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I understand that, I was just trying to learn a bit about fault finding, not saying I wanna know everything someone knows/has learned, etc

ok fair enough. I'm not gas safe registered at the moment so I understand your point there, was just trying to learn a bit about boiler fault finding

I know a bit about boilers and fault finding but not the methodical system/way. I'd like to learn though.

dave18 - what do you mean? I am just interested in learning about boiler's/fault finding. I appreciate your help btw.

yeah i agree. what's your methodical system :lol:

if he saying h/w is on all the time (boiler firing) - could be a stuck DHW flow switch.... but I agree diverter valve passing is allowing rads to heat up in pre-heat mode

thanks dave18 for your help. if the secondary heat ex was blocked - on the ch side (not cold going in - hot out) then i've heard the water will run hot then cold hot then cold like that and could even overheat lockout. if the water slow from a combi was poor it could also be a blocked/part...

check to see if the ballvalve is leaking/dripping. then if not, is the big tank's water level higher than your small tank's level?

I know that, but also it could be down to a sludged up system, as newer boilers have smaller heat ex's and they can get blocked. About the pump: do you mean the pump failing/slowing...

I was just asking in general, about the main heat exchanger and causing the boiler when on, after afew minutes to overheat/lockout.

What would you say the best/easiest way is to fault find a sludged up heat exchanger that could be causing no/poor circulation and boiler overheat/lockout?

ok, sorry, didn't realise!! :oops:

The poster needs an boiler breakdown engineer to check everything in the boiler and to find/replace the part ideally. One last point dave18: what would you say the best/easiest way is to fault find a sludged up heat exchanger that could be causing no/poor circulation and boiler overheat/lockout?

thanks for your help.

dave18 - of course it would, soryr for that one of those lazy days!! :lol: I agree with you about the CH flow seitch being stcuk in the ON position, and if the pump was faulty the no circulation would cause the boiler to overheat (on CH or DHW demand) - so 2 faults in 1 there. (as if...

One last point: At what point does the thermistor/stat usually come into play, ie is it before pump/fan running/proving or after, as if the stat/sensor was stuck open (infinity reading) then the boiler wuldn't fire.....

dave18 - it's OK I was just trying to work this one out! I know a bit but am no expert obviously. One point though : if the CH was turned OFF at the timer/stat etc - and the CH flow switch was STUCK in the ON position - would the boiler still fire then continuously, or does it have to...

sorry that's what I meant so the water only circulates through the main heat ex and dhw heat ex, not the ch circuit/rads etc

dave18 - yes I understand that. If the CH flow switch was stuck in the ON position though would the boiler not permanently fire/cycle on-off the boiler stat without demand from CH

gas valve could be faulty.... or fan could be on way out and only working at low rate for pilot. APS could be faulty; could be an ignition problem/fsd problem, could be a lot of things. Need a boiler engineer to look at it