Boiler fires up for water but not heating

[tofit]

Hi - I have a Worcester Bosch 24SBi Boiler set up as a Y plan system with the boiler used as the wiring centre and with a built in mid-position diverter valve and a built in digital programmer. The boiler heats water in a cylinder which has a thermostat, and the central heating which has a room thermostat. You can see on the digital programmer when either or both thermostats are calling for heat. The system has been working fine for about 5+ years.

Just recently the central heating stopped working, even when the thermostat called for heat. The water is still heated. I thought it was the mid-position diverter valve and put in a replacement one. Unfortunately this has not worked and there is still no central heating.

I have thoroughly bled and got all air out of the system, but still no joy. Any suggestions as to what to try next?

Thanks.

 

[stem]

Just recently the central heating stopped working, even when the thermostat called for heat.

I have some suggestions you could try, if you are able to use a multimeter safely.

1.The thermostat could be "calling for heat" but not actually providing the necessary control signal to the motorised valve to move it to the central heating position. This can be checked by testing the wiring at the motorised valve with a multimeter to see if there is a live present on the 'central heating required' wire (usually white in colour, but check with the valve instructions) If there is not a live present when the thermostat is 'calling' the thermostat may be faulty, if it has batteries, a fresh set may be required (I have known this happen even when the stat is not displaying a low battery symbol), or the programmer may not be providing the thermostat with a live signal to be switched. This too may be checked with a multimeter.

2. The neutral connection to the motorised valve has failed, or been lost. If so, the motorised valve will sit at rest in the hot water only position.

3. Does the heating come on just for a brief period whilst the hot water is calling for heat? This can easily be missed as it may not be 'on' long enough to heat up the rads fully. If this is the case, it suggests that maybe the 'hot water not required' signal from the programmer or motorised valve may be missing, this signal is responsible for winding the motorised valve fully across to the central heating only position when hot water is not required (Usually a grey wire at the motorised valve)

 

[tofit]

Thanks Stem - will do those tests and let you know how I get on