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Grant Combi MkII - Overheating after using hot water

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6666nick

Hi All ,
My first post so please be gentle!.

I have a Grant Combi mkII which has been giving me an over heating problem after using hot water recently , prime example is i have a shower in the morning before work hot water is fine , by the time i've dressed and come down stairs the boiler has gone in to 'Overheat' , eventually once the boiler has cooled i can use the reset and the boiler will work fine , this does not happen every time the hot water is used.

This has been happening for few weeks now and on the advice of 'a mate' i changed the expansion vessil which i thought had cured it as it had not over heated until this morning when it did it again after i had a shower.

Any suggestions please?? - i was thinking either partially blocked heat exchanger although hot water performance when running is 100% so maybe not that , or some kind of issue with thermostats / temperture settings

Thanks.


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If the hot water is fine I would disregard the pump. I would be looking at the thermostats, I am afraid you would need an engineer, you would need to check that the tank stat is cutting in at I think it is about 78deg (or thereabouts), it could be that the overheat stat is cutting in too early. There were one or two problems with the stat on these in the early days especially with the control stat which you sometimes had to tweak a bit. A stem thermometer is the answer to check them with, I carry one in the van
 
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kimbo said:
If the hot water is fine I would disregard the pump. I would be looking at the thermostats, I am afraid you would need an engineer, you would need to check that the tank stat is cutting in at I think it is about 78deg (or thereabouts), it could be that the overheat stat is cutting in too early. There were one or two problems with the stat on these in the early days especially with the control stat which you sometimes had to tweak a bit. A stem thermometer is the answer to check them with, I carry one in the van
Click to expand...

Kimbo - I have a stem thermometer that i can borrow from work as i am a building engineer at a hospital so i'm resonably familar with the engineering side of things , its just knowing where to look for the problem. Are the stats sets points adjustable??
 
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If you let me have your email I will send you the relevant copy of the service manual, its far easier than me trying to explain it. It is probably better getting an engineer out but that will be up to you.
The boiler may be firing prematurely on the store stat in which case it will overheat.
 
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Could it be sludged up? As large amounts of sludge can store a lot of heat and give it off after the pump stops. That brings us to the pump overrun.

Would not be the first one where the pump overrun has been disabled or broken. Is the pump still going for a while after the HW gets turned off?
 
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If you are doing measurements check your hot water on the taps as well for temperatures. How is your general setting for the flow temperature? Have you tried lowering that a bit?

Sorry for AddOn: When was the last service on the boiler?
 
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