Fitting an L641B Pipe stat to a Honeywell S Plan System

11 Jul 2006
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United Kingdom
Hi All,

I want to fit a Honeywell 641B pipe stat to a Honeywell S plan system, which does not include a frost stat. However, I want to use the pipe stat, so that it switches the system ON when the temp drops.

I have been looking at the many Honeywell electrical diagrams, but cannot find one that directly tells me explicitly how to do this.

My best guess, based on what I can find, is that it should be connected as a 2-wire unit across terminals 1 & 5 on the junction box - which would correspond to live and the input to the motorised valve, but I am not sure.

If anybody is fully clued up on this and could help I would be very grateful.


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Why are you installing a pipe stat if there is no frost stat?

What is the make and model of the heating appliance?


The boiler is a Potterton Pro Max 30, which is fitted in a garage. There was a frost stat, which was continually deciding that the temperature was too low and hence the boiler was frequently coming on throughout the night, even in some moderate temperature conditions.

The idea was to replace this with a low level pipe stat, which offered some control over when the boiler would cut in. The 641B appears to fit the bill, but the wiring is not at all clear to me.

At what temperature do you propose to set the pipe stat???

Pipe stat terminals will go across perm live/motor valve brown.
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Around 5 degrees. This is about where the frost stat is set, but this has kicked in too frequently. Two people have advised me that the pipe stat is a viable working solution, even though the two would appear to be equivalent when configured in this way. Are you implying that you know for a fact that it is not?
Use frost stat and pipe stat.

Wire as follows:
Live - Frost stat common
Frost stat (usually terminal 3) - Pipe stat common
Pipe stat (usually terminal 2) - Heating zone valve motor live (usually brown)

Pipe stats are usually fitted to the heating return and set at 25°C.

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