Boiler Frost Protection

[Mattvw]

Hi all,

I have an 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?

many thanks,
Matt

 

[Mottie]

Are you sure it’s not the pump/boiler overrun?

 

[Mattvw]

Hi, thanks for your reply, it’s only been doing it since we’ve had the cold weather. I’m not an expert on this sort of thing, but I’m assuming the over run would only ever be straight after the heating has stopped? It can sometimes kick in 5 minutes after the heating has stopped, just for a minute. Does that make sense? It’s not always straight after, sometimes 5 minutes.

edit: just read up what pump over run is, and I can confirm it’s not just the pump running, but the boiler as well...

 

[Harry Bloomfield]

What happens if you adjust the pipe stat to a bit lower temperature, say 10C?

 

[Mattvw]

Hi Harry,
That did cross my mind, but the boiler manual actually says 25! I might give it a try though just to see. What I don’t get is that how can the pipe stat sees the heat from the frost protection cycle in about 1 minute, but cannot detect the hot pipe from when the central heating has been running a couple of minutes prior?
Perhaps I should try and lag around the stat??

 

[Harry Bloomfield]

What I don’t get is that how can the pipe stat sees the heat from the frost protection cycle in about 1 minute, but cannot detect the hot pipe from when the central heating has been running a couple of minutes prior?
Perhaps I should try and lag around the stat??

My best guess at the moment, is that the pipe is cooler that the 20C. Might the room temperature have been met, boiler shut down and the pump continued to run dissipating the heat around the radiators, on pump over-run?

 

[Mattvw]

Thanks Harry,

I really don’t think it’s pump over run, I keep wondering if the pipe stat isn’t making good contact with the pipe, but then the counter argument to that is if it wasn’t then the frost protection would keep running and running!
It’s the oddest thing. Thank you for your advice.

Matt

 

[Terrywookfit]

What boiler is it.
Is the pipe stat on the return near the boiler.
What do you n mean by prv whistling.
Does the frost stat open the htg valve?

 

[Terrywookfit]

My best guess at the moment, is that the pipe is cooler that the 20C. Might the room temperature have been met, boiler shut down and the pump continued to run dissipating the heat around the radiators, on pump over-run?

Frost protection overrides any timer or room stat .
I can only think of one oil fired with pump overrun.

 

[Mattvw]

Hi Terry,

the pipe stat is on the return, right close to the boiler.
The frost stat doesn’t open either the htg valve or the HW, instead it deadheads the pump, lifting the prv. I can hear it, it’s a different noise to the hot water (quiet) and the heating (pipes expand in joists and can be heard).
Edit: it’s a grant boiler. Not sure what model, but only two years old.

 

[Harry Bloomfield]

the pipe stat is on the return, right close to the boiler.
The frost stat doesn’t open either the htg valve or the HW, instead it deadheads the pump, lifting the prv. I can hear it, it’s a different noise to the hot water (quiet) and the heating (pipes expand in joists and can be heard).

Have you any means to monitor the temperature of the return pipe where the stat is located?

 

[Mattvw]

Good question Harry. I’ll have a think...

 

[Mattvw]

I just timed it, 20 minutes between cycles, and heating has run between those two cycles...surely the pipes don’t cool down that quickly!

 

[ianmcd]

lifting the prv

That makes absolutely no sense

 

[Terrywookfit]

In Frost protection mode the Timer and star are by passed