Frost stat kept the C/H running the other morning?

[YorkshireMidge]

Woke up the other morning to find the house really really hot - much hotter than the room stat would have allowed.

Ours is an old Y plan configuration on a condensing boiler. I made sure both H/W tank and room stats had actually clicked off as I would have expected given how hot everything was - and that was the case, but by the time I'd found my wiring diagrams to try and start diagnosing things, boiler and pump had finally stopped running of their own accord (controller for H/W and C/H still on) - and there has been no repeat since.

Our boiler is in an integral garage which can still get pretty cold - which made me wonder whether the frost stat had kicked in. That sits between live, and the demand for heat connection into the boiler. The frost stat is really old - it is the only bit of the control circuitry that has never been replaced. So I'm minded to swap it anyway in case the contacts had stuck together or something - and then finally pinged open when the temp in the garage had risen a bit to force the contacts apart.

Whilst I do this, I was thinking it made sense to review the setup we have as I'm not clear whether it is appropriate given where the boiler is and I was looking for a bit of advice.

The boiler is a Glowworm 18 HXI and the manual says it has it's own frost protection - i.e. it says

"This device operates the burner and system pump when the temperature inside the boiler falls to 3 degrees C. Any other exposed areas of the system should be protected by a separate frost thermostat".

Now as I understand it, the Honeywell motorised valve parks itself in the hot water position when there are no inputs - so if the boiler frost protection operates, it will circulate through the H/W tank. But that will also be the case if the external frost stat is activated (unless the controller is on for heating at the time) - so the only purpose of the latter would seem to be to allow the frost threshold to be varied (it is set at about 4 degrees currently). It also seems to me that the boiler running will have little impact on the air temp in the garage - so perhaps I should be fitting a pipe stat on the return pipe to the boiler and wired in series with the external frost stat so the latter isn't causing the boiler to run uncessarily. I suppose it is possible that the lack of a pipe stat is what caused the issue I saw that morning too - but we haven't had any problems in the many years it has been like that.

Thoughts appreciated. Thanks.

 

[Harry Bloomfield]

The boiler is a Glowworm 18 HXI and the manual says it has it's own frost protection - i.e. it says

Then you have no need, for a separate frost stat at all. Disconnect it, or replace it with a pipe stat.

Your present frost stat will sense the general temperature in the garage, and while ever the temperature in the garage is below the operating temperature of the stat, the boiler will run. With no radiator in your garage, that run time could be a very long time, just wasting gas.

Your boilers built-in sensor, will be much better, it will switch off, as soon as the water in the boiler, has no risk of freezing, but should also protect your pipework too.

 

[YorkshireMidge]

Thanks Harry. I'm fairly certain the frost stat dates back to the original boiler from when the house was built so I suspect it was just left in circuit when the boiler was replaced. I suppose it could be argued that its belt and braces if the boiler's internal frost protection was to fail.

When you say replace it with a pipe stat, presumably you mean something that has a low operating range such as a Honeywell L641B. Some of the other pipe stats (e.g. Drayton PTS1) have a much higher range which would make sense if they were being wired in series with the external stat in order to turn the boiler off when the pipework was warm anough to avoid a freeze.

 

[Harry Bloomfield]

When you say replace it with a pipe stat, presumably you mean something that has a low operating range such as a Honeywell L641B. Some of the other pipe stats (e.g. Drayton PTS1) have a much higher range which would make sense if they were being wired in series with the external stat in order to turn the boiler off when the pipework was warm anough to avoid a freeze.

It would only be needed, if you have a lot of pipework in that garage, which needs protecting, but likely it would not be needed. Your boiler's built-in protection, will respond sooner, because your boiler's flue allows cold outside air into the boiler. So a pipe stat, would be just 'belt and braces'. The pipe state, would simply connect exactly the same as your frost stat was connected.