Boiler Frost Stat causing problems

[john306]

Hi, I've got a Glowworm boiler (not combi) in garage with a frost stat beneath the boiler itself.
Since before Christmas, the overheat stat has been tripping quite a few times with F5 on the display.
I've thought I'd narrowed it down last week to a problem with the frost stat, which I replaced but the problem remains....
If the boiler fires up normally for hot water and/or heating, everything is fine, the temperature display on the front of the boiler will slowly rise up to required temperature.
If I dial in a temperature on the frost stat myself to test it so that it kicks in, the temperature on the front display of the boiler immediately rises to above 80 deg, there is a banging /gurgling noise from the boiler and the boiler shuts off.
I'm assuming the really cold weather we had overnight Christmas was causing the frost stat to activate and shutting off the boiler and no water/heating the next morning until the reset was pressed. Additionally I was woken some nights with the pipes banging/gurgling.
This doesn't happen with normal heating/hot water, everything is quiet and works as it should.
Any advice what may be the cause please?
Thanks.

 

[stem]

It appears that when the frost stat fires up the boiler that there is insufficient circulation of water, so that the heat produced cannot be dispersed and causes an overheat trip.

You don't mention anything about the system that boiler is connected to, which is likely to be the cause of the problem. For example, if you have 2-port motorised valves (aka S-Plan) the heating motorised valve will also need to be opened to allow circulation of water through the system. In this case, if the frost thermostat has been wired directly to the boiler causing it to fire up, but is not also opening the motorised valve to allow water to circulate it would cause the problem you describe. Also, what is controlling the pump? and is it running when the overheat trips? Some boilers control the pump directly others don't and rely on the external controls to operate it.

If you could provide some additional information about your system such as the boiler model and details the pipework and controls (ie motorised valves) connected to it, someone may be able to give you more specific help.

 

[john306]

Thank you for reply.
System is S Plan, I don't know what's controlling the pump or if it's running when it trips, as boiler is in garage, pumps valve tank etc are in airing cupboard upstairs.
The boiler only runs for 10 secs or so before it cuts out.
Boiler is Glowworm 18hxi.
I've been in this house 3 years and boiler was supposedly fitted a few years before that.
Only had the problem this last month, never a problem before. Nothing altered/changed on boiler since we've been here, just servicing, last time September and that just involved a gas analyser, I don't even think he took the cover off the boiler.

 

[stem]

With the Glowworm 18hxi, the boiler does control the pump, and it also provides a pump overrun, this requires a provision to allow water to circulate even when the motorised valves are closed. If this wasn't present, then the boiler would trip during normal use when the motorised valves close which you say it doesn't.

There is also a dedicated boiler connection for a frost thermostat, so it would be difficult to get wrong.

So that rules out both of the things I thought it could be. Maybe someone else will be along with some other ideas.

 

[john306]

Thanks for trying to help stem, what you've said makes sense.
Just need to hopefully find a solution now.

 

[Hot&Cold]

your boiler has auto frost protection already built in,if you have additional vulnerable pipework then additional frost protection is required and a suitable open bypass or other flow arrangement is required.

An onsite inspection by a heating technician will find the best solution.

Please remember when the boiler is at F5 you have no frost protection

 

[john306]

your boiler has auto frost protection already built in,if you have additional vulnerable pipework then additional frost protection is required and a suitable open bypass or other flow arrangement is required.

An onsite inspection by a heating technician will find the best solution.

Please remember when the boiler is at F5 you have no frost protection

Thanks, yes I read about the built in frost protection, I assume they put in an additional frost stat as there is pipework coming out of the top of the boiler then through the wall into the house, the garage is built on the side of the house. That section of pipework is insulated though and wouldn't get any colder than the boiler.
I'm just curious as to why there is banging/gurgling when the frost stat kicks in and the boiler tries to run, as opposed to coming on with the timer and all is quiet and well?
I was hoping it was just a replacement of the frost stat or dodgy connection maybe, or someone had this problem before getting someone in.

 

[MeldrewsMate]

My opinion is that Stem was on the right track.
It may be that the pump has not been powered from the boiler, rather it has been connected to the S plan valve outputs. You will need to take the output from the external frost-stat to the heating zone valve directly, bypassing the roomstat.
Additionally you should power the pump from the boiler to give pump overrun, and for proper operation you should install a bypass to give circulation during overrun, an auto bypass is preferred.

 

[john306]

My opinion is that Stem was on the right track.
It may be that the pump has not been powered from the boiler, rather it has been connected to the S plan valve outputs. You will need to take the output from the external frost-stat to the heating zone valve directly, bypassing the roomstat.
Additionally you should power the pump from the boiler to give pump overrun, and for proper operation you should install a bypass to give circulation during overrun, an auto bypass is preferred.

Ok thanks, but you've lost me with most of your reply.
All I can say is that we have been at this house 3 winters and nothing like this has ever happened before, and as I said previously, nothing has been added/taken away from the system so again I'm assuming it was all working correctly previous years when the frost stat kicked in.

 

[john306]

Another night where the pipes were banging/gurgling.
Anyone have any idea why this only happens on a freezing night when the frost stat kicks in?
Why would it be any different to the boiler kicking in on the timer?
Is there a different sequence of events when kicking in from frost stat as opposed to normal operation?
Thanks for any help.

 

[stem]

Possibly because the frost stat is wired directly to the boiler and is not opening one of the motorised valves, which would be opening under normal operation.

As a test you could try turning off the heating and hot water at the programmer and then physically side the manual operating lever of the heating motorised valve to the manually open position and latch it in place. Now turn up the frost stat until it kicks in. Do you get the banging and gurgling now? If not, you have found the problem.

 

[john306]

Possibly because the frost stat is wired directly to the boiler and is not opening one of the motorised valves, which would be opening under normal operation.

As a test you could try turning off the heating and hot water at the programmer and then physically side the manual operating lever of the heating motorised valve to the manually open position and latch it in place. Now turn up the frost stat until it kicks in. Do you get the banging and gurgling now? If not, you have found the problem.

Bingo, you've got it.
Tried what you said and it fired up and worked without any noises.

If it worked correctly over the last few years what could be the cause of it not doing so now?
Are there any fuses, boards, wiring that is solely for use with the frost stat, or is it possible to do away with the frost stat entirely, as you mentioned the boiler has a built in one anyway that should protect it?

 

[stem]

Very odd. Has the wiring been changed? Was the system previously a Y-Plan with a three port valve that has been converted to S-Plan? Has a by-pass been added or adjusted? Anything else changed since last winter?

There will (should) only be one fuse protecting the whole system so if that had blown then nothing would be getting power, and in any case the frost stat is getting power as it's firing up the boiler, and the motorised valve is working OK under normal circumstances.

The boilers integral frost stat will protect the boiler (if it has one fitted, not sure if yours does or not) but unfortunately that won't open the motorised valve either so will cause the same scenario. And sometimes the frost stat is located away from the boiler for example to protect exposed pipework elsewhere.

 

[john306]

Very odd. Has the wiring been changed? Was the system previously a Y-Plan with a three port valve that has been converted to S-Plan? Has a by-pass been added or adjusted? Anything else changed since last winter?

There will (should) only be one fuse protecting the whole system so if that had blown then nothing would be getting power, and in any case the frost stat is getting power as it's firing up the boiler, and the motorised valve is working OK under normal circumstances.

The boilers integral frost stat will protect the boiler (if it has one fitted, not sure if yours does or not) but unfortunately that won't open the motorised valve either so will cause the same scenario. And sometimes the frost stat is located away from the boiler for example to protect exposed pipework elsewhere.

Hi, nothing at all has changed/been altered.
Only thing done to the boiler was the service last September, I let him in garage and we chatted while he put the probe into the flue and printed me out the results.
The frost stat is located 6 inches beneath the boiler itself on the garage wall.

You've worked out the cause of the problem and I thank you for that.

 

[john306]

While we've got this cold weather this problem is winding me up.
Can I ask who is the best person to get around to try to solve my issue, would it be a Gas Boiler Fitter/Engineer, or an Electrician.
Thanks.