Promax FSB HE - central heating not working but hot water still fine.

[terryplumb]

Only testing voltages would ascertain what's at fault ,but I would also suspect the actuators at fault,motor probably.

 

[EverythingsBroken]

Sounds like the valve actuator. Putting in manual means just that, it doesn’t prove any electrical microswitch failure.

But if the valve actuator had failed, would that prevent the boiler from firing up?

If I put the HW valve into manual, that doesn't cause the boiler to fire up - I still have to turn on the HW at the controls to get the boiler to fire up.
If I put the CH valve into manual, the CH controls do nothing, I only get CH if I turn the HW on (which causes the boiler to fire up).

So with both valves in manual mode, the CH and HW don't behave the same, so I assumed this meant that replacing the actuator would make no difference. Unless it's set up so that the actuator is sending a signal to the boiler to turn on when it activates which doesn't happen if it's broken? (I don't know what tells the boiler to turn on).

 

[CBW]

Unless it's set up so that the actuator is sending a signal to the boiler to turn on when it activates which doesn't happen if it's broken? (I don't know what tells the boiler to turn on).

Bingo! The microswitch inside the actuator is what sends the signal to the boiler to fire up, the demand has to come from other controls, so programmer requests heating/hot water or both, these send feeds to the thermostats which then if needed (as in not satisfied) send a switch live to the motorised valve which is driven open and only when the microswitch is made is the boiler then fired up (switch live to the boiler). Putting them into manual as I said does just that, but when the heating one is open manually it receives the heated water (because there is nothing stopping it) from the hot water demand until the hot water is satisfied