central heating

[wmvej01]

Cold radiators, The system is on an electonic timer for water & heating, it would appear that the radiators will only work when the system is heating the water, once the water is hot enough the radiators go cold ! pump seems to be ok as does the thermostat also no air in rads or pump, header tank also ok, not sure about the y valve, system has been ok for a couple of years and timer also seems ok, Help
Thanks

[ChrisR]

Either a faulty 3 port valve or it's wrongly wired, which is very common.

see http://content.honeywell.com/uk/homes/Catalogue/Sundial%20Plans/3.4%20Y%20Plan.pdf

[MANDATE]

I would agree with ChrisR.
Sounds as though the valve is either moving to the CH position but not firing boiler because there is no output voltage from the valves orange wire due to faulty microswitch within valve head.
Or the valve is not moving to the CH position to trigger the microswitch, this could be a fault in wiring where there is no power on the grey wire of the valve which is needed to drive the motor from mid position to CH position.
Or motor within valve faulty and unable to overcome the resistance of the fairly strong return spring.
You should be able to remove valve head off spindle and try the various positons and verify the various movements. Only expect valve to move after few seconds delay. Also! if at CH positon and you switch off both HW and CH dont expect valve to move at all. It will not move from CH until HW is called for. Of course if all power is removed from programmer, the grey wire becomes dead and in this condition valve will return to start positon ie HW.