Frost Protection

[Mattvw]

Hi All,

I’ve posted this in general plumbing and realised perhaps that isn’t the correct place. I’m new to the forum and can’t figure out how to move my thread so am reposting here, so apologies for that.

I have a 2 year old Grant oil fired boiler situated in the garage which is uninsulated. It has a Honeywell frost stat set to 4deg and pipe stat set to 20deg.

In the cold weather we’re having it’s coming on for about 1 minute every so often as expected to do what it’s supposed to. There’s something I just can’t get my head around though. I’m sat here and the heating has been running, it’s just cut off due to the thermostat in the hall making desired temperature, and then 30 seconds later the frost protection is kicking in (I know it’s the frost protection as it’s s-plan and you can hear the prv whilstling).

I can’t get my head around why it is kicking in, the pipe is obviously hot as the heating has just been running and I’m assuming the pipe stat must be working as it only runs for a minute or so? Has anyone got any ideas why this may be or what may be wrong?

thanks,
Matt

 

[John.g]

I imagine the frost stat only operates when the boiler is switched off so when the water temp falls to 5c it switches on the boiler and, depending on the stat hysteresis, switches it off at ~ 15C. Maybe the Grant stat is measuring the air temperature? but would still assume that the pipe stat still switches it off.
I think that the frost stat (situated near the boiler casing) is to protect the boiler and switches on the boiler when the air temp drops to 4/5C, it is recommended to have a pipe stat on the return to the boiler and when that reaches 15/20C it switches the boiler off but would then have to open a zone valve as well. AFAIK the basic frost stat just works on air temperature but you then need a ABV, this is your set up??.

 

[SimonG]

The pipe stat de energises the frost stat when it's at it's set point.

So power into pipe stat, then to frost stat and finally to boiler.

Need to check operation of pipe stat and frost stat to find the fault.

 

[exedon2]

Grant wiring instructions for frost stat show permanent live to frost stat switched live to pipe stat feed from that to heating (normally 2 port valve)
Not forgetting most frost stats need a neutral as well .
Probably best to check wiring is correct first.

 

[John.g]

I think you can also buy a 2 deg grant frost stat.

 

[exedon2]

I think you can also buy a 2 deg grant frost stat.

Thats the one that's built into the external boilers though not the combi

 

[exedon2]

And to be honest we have had to change a few either not working at all or on all time!