Combi boiler overheat or overrun problem

hockey2019

Hi all , I have a Greenstar 35cdi Combi boiler - have just started to have a overheat problem - it doesn’t lock the boiler out or get any alarms not that hot - first thing in the morning central heating comes on boiler fires up but the temperature keeps rising quickly to above the heating temp and cuts out - stays hot at 65 - 88 on read out - can get it going by turning hot water on and off until it seems the pull comes on then temperature drops quickly - then when heating is on - use hot water works fine turn off then the temp rises really quick again - checked the pump casing feels really hot to touch 

nofoollikeold

1. Sounds like a circulation problem on the heating side.  Boiler fires up, sends water around, but poor circulation means the system can't get rid of the heat, so the flow temperature rises and the boiler cuts out.  Open a hot tap, cold water runs through the secondary heat exchanger and cools the too hot water in the boiler.  
2. If so could be:
2.1 The circulating pump getting weak.
2.2 Sludge in the system.  Do you have a filter such as a Magnaclean, installed?
2.3 Too many radiators turned off and the auto-bypass not working properly.  
3. I think you are going to need a heating engineer to diagnose it properly.

FreeBear

Can you hear the pump running when the boiler fires up ?

It could be the pump is sticking and in need of replacing. The other fault might be a stuck diverter valve. In either case, you will need a Gas Safe engineer to pop off the cover to take a look.

hockey2019

Thanks for the tips - it’s very strange boiler is intermittent- today boiler for heating started - then put ran hot water - boiler slows keeps the boiler running very slowly and the temp rises - then it’s to high for the heating to come back on the temp stays high needs to be circulated to cool down - should the pump start before and after hot water to dissipate heat ? 

FreeBear

You may also have a partial blockage in the P2P heat exchanger. Is there a magnetic filter on the CH return pipe ?

Albermarle

The pump should have 3 speed settings. Try it on the highest setting ( unless it is on it already) and see if it makes any difference. 

hockey2019

Is the speed change on the black electrical housing - is it accessible without opening up our pump housing 

Albermarle

hockey2019 said:

Is the speed change on the black electrical housing - is it accessible without opening up our pump housing 

Yes should be just a button or a lever or a dial. No need to open up the housing or anything technical.
Unless it is very old/a cheap one it should have 3 speeds. Normally the middle speed is OK for most houses but if you have a bit of a blockage a higher speed might just clear it. Or you may realise the pump is not working properly, or most likely it is another problem. Just worth a try.

hockey2019

Thanks will try that - should the pump start straight away when heating is called for - as it seems that the boiler fires up but - water probably not being circulated so heats up and boiler stops heating as it reaches temp then stays at that temp for a while only slowly dropping 

hockey2019

But if you turn water on and off the pump starts circulates the water and temp drops 

Albermarle

hockey2019 said:

Thanks will try that - should the pump start straight away when heating is called for - as it seems that the boiler fires up but - water probably not being circulated so heats up and boiler stops heating as it reaches temp then stays at that temp for a while only slowly dropping 

I think the pump should start first, how do you know it is not working ?