Potterton performa 28 doesn't regulate CHW temp

[johnjames86]

My potterton performa 28 doesn't regulate the CHW temperature very well. I have replaced the thermistor (a regular fault) with new part no. 5108265 and the behaviour has changed from cycling low to >80degC and (I think) dropping out on the safety thermostat as there is a loud simmering just before turn off.

Now, at low temp settings the gas comes on at low and after 10 secs or so modulates up over the next 5 or 6 secs and then reaches a simmering sound then turns off. This repeats all the time.

At much higher temps the result is much the same except that the gas modulates back down instead of turning off and will stay low for quite some time before turning off. Any ideas?
_________________
JohnJames86

[HSP]

pump?
test ohms on L and N when not in motion
should be 150-300 k ohms
_________________
Dont cha wish your bouyfriend was a plumber like me !!!

[johnjames86]

Hi HSP

Thanks for reply. Measured pump resistance. As suggested it lies around 180-190 accord to my Fluke DMM.

Whilst I was inside the case I checked the return filter and the pump but they're both very clean. Someone had suggested that the water flow rate could have been low because of debris ... at least that's one option dealt with. Since I replaced the CHW thermistor it is better behaved but the temperature still swings a lot when the CHW temp dial is set above about two thirds (80 top temp down to 30/40 at re-ignition). Makes the pipework tick quite loudly. If I choose a dial setting to give a max of 60/70 it stops the ticking but I don't think it will cope when it gets much colder.

I still think the gas rate for CHW is to high!
_________________
JohnJames86

[Agile]

What do you mean by CHW ?

If you mean DHW then the fault will be a blocked plate HE and it also sounds as if you have some lime scaling in the main HE as well.

If this is the case then replacing or cleaning the plate HE and chemical cleaning or replacement of the primary HE will be required.

Tony

[johnjames86]

Hi Agile,

CHW = Central heating water.

As I said, I've checked the pump and the return filter and the are very clean, but the boiler temp swings from low (40-50C) and turns off at around 80C when the boiler simmers? and stays off for a good few minutes (maybe 10-15 mins) when the temp drops back to 40-50C.

When showering the boiler maintains a good temperature and we have an Aqualisa valve with a very good flow - no problems at all. Guess the pump must be coping well.
_________________
JohnJames86

[Agile]

WEhen you replaced the sensor what did you use apart from the sensor?

What you call simering is what we call kettling and is caused by lime compounds in the primary HE.

Tony

[johnjames86]

Hi Agile

Not quite sure what you meant

[WEhen you replaced the sensor what did you use apart from the sensor? ]

I bought a new thermistor, removed the old one from its pocket and replaced it. As I said, the boiler seems a little better behaved, The temperature swings are less but the "kettling sound is there just before the gas goes out. It doesn't modulate down, just goes out.[/quote]
_________________
JohnJames86

[Agile]

So you did not use any thermal conductivity paste?

A CORGI would have checked the minimum and maximum gas settings of the gas valve as well.

Tony