Potterton Suprima 50L heating not coming on

[wahsco]

Hi I have a Potterton Suprima 50L that is 8 years old. Recently the heating has been playing up. Hot water has been working fine. Heating only seems to come on at the same time as hot water request. Have recently changed out MEP 2 controller as display had almost completely vanished and thought this might have played a part in the fault. Also changed out room thermostat as thought this might have been an issue. Still no heating! Checked the swl at the thermostat. Working fine. Checked value of swl at boiler with only heating switched on (& thermostat turned up) 40v regardless of thermostat on/off - should this request value not be 240v when on and 0v when thermostat off? Does this mean that my new MEP 2 is also faulty?

Any help or info would be very much appreciated.
Cheers
Stuart.

 

[Carmine]

Check the motorised valve in airing cupboard to see if it's in the heating position.

 

[ianmcd]

As above more likely to be an external control problem , most likely a faulty mid position valve