Potterton PTT2 HWC Thermostat... faulty?

[Scarum]

This refers to vented Y-plan system. Potterton Suprima 30 is the boiler: boiler stat is set to 1/3rd on dial. I am getting nice hot water out of my taps.

The HWC thermostat is a Potterton PTT2 and is set at 60 degrees and when heating has been on for a short period I can hear kettling (boiling?) only in the pipework in and around the HWC, pump and 3-port valve. No such noises when valve set to rads only (at first I wondered if it was trapped air). The set temperature on the HWC stat is never met no matter how long I run the boiler, unless I drop the temp down to below 50 degrees.

I have run this test. Boiler off for 3 days and HWC let to go cold. Then boiler on and run to heat HWC. No noises at all for 30 minutes until the HWC heats up.

Replace the stat?

 

[45yearsagasman]

Your boiler stat needs to be set higher. If the Cyl stat is set to (say) 60c then it follows that for the set point to be achieved the boiler output must exceed 60c. Try that first before changing parts. You need to be taking accurate temp readings of boiler flow temp and cyl water temp.