipe and this is what SE mentioned about pipes with heating cables. I am not a heating expert either as the heating guys who pop in here are very busy at this time of the year you may wish to consider the points made by us knowledgable amateurs.

DWD

rautospoon · Post by **rautospoon** » Thu Jan 09, 2025 1:19 pm

I have no worries regarding my condensate pipe - it is well protected by Armaflex HT.

I will "live in hope" for a considered reply to the points I have raised, from an experienced heating engineer !

dewaltdisney · Post by **dewaltdisney** » Thu Jan 09, 2025 1:40 pm

The important point you are missing is the frost stat only comes into play if the boiler is not in normal operation. If you are living there with normal winter heating use then it is unlikely ever to come into play. In sub zero temperatures you would sensibly leave the heating on 24/7. Your questions are irrelevant and only come into play if the property is unoccupied during your yearly skiing trip to Aspen and the temperature falls below zero during your your absence.

DWD

rautospoon · Post by **rautospoon** » Thu Jan 09, 2025 2:08 pm

I have lived in my present property for 31 years and for the last 20 years or so my boiler has been in my loft (previously the garage). If the temperature in the loft falls below 3C the boiler "kicks in" until such time as the temperature in the return pipe reaches 20C. Obviously this is usualy during the night when the boiler is not in normal operation. This has been the norm for the last 20 years or so.

I believe frost stats are fitted in most situations where a boiler is fitted in "say" a loft or garage and I cannot believe that many people, with such fitted, leave their heating "on" 24/7 !

I normally "drain the system" when visiting Aspen for a prolonged winter break - this avoids invalidaing my household insurance.

dewaltdisney · Post by **dewaltdisney** » Thu Jan 09, 2025 3:15 pm

Just as an aside I have my heating on 24/7. The intelligent room thermostat controller can be set for various temperatures throughout the daily period. I have found that keeping an even house temperature means the boiler has short burns and overall the gas use is comparable to the off day/night cycle. I think my gas averages £100 a month through the year.

DWD