Honeywell ST9420C Issues

[Rattlymatt]

I have recently had issues with my central heating and water system. When the hot water is on, the radiators are hot. I thought this was the Honeywell Valve head, so I replaced it. Problem still there.

I then spotted that the ST9420C programmer wouldn't fire the heating (the water was off) when trying +1 hour or when set to continuous (it doesn't work on auto or once either). The hot water does fire. I then remembered that the +1 stopped working earlier in the winter, but forgot because I relied on the auto program to heat the radiators.

Could this be a duff programmer?

 

[Rattlymatt]

Sorry I should have put this in the central heating forum

 

[SteveA1962]

Hi, I am a carpenter rather than a plumber, but would say I have decent other diy skills. I have a similar set up - same programmer but a drayton diverter.

I've read somewhere that the main capacitor can fail on particular batch numbers of the st9400 series. Some users seem to make a temporary difference by turning off the back light option to reduce consumption. This problem seems to be more prevalent when trying to turn on central heating and water at the same time. Permanent fix is swapping the main capacitor on the pcb (if you are good with a soldering iron) or replacing the unit.

Quite easy to replace as they are on a standard backplate.

However, if the radiators are hot when only the water is on then I would then still suspect the diverter valve. This could either be the motorised head or the actual mechanical valve. If you have replaced the head then likely to be the head. I have freed up before by greasing the spindle and turning with piers.

Hope this is some help.