Potterton Performa DHW/CH Problem

[madrid2000]

Hi All,

My 5 year old performa has just developed a problem with both the DHW and CH. When there is a demand for either HW or CH the boiler fires in cycles (i.e. fires then runs cool). When the boiler does fire initially the flame doesn't modulate at all hence the thermistor I assume keeps telling the flame to shut down.

I've disconnected the thermistor and run the DHW, and not unexpectedly, the water has got excessively hot on the prim. flow.

Having followed a couple of similar threads I just wanted some guidance on whether the thermistor is the fault (does it directly modulate the flame?) or should I be looking at scaling of the primary HEX?

Thanks in advance,

D

 

[Agile]

My initial expectation is that it is a combustion related problem which would normally require a gas trained engineer to deal with it so consequently I am unable to give detailed advice on an open forum.

However, are you suggesting that by disconnecting the DHW thermistor the flame stays on?

Tony

 

[madrid2000]

Hi Tony,

I'm very recently qualified, but will be working with my boss to resolve this.

I've recently chemically cleaned the system, and replaced the diverter valve on the boiler. This issue has occured within of a week of this being done.

Yes, when I detach the thermistor the flame stays on-which points to there not being a problem with the flame sensing device.

regards

 

[ChrisR]

I've recently chemically cleaned the system, and replaced the diverter valve on the boiler

I do think I'd have tried a new thermistor by now!
You can try swapping them to see if anything changes.

If you didn't "chemically clean the system" including all the boiler's bits. you could have filled it with debris from the system. For example if the round depression in the middle of the pump is blocked with flakes , the circulation would be poor and the boiler would get hot and shut down v quickly.
Diagnoisis is trivial - measure what temp the outlet from the main hex gets. You could also measure the resistance of the thermistor, which should be about 10k Ohms cold dropping as it heats to a couple of k Ohms hot.
To be honest, if you can't work this sort of test out for yourselves, you aren't best suited to mending boilers!

 

[madrid2000]

Chris,

Thanks for the advice and self-righteous comments.

Regards,

 

[Agile]

Chris,

Thanks for the advice and self-righteous comments.

Regards,

I dont think saying things like that are going to enamour you to the experienced people who could be helpful to you!

If you are gas qualified then you should have stated that at the outset. You should post trade questions in the Combustion Chamber part of this aite where we can freely give advice on gas related matters.

Tony

 

[shawd]

Hi, to tell if the boiler can modulate at all go to the gas valve and chek with manometer that the burner pressure is at 12 mbar when hot tap running then disconnect a modulating wire it sits next to another wire at front of valve and then read what you have got ,it should be about 2 mbar , but do not do this if you are not competent to do so.
This test would rule out gas valve fault

 

[madrid2000]

Great thanks I'll try that to establish whether the gas valve is the problem.

Sorry Tony and colleague, I didn't mean to appear rude. I appreciate all the voluntary advice provided free on this side.

Regards