Water only gets hot when central heating is also switched on

[jazz-uk]

Last year I had the diverter valve changed. Everything seemed OK until a couple of weeks ago.
I have a traditional water and central heating system run by a 20 y/o Chaffoteaux & Maury boiler.
Problem is if water only is switched on, the pump starts but the boiler doesn't heat the water. You can hear the water pump through the boiler but the boiler doesn't flame up.
If heating only is on, house heats up nicely. If both are on, then I get both hot water and heating.
PS - Hope to change the whole system next summer, but need this sorting out soonish - thanks

 

[oilman]

What make and model diverter valve?

 

[jazz-uk]

Hi,

I couldn't see a make or model on it, but its a blue unit (and brass fitting) fitted by British Gas as part of their service contract.

It has the words "Mid position activator/actuator code" on a white plate on the housing. Its fitted upside down and had a black wire on what is the bottom of the unit with a little black clip on the side and a big clip on the top.

Sorry can't provide more information.

Jaz