Problem with Glow worm boiler

[niamhinho]

I have recently replaced a radiator in an upstairs room, and I'm now having problems with the boiler overheating.

If we just have the hot water on, everything is fine, however, if I switch the central heating on as well, all is OK for a few hours, but after a while the boiler shuts off, and I have to press the overheat cutoff reset button to fire it up again.

The boiler is a Glow worm Ultimate 120FF and is about 7 years old. Prior to me changing the radiator, there were no problems.

 

[ChrisR]

Air lock? Close all but one rad - work your way round all rads one at a time to try to force it out.

You didn;t put a trv on the new rad did you?

 

[niamhinho]

Thanks for the reply Chris.

I did put a TRV on the new rad, but there was one on the old one, so it was like for like. Will this cause a problem?

 

[niamhinho]

I've gone round each of the radiators in turn, but the problem still exists. All of the rads get extremely hot, so there doesn't seem to be a problem there.

The one thing I did notice though, was the bathroom rad is now 'banging' when the TRV opens or closes. This never happened before I changed te rad in the bedroom. Could this be linked to the boiler overheating?

 

[breezer]

at least one rad should NOT have a trv (usually bathroom)

 

[masona]

What number is the boiler stat setting at ?

 

[Porker]

Probably not your problem but worth mentioning. I have a 120FF and experienced identical issues to yourself. I did have an airlock which took regular bleeding every day for a week to stop the banging but my main problem was that I had inadvertantly altered the timing of the HW and CH at the controller. I found the only remedy was that the HW has to be ON when the CH was ON. The only explanation I can think of is that there is no cooling path for the water when all the TRV's are closed. I also have a bypass valve fitted which I believe you need if you have all TRV's. What I still don't understand is that the circuit is so small with only the bypass valve, I can't see how much heat can be lost.

 

[breezer]

I can't see how much heat can be lost.

not being a heating engineer, i do not think it is the heat that is the problem. (may be wrong)

Put it this way, all rads closed, pump still running it is only pressurising things since it has only a flow and no return, by having no trv or by having a bypass valve the water can flow AND return

 

[niamhinho]

All rads have TRVs fitted, and as far as I'm aware, there's no bypass valve, although I'm not too sure what I'm looking for.

As I've said, the system is exactly the same set up as before I changed the bedroom radiator, and that one heats up fine when the boiler is on.

The boiler thermostat is set mid way between min and max, although I have tried turning it both up and down with no effect. Could it be the stat that needs replacing?

Do you think the banging bathroom rad is connected to the overheating problem?

Thanks for all the help so far

 

[Porker]

Breezer - you could be right. I'm not a plumber or HE. But given no other changes to a system there are only a limited number of causes I guess.

This is really just my guess from many days head scratching my own system. BTW I had bleed valves on the pipework at the top of the boiler because the pipework was configured in a way that an airlock could occur there.It may be worth seeing whether you have.

Another theory - your new TRV closes earlier than your old one. The boiler is fired due to demand from room stat. (Assuming no bypass valve) but there is now no path for the water. The boiler heats up what is in it and eventually trips out.

Have you tried opening another TRV up fully in another room and seeing whether that cures the problem.

 

[Tony6]

Just for your information new regs state hallway

at least one rad should NOT have a trv (usually bathroom)

How this works with the thermostat normally in hallway i dont know?

 

[breezer]

How this works with the thermostat normally in hallway i dont know?

thats easy.

hall is not too hot or too cold.

if stat in say lounge, heating will turn off when lounge hot, but remainder of house will be "cold" because heating has now turned off.

if stat in hall, heating can run and run, as each trv closes so hall gets hotter quicker, until stat turns heating off.

front door opens (some one in or out) hall cools stat comes in bringing on boiler (but it wouldn't if it was in lounge)

that's my theory

 

[ollski]

If you have no bypass and all trv's then the banging is probably thermal shock when the burner turns off trying to burst your heat exchanger. It could also be that the trv is a uni directional model fitted on the wrong end.

 

[Tony6]

No you can have room stats

if stat in say lounge, heating will turn off when lounge hot, but remainder of house will be "cold" because heating has now turned off.


if stat in hall, heating can run and run, as each trv closes so hall gets hotter quicker, until stat turns heating off.

If the hallway gets hotter quicker and stays hotter the thermostat will cut out and leave you cold in the front room .If you want more heat in the front room?

 

[croydoncorgi]

Bypasses, radiators without TRVs providing a bypass, automatic bypass valves...

all of these do the same thing and for most boilers are ESSENTIAL.

ESPECIALLY with a 'low water content' boiler (aluminium heat exchanger, physically small), the boiler builds up quite a lot of 'latent heat' when it's running. This heat acts like a flywheel - if everything keeps going (pump, burner, flow of water round the plumbing and through the rads, ... - it all stays in-balance and the boiler is happy.

But if the pump stops or the flow through the rads is stopped or slows down then the main thermostat in the boiler, and / or the secondary overheat stat, will operate and switch off the burner. OK - so there's no more heat going into the boiler but there's LOTS already in there. If there's no water flow to carry it away, the boiler will overheat and boil - producing banging noises, water and steam out of the emergency discharge pipe from the Pressure Relief Valve (PRV), pressure loss and general Bad Things for the boiler.

Usually, all that's needed to prevent this is a loop through the plumbing to allow the pump to carry on taking hot water out of the boiler through the Flow pipe and cool it down a bit before putting it back in through the Return while the boiler cools down. This, by the way, is also why most boilers have 'pump overrun', so that the pump carries on going for a couple of minutes after the burner has switched off. In many cases, the pipe run between boiler and HW cylinder is actually a big enough 'heat sink', which is why you'll often find an 'open bypass in the plumbing near the cylinder. (Note that if there's a valve on the bypass, it should be adjusted to allow enough flow to dissipate the heat.) Another bypass design uses a radiator or, more often, a towel rail as a bypass - which is why the control valves on whichever MUST NOT be turned off. Other systems include an 'automatic bypass valve' which is a special spring-loaded valve linking flow and return some distance from the boiler. This is adjusted so that if all the other paths between flow and return are closed (by TRVs, zone valves, etc.) it opens to provide the bypass route to cool the boiler.

Combis are a bit different, mostly I guess because they work much harder in HW mode compared with CH. Some have 'internal bypasses', others actually use the fan rather than the pump to dissipate heat. Some include a jumper option to select either pump or fan (Ariston Microgenus owners take note: this model is a bit susceptible to overheating in HW mode when the heating is completely off and seems to work best with fan overrun selected.)

End of lesson on bypasses.