Boiler overheat trips when it gets cold out (Potterton Profile) (Ed.)

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Wonder if someone can point me in the right direction.
Potterton Profile 80E (I think). Lots of rad stats, but towel rail in bathroom doesn't have one. Also have a wiser room stat in lounge but ordinary rad stats which are fully open there, i.e. there are at least a couple of paths for water to always circulate.

However, when it gets cold out / the boiler is on quite a bit then after a while the overheat trips and I have to reset it. The heat control on the boiler is set to about 5 (i.e. not max).

So why does the boiler being on quite a bit eventually cause the overheat to trip? I'm guessing that it being cold out means the boiler is on quite a bit of the time so doesn't cool down enough, but surely if hot water is returning to the boiler then it shouldn't be heating it further? I'm doubtless not understanding how it works.

Wondering if the stat needs replacing or if the heating control may be having no effect (always on max), or is the pump on too low a setting (it's quite noisy if I put it on fastest and keeps me awake on cold nights as the boiler is in the garage and gets cold so sometimes kicks with frost stat) or what...

Thanks
 
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Edit, having reset the trip about 30mins ago and the boiler having been on since I notice one rad is now at about 73C which seems quite hot for 5? But I'd still expect the boiler to stop heating hot water if it is getting back to it too hot! Other rads are mostly <60C.
 
If it’s overheating then I’d say it’s a circulation issue.
 
Does it trip because the water leaving the boiler is too hot or water entering it is too hot? Either way, why doesn't the flame simply cut out and it circulate existing hot water?
Thanks
 
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Does it trip because the water leaving the boiler is too hot or water entering it is too hot? Either way, why doesn't the flame simply cut out and it circulate existing hot water?
Thanks
Surely the flame does cut out when it gets to 5 and the pump carries on circulating the hot water. Have you tried running it at 4
 
Actually after my last post I've turned it down to 2 - just to see what happens while we have a cold spell. When I turned it down a bit far to 1 I think the flame did cut out, so hopefully that's all working ok. I wasn't sure if the flame should reduce in height or whether it just burns for less time. But yes, the pump does carry on.
 
If you have a Profile, it is an excellent boiler but very old now. You may have a lot of sediment or scale in the system. Does the boiler ever make banging noises?

Has the system ever had a clean?

Look in the feed and expansion tank in the loft. Is it warm? What colour is the mud, and how deep?

You say the pump is noisy. Do you mean like a worn-out bearing?
 
Yes, I was advised when I first got this boiler serviced it was a good model and to just keep it going. But as you say, old and not condensing etc..
The boiler normally makes nothing more than a gentle roar if that makes sense, no unusual noises. The system was half drained last year. We've only been here 2.5 yrs so no idea about clean - although it does have a magnaclean which I have previously emptied.
I'll have to go into the loft to check the expansion tank.
The pump was changed last year so it's not bearings etc. (previously it was very noisy), it's just the airing cupboard where the pump is, is adjacent to the bedroom so the sound of a motor running hard during the night is enough to wake me etc., it's also quite audible in the lounge below when the pump is on full speed. If I drop the speed from 3 (max) to 2 it basically becomes mostly inaudible.

If it makes any difference, the C/H was a retrofit (previously hot air), so all the rads downstairs are fed from pipes coming down and going back up. But it all seems to work otherwise.
I have recently had to have 2 TRV bodies replaced as they'd jammed shut - I suspect the rest need replacing (a spring job), although the rads are hot so they aren't currently stopping water passing through.

I had wondered whether the boiler could be over-speced for the house (an 80E in a small 4 bed detached) so it being on 5 could result in too hot water being returned and being overheated? But may be talking rubbish.

I'm not quite sure how all this should work, on the one hand I read to balance the system I should partly close valves to get a temperature drop between in and out, but then if you close things too much you could be in for flow problems and overheating. It is curious that I seem to primarily have problems in cold weather though, as though something is gradually getting hotter and hotter until it trips.

Thanks
 
Perhaps turn your room thermostat down so the boiler stops firing up for as long, depends whether your stat is in an area where a cooler control temperature is acceptable
 
Interesting idea. The room stat is in the lounge, but it's all programmable so I could perhaps change it so instead of e.g.
Overnight 15C
8am 18C

it becomes
overnight 15C
6am 16C
7am 17C
8am 18C
 
I would have thought it was a circulation problem, due to sediment or pump, or maybe the main thermostat, which should stop the firing before it reaches the overheat temperature.

I used to have one of these and I don't think the pump had a run-on to cool the heat exchanger after demand stopped.

I gave mine a clean with X400 and a huge amount of black sludge came out, the Magnaclean continued catching the declining residue that had been disturbed for months. It originally had gravity circulation to the HW cylinder.

Some heat engineers are very fond of Profiles, and understand them well. Try to find one in your area. They may not be very young.
 
P.s.

Just to check

The room with the thermostat must have a radiator with no TRV.
 
That's interesting, so is the possible scenario that hot water is returning to the boiler, the room reaches temperature so the boiler cuts out and the pump immediately stops. So if the boiler then restarts fairly soon it will heat already hot water and overheat.
Although the room with the thermostat does have TRVs, they are set to max.
 
No, it's because the big iron heat exchanger holds a lot of heat, which is taken away by the flowing water.

Take at least one of the TRV heads off. It will have a retaining ring or nut. No water will come out.
 
Presumably I can test this. Create demand e.g. 25C for 15mins and then change temperature down to 15C. The boiler will stop immediately, but see if the pump stops at the same time. If it does then that could well be the problem.
I have a towel rail with no TRV and currently at least one rad in the house doesn't have a TRV head on it, so there should always be the possibility of circulation.
 

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