Performa 28, gas won't modulate on hot water

[woodtop]

Hi, have a problem with a Performa boiler. When hot water running the gas does not modulate, temperature goes up to 80 on the boiler regardless of temp set on dial and then turns off, cools a bit and relights. If the central heating is running the gas will modulate. Have removed and cleaned the hw heat exchanger and removed and cleaned the thermistor. Checked the readings for the thermistor at the boiler which seem to show its working and have continuety on the thermistor. Gas valve also seems to be giving correct readings. Spoke to Potterton technical who suggested changing the thermistor then main board. Just wonder if it's a regular fault or if someone has over thoughts, before the expence of a new board.

 

[stevie]

What reading did you get on the thermistor ? you should get a resistance reading ( normaly between 1,000 & 30,000 Ω) not continuity , remember to switch through yourΩ values if your not using a auto ranging meter.

 

[Billy Bob Bob]

thermistors on a performa/105 must be 11k cold

 

[woodtop]

Sorry, it was an ohms reading but didn't record it, think it was 4.6K ish across the terminals of the thermistor, boiler wasn't cold because I'd been playing with it for quite some time

 

[Billy Bob Bob]

needs to be checked when cold the resistance decreases with temperature

 

[woodtop]

Thanks gas man, so if I'm getting 4.6 hot it'll probably go to about 11 cold indecating its sound and possably main board

 

[Billy Bob Bob]

Thanks gas man, so if I'm getting 4.6 hot it'll probably go to about 11 cold indecating its sound and possably main board

probably but this is not a common fault on this boiler i would not be looking at the pcb yet is it dumping down?ch pipe hot in dhw only?

 

[woodtop]

Not sure, that was a question the Potterton guy asked, but the heating pipes were hot due to checking modulating in heating mode. Bit confused on that one, if it was I'm thinking it might help the boiler to modulate because it does when ch's on and wouldn't get hot so quick because it's also going round the heating system slightly

 

[Billy Bob Bob]

try it when its cold if its dumping down to ch when in dhw mode your diverter or plate heats faulty,dont assume by cleaning out a plate heat its not blocked in the middle,the rubbish welds its self to the innards

 

[woodtop]

I did soak it in cleaner for about 2hrs to try to losen any deposits, and the system had cleaner in for a day before removing to clean. I'll spend some more time on it tomorrow and check the heating pipes from cold. Thanks again for your time and help