Heating starts only after hot water is on

[kevin12345]

Hi,
A new problem that has just started with my heating and hot water.
I have a conventional glowworm boiler ultimate 40FF and a 3 port heating hot water valve.
Until yesterday everything worked fine but now my heating switch will not fire up the boiler (the 3 port danfoss valve stays in the hot water position.)

When I switch hot water on it starts the boiler up OK.
When I switch Heating on with hot water on after about 10 minutes the valve moves and the heating starts to work.

If I then turn hot water off the heating will continue to switch on and off OK, but if I switch the hot water on by itself again so the valve moves back to just hot water and then turn hot water off then I can not start Heating on it's own again.

I thought it might be the 3 port Actuator head but I have just changed this for a new one and I still have the same problem, the valve on the 3 port valve was turning reasonably freely behind the actuator head.

Anybody got any ideas please?
Thanks,
Kevin

 

[ALCPlumbing]

If you've replaced the MV head and it works intermittently I would be looking at the programmer/timer or the heating thermostat. It sounds like one of the relays is on the way out. Is your thermostat wireless? If not it's probably the programmer. If it is it could be either. You really need someone to come and have a look unless you have a multimeter and know how to use it. When you turn the heating on and it isn't working this is when you should check for where the break in the circuit is. If there isnt 230 volts feeding the thermostat when heating is on at the timer then you know it is the programmer. If there is and the thermostat is calling for heat but there isn't 230v on the white to the 3-port then you know it is the thermostat. If u just have the heating on without the hot water there should also be 230v on the grey to the 3-port. This will be from the HW off terminal of the programmer.

 

[kevin12345]

Thank you for advice.
I have a wireless thermostat that I put in about 18months ago. I have just looked at the battery on it and although the light is not flashing it is showing battery practically empty. I will go get new batteries for it and see if that helps. Do you think that could be possibility?
I have heating on now with no hot water on but I think as soon as I have just hot water I will be back in the same boat that it will not be able to turn heating on without having water on.