Combi Boiler boiling water!

[allymorrison70]

Hi

I've got a heatslave 18/25 external boiler. Burner seems to stay on too long and heats the water excessively causing the boiler over heat and high limit stats/thermistors to trip. These stats are doing there job so pretty sure theyre working correctly. Does anyone one know what stat switches off the burner when the water hits the correct temp? Pressumably its one of the control stats for the hot water and/or central heating that could be faulty? Any help is appreciated.

cheers

 

[Johntheo5]

Does this happen if CH on or only after using DHW, the burner may then run on to reheat the (primary) store up to temperature.
Also, if a sealed system is the system pressure ~ 1/1.5bar or if open vented. is the small feen&expansion cistern "full".

 

[allymorrison70]

I couldn't say for sure if it happens on DHW or CH or both. It happened today while I was trying to investigate the issue, I think I might have turned the hot water stat up to get the burner on. Burner remained on until the water expanded so much the relief lifted then it tripped. Burner didn't stop firing even when I turned the DHW and CH knobs down all the way.

 

[Johntheo5]

Put both the DHW & CH settings back to normal then open a hot tap, but not fully, the burner should then cut in/out occasionally, if no problems after say 10 minutes, shut the HW tap and see what happens.
Ensure CH switched off, first.

 

[allymorrison70]

Ill give that a go tomorrow and see how it goes. Ill post again with the results.

 

[Johntheo5]

Also, worth a read ........

Worcester 12/14 won't turn burner off​

• Thread startericeman66
• Start date13 Jul 2010

 

[allymorrison70]

Interesting read. Ive had issues with the flow switch before, I've changed two already. I didn't think that could be the issue but I guess it makes sense that it could be. I have a spare at least.

 

[allymorrison70]

Would you happen to know resistance values expected if I had to test the control stats and pipe stat?

 

[Johntheo5]

Maybe these??

 

[allymorrison70]

So finally got to the boiler today. Turns out a faulty flow switch was causing all my issues. Luckily I had bought a spare, so im back in business now! Cheers for the advice.

 

[Johntheo5]

So, on flow switch no3 now?.

 

[allymorrison70]

Yes, number 3 now. They don't seem to be very good. Or is my system breaking them!

 

[Johntheo5]

It must be sticking open (if its a paddle type) to cause the overheating, I'm sure I read somewhere that someone used to shave a bit off the paddle to ensure it doesn't stick, have you inspected the removed one to see if its a very tight fit in its housing?.

 

[Exedon]

It must be sticking open (if its a paddle type) to cause the overheating, I'm sure I read somewhere that someone used to shave a bit off the paddle to ensure it doesn't stick, have you inspected the removed one to see if its a very tight fit in its housing?.

It is known to have to get stanley knife out and gently trim/scrape a bit of paddle.
I live & work in a area where the water is described as aggressively hard untreated that can certainly cause flow switch problems.

 

[allymorrison70]

The paddle was free in the one I removed. I wonder if the switch had just become faulty. I live in a soft water area yet there were some significant hard deposits within the housing.