Green star Combi boiler overheat problem

hockey2019

Have noticed the heating does not come in all the time - opened flap to watch temp - 2 problems when heating is on pressure gauge roses from just above 1 bar to 3 bar - and when hot water tap is run for a short burst the boiler temp climbs quickly to 80-90 and does not drop and the heating will not come back on - can get temp back down by running hot water tap until boiler fires up and then the temp drops quickly and the heating will fire up again 

ThisIsWeird

The pressure issue is classic faulty expansion vessel problem. The EV is designed to accept the expanded hot system water, and return it when it contracts again, ergo the pressure should remain constant. 
At 3 bar, a safety valve usually opens, and allows this excess pressure to be dumped via a discharge pipe. Once that happens, your system won't have enough water in it, so the cold pressure is often then too low.- a vicious circle.
You need a GasSafe plumber to sort issue for starters, and then they can check if anything else is amiss.
You may be better off with a WB fixed price repair - that'll usually be around £300, but should fix everything that's wrong. Look it up, and check that EVs are covered.
How old is this boiler?
Do you find you need to top up the pressure?

hockey2019

Boiler is 10years old - no I do not need to top op pressure - the heating side works fine - the temp o my raises when the hot or cold taps are run for a small time - then if you run the hit for a while the boiler kicks in and the water seems to circulate and the temp drops quickly then the boiler and heating starts again normally - thanks for the reply - where to I find the fixed price service and fix does this include parts aswell 

ThisIsWeird

If you Google 'fixed price boiler repair', you'll find a few companies do this. Folk on here will likely advise on the best. Also see if WB themselves do it.

I'm astonished your system doesn't need topping up. It must reach 3 bar, and then no more! But, that's still too high, so summat is amiss - mostly likely the EV needs an air top-up.
No idea what's going on with the temp issue. I can't even speculate if parts are needed, so FPR perhaps overkill as a first step.
See what other folk think.

FreeBear

hockey2019 said: 2 problems when heating is on pressure gauge roses from just above 1 bar to 3 bar - and when hot water tap is run for a short burst the boiler temp climbs quickly to 80-90 and does not drop and the heating will not come back on - can get temp back down by running hot water tap until boiler fires up and then the temp drops quickly and the heating will fire up again 

Check the DHW and CH max temperature settings - If you have dials numbered 1 to 6, try 3 for CH and 4 for DHW.

As for the pressure - Bleed all the radiators first. If the problem persists, time for a Gas Safe engineer.

hockey2019

Thanks - can I check the EV - do I need to drain system to check I have seen that I can pump it up with pump - can I do this with out draining system ? 

ThisIsWeird

Provided you can gain access to it without opening a panel that affects the combustion area, then it can usually be checked quite easily.
It should have a Schrader valve on it, like a car tyre.first test is to remove the safety cap and briefly press the valve pin - see what happens. 
If air only hisses out, then good chance the EV is ok, and just needs a small pump up. If nothing comes out, then the air side is completely empty so it needs a darned good pumping. If any water comes out, it's kaput and it needs replacing - or a remote EV fitted somewhere else.
To pump it up needs the boiler turning off, and a rad bled until the pressure is zero - you don't need to 'drain' the system. Open a bleed screw until no more water comes out. Leave it open as you pump up the EV - more water will come out with each pump. Pump until air side is between 0.75 and 1 bar (see if you look up  specifically for your boiler). Close the bleed screw, and refill the water side to 1 bar.

FreeBear

ThisIsWeird said: Provided you can gain access to it without opening a panel that affects the combustion area, then it can usually be checked quite easily.

The EV will be internal to the boiler on a Greenstar. So to gain access will require breaking the seal of the combustion area. Gas Safe engineer territory I'm afraid.

ThisIsWeird

This suggests that it may be accessible from above, without the cover being taken off, but I'm not sure - 2:20

https://www.youtube.com/watch?v=kQWYbuSjVkM&t=141s

hockey2019

Thanks for all the responses will try the EV pressure first - will this help with the over temp issue ?

ThisIsWeird

hockey2019 said:

Thanks for all the responses will try the EV pressure first - will this help with the over temp issue ?

No. Unlikely. I can't see a connection. But the pressure does need sorting because if it exceeds 3bar you'll have more issues!

What are your DHW and CH temp settings at? How low have you turned them down?
Do you have a magnetic filter? If so, when was it last checked?
If you have system sludge, then possibly your P2P exchanger or diverter valve is affected, and doesn't allow a full flow if water through them. This can cause overheating inside the boiler, where the main exchanger cannot shift its heated water fast enough.
But, pure guesswork.