Installing a pipe stat in series with existing frost stat

26 Jul 2015
Reaction score
United Kingdom
Hi everyone, I currently have my boiler installed in an outhouse attached to the kitchen which has no radiators and can get very cold in winter. There is already a frost stat installed, with the obvious problem that when the temp falls below 3 degrees the frost stat kicks in but doesn't then go off as the air temp in the room never rises. I would therefore like to install a pipe stat in series with the frost stat so that when the return pipe temp reaches a set level (say 20c), it stops the frost stat from calling for further heat.

I would like to run my plans by you guys so you can let me know if the following is correct...

I plan to buy a low limit pipe stat. My current frost stat is wired up to the mains (it's a T4360A with mains connected to terminal 1) and terminal 3 is connected directly to the call for heat terminal on my boiler (a Baxi 105 e with terminal box connections L, N, E, 1 and 2; 2 being the call for heat terminal).

As a side note, I was surprised there was only 1 connection from my frost stat to my boiler, but I assume this is fine as mains power is supplied from elsewhere. Is this correct?

My plan would be to simply disconnect the frost stat from boiler terminal 2 and connect it instead to terminal 1 on the pipe stat (an L641B1004). Terminal C on the pipe stat would then connect up to terminal 2 on the boiler where the frost stat was originally connected. Based on this, there should be no call for heat when the temp is above the safe temp as terminal C on the pipe stat will be connected to terminal 2 on the pipe stat in this instance. But, when the temp falls below the safe level terminal C switches to terminal 1 on the pipe stat and you have your circuit (assuming the frost stat has detected a below limit temp too).

Does this all sound correct? Am I right in thinking that with this setup there will only ever be a power supply to the pipe stat when the frost stat detects a temperature below its set limit?

Many thanks guys.
Sponsored Links
As a follow up my plan is based on this circuit diagram, incorporating the frost stat and the pipe stat in series.


  • Screenshot_2015-07-27-08-55-58_1.jpg
    50.6 KB · Views: 13,585
yes that looks right
I'd fit pipe stat to return and ensure bypass is adequately open.

Doesn't matter. Which stat is first in series. They work in series

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links