Why are both frost stat and pipe stat needed?

[nishnish]

Hi,
Rather than fiddling with the timer during a freezing period, to make the heating come on every 30mins for 10 mins or the like, I'd like our old WB240RSF boiler to protect itself.
I see both frost stat and pipe stat are recommended.
Why not just a pipe stat?
After all you want to protect the CH pipes against freezing, whatever the ambient temp is in the cellar.
What purpose does the additional frost stat serve?
I'm probably missing something obvious to the more experienced on here?
Thanks.

 

[doitall]

Frost stat turns in on, and the pipe stat turns it off when the return pipe is at set temperature.

 

[MrV]

The "Wall/air" stat and "Pipe" stat contistute double protection in order to conform to Part L building regulations and MUST be wired in series from a permenant live and must override time controls in the event of freezing.
This means that BOTH stats must call for a demand before the boiler will operate thus reducing the possibility of speradic operation that ONE stat is more likely to offer.
If only one stat therefore calls and the other doesn't, the boiler will not fire.
This is all in the name of conversing energy.

 

[Terrywookfit]

Or even conserving energy

 

[gaswizzard]

Frost stat coupled with pipe stat was good practice long before the part L suits were born.

 

[TicklyT]

A pipe 'stat alone will only monitor the temperature of a small section of one pipe. Unfortunately water is a poor conductor of heat, so the temperature may vary greatly just a short distance away from the pipe 'stat.
A wall 'stat better monitors the general environment around the boiler.

The wall 'stat turns the boiler on when the air temperature drops low enough for freezing to be a hazard. Once the heating pump starts, the pipe 'stat more accurately reflects the temperatures in the whole system. It can then shut the boiler down as soon as the system is warm enough to avoid freezing.

Without a pipe 'stat, (and a relatively poorly insulated and unheated boiler room), the boiler will continue to run until the frost stat is satisfied by the waste heat from the boiler and pipework slowly warming it's environment, regardless of the temperature in the conventionally heated parts of the house, and that may take a considerable amount of time.

The temperature inside our house exceeded 26° overnight last winter when the door to the outhouse where the boiler lives was accidentally left open (frost 'stat fitted, but no pipe 'stat )

 

[nishnish]

Thanks guys.
Just having a further think about this. Isn't the combination of stats so that when the criteria are met, that the boiler turns on for a reasonable burn time, enabling the heat to reach the far flung corners of the system? With just a pipe stat, the boiler may only fire for a short time. Effectively just going on and off repeatedly for short periods. The heat not getting around the system.

What temps do you guys set the frost and pipe stats at?

 

[D_Hailsham]

What temps do you guys set the frost and pipe stats at?

Honeywell recommend 5°C for the frost stat and 25°C for the pipe stat, which should be fitted to the return pipe.

The two stats should be connected in series between terminals 4 and 5 of boiler terminal block X4

 

[mickyg]

Honeywell recommend 25°C for the pipe stat

Just a shame they cant make one that actually switches off at 25 C

 

[shagty]

i understood 2 stage protection to be as follows:

1. air stat opens all 2-port valves and engages circulating pump(s) to move static water which is more vulnerable to freezing.

2. the pipe-stat enabled the burner if the system water temperature continues to drop. when the burner is fired and safe water temperatures are restored in the circuitry, the burner is disabled.

 

[shagty]

Honeywell recommend 25°C for the pipe stat

Just a shame they cant make one that actually switches off at 25 C

could there be a market for electronic pipe thermostats

 

[mickyg]

Honeywell recommend 25°C for the pipe stat

Just a shame they cant make one that actually switches off at 25 C

could there be a market for electronic pipe thermostats

absolutely

 

[D_Hailsham]

i understood 2 stage protection to be as follows:

1. air stat opens all 2-port valves and engages circulating pump(s) to move static water which is more vulnerable to freezing.

2. the pipe-stat enabled the burner if the system water temperature continues to drop. when the burner is fired and safe water temperatures are restored in the circuitry, the burner is disabled.

Not quite.

The frost stat closes as the air temperature drops; the pipe stat opens as water temperature rises. The two stats are in series, so there will be a circuit through them if the air temperature is below, say 5C, but the water temperature is below, say 25C. If the air temperature is above 5C or the water temperature is above 25C there will not be continuity through the two stats.

On a combi boiler, the pair are wired in parallel with the normal programmer and room stat.

On a system or OV boiler they are also wired in parallel with the programmer and room stat, but the "switched live" output will go either to the valve white wire (Y plan) or to the CH valve brown wire (S plan).

 

[nishnish]

The two stats should be connected in series between terminals 4 and 5 of boiler terminal block X4

Ok. I have a T4360A1009 frost stat and an L641B1004 low limit pipe stat.
It would be simpler to wire if I could run a dual core to the frost stat and then a dual core from the frost stat to the pipe stat. The frost stat has an extra terminal 4 in there which is not mentioned in any literature. Is that not connected?
If so, it would be a convenient terminal to join the return from the pipestat to the return to the boiler.
I propose two dual core cables; A&B plus C&D

A WB240 X4 terminal 4 to T4360 T1
B WB240 X4 T5 to T4360 T4

C T4360 T3 to L641 T1
D L641 C to T4360 T4

Or do I have to use an inelegant terminal block?
Cheers.

BTW Last page here:

http://www.honeywelluk.com/documents/Full-Specification/pdf/787.pdf

on bottom left has T4360/L641 terminal explanations so you don't need to dig them out to understand my proposed wiring.

 

[D_Hailsham]

Ok. I have a T4360A1009 frost stat and an L641B1004 low limit pipe stat.
It would be simpler to wire if I could run a dual core to the frost stat and then a dual core from the frost stat to the pipe stat. The frost stat has an extra terminal 4 in there which is not mentioned in any literature. Is that not connected?
If so, it would be a convenient terminal to join the return from the pipestat to the return to the boiler.

Does terminal 4 actually have a screw and clamp - the drawings in the instructions don't show them?

If so, can you see if anything is connected to T4 or can you borrow a multimeter to check continuity to the other terminals (do it with stat on min and on max).

If there is a screw and clamp and no continuity to any other terminal, your wiring scheme would work.