Gas boiler gone into overheat?

macman

twopenny said:

Thanks Lesalanos, another bit of info to add to my understanding

I'm desperately needing a break from this but it's tapping/knocking away in the ceiling making it difficult to ignore.

Thermostat in garage set at 60.

So the boiler is firing every 2mins or so, for 30 secs.

The radiators are just warm trvs on 2-2.5 which is lower than normal but the house is warm enough. I'm not a hot house flower.

The tapping noise in the ceiling is in between this mainly on shut down. No wonder it's driving me nuts!

I need to time it with the trvs set higher to see what the difference is.

But now I need to cook and have a shower..........

I'm thinking 2 things. A thermostat somewhere is not functioning right or there's air in the system. No one has bled the rads though they seem ok.

Is it alright to bleed them when the system is on? I used to do that with my last house as I could then see if the heat was even but that was a different kettle of fish.

No, it's a frost 'stat and it's set at 6C. Not 60C. It will fire the boiler when the air temp drops to 6C, to protect the pipework from freezing.

plumb1_2

Frost stat best set at 5

ThisIsWeird

Ok, the black knob on the boiler sets the boiler's flow temp. '3' will probably be something like 'nicely hot' - around 65oC - and this will be fine for most circumstances. 4 will be getting 'ouch', and 5 deffo. You'd turn it up higher only in cold weather, if the rads are struggling to heat the house, so the rads would then become 'ouch-to-scalding' hot.

The rad's TRVs are completely different. These open and close the supply to the rad depending on the room temp. '3' on a TRV equates to around 19oC, I think, a usually adequate room temp.

If you have a cold room and open the TRV to, say, 3, this will open the TRV valve and it'll receive a good flow from the boiler - the rad will be 'on'. If the boiler control is set at 2, then the rad will heat up 'hottish', and start to warm the room. When the room temp reaches ~19oC, the TRV will start to close and keep the room at that temp. That's all fine.

What difference will it make if you turn the boiler knob to 4 instead of 2? This time the rad (@ 3) in the cold room will come on 'ouch' hot, so will heat the room more quickly than before. The rad will still go off at 19oC, because the TRV is set to the same '3' as before. 

Why would you have the boiler at 4 instead of 2, then? Only if the weather is so cold that a merely 'hottish' rad just can't manage to heat the room, so the boiler output needs turning up. The TRV remains the same.

Why not leave the boiler at 4, then? Because with a modern condensing boiler, it's less efficient = waste of energy. You are always better of having the boiler flow as low that will still do the job of heating, but no more.

ThisIsWeird

You mention the rads come on 'scalding hot'? What is the boiler's knob set to for this? See if it's still ok heating your house when set to 2, and how hot the rads feel.

Bear in mind that the TRV affects the rad too.