Main heating not working, but everything else does.

[baggielad]

Issue with main radiator heating. The system has two 2 motorised valves one for radiators and the other the underfloor heating. Underfloor and hot water work fine. Initially with the rads when calling for heat the pump would run but the boiler did not fire up. Checked the motorised valve and it was closed so manually opened it and boiler fired up and radiators warmed up, so assumed it was a burnt out motor.

However, this morning when calling for heat nothing happened so checked the wiring. There is no current going to the motorised valve, so could this suggest the receiver is faulty as it's not giving the signal to the boiler or the motorised valve to turn on? Could I test this by bypassing the receiver, but not sure on the wiring. There are neutral and perm live then a grey cable and a black cable. The black cable is wired to the motorised valve and there is no voltage across it, but there is 240v on the grey cable.

The thermostat/transmitter is a wireless Drayton Digistat and the display suggests that's working and the lights on the receiver also suggest it's working too. However, if you press the override button on the receiver the green light comes on, but nothing happens.

Any advice gratefully received.

 

[stem]

The motorised valve should be wired to the thermostat receiver, so that when the thermostat switches the heating 'on', the valve opens. When the valve is fully open it operates an internal switch that operates the boiler.

Usually, the Drayton Digistat works in conjunction with a SCR receiver. If so, the wiring connections at the receiver are:

If yours does have an SCR and you are electrically competent to do this safely, moving the wire from terminal 3 (call for heat) and putting it in terminal 1 (common) along with the wire already in there will by-pass the receiver and put the heating permanently on. If it does, it's the SCR that is faulty. If it doesn't come on, it's the motorised valve or wiring to it.

It's generally best to do the test in isolation (IE with the other zone and hot water off) as they will still operate the boiler if their motorised valves are open.

 

[baggielad]

The motorised valve should be wired to the thermostat receiver, so that when the thermostat switches the heating 'on', the valve opens. When the valve is fully open it operates an internal switch that operates the boiler.

Usually, the Drayton Digistat works in conjunction with a SCR receiver. If so, the wiring connections at the receiver are:

View attachment 289911

If yours does have an SCR and you are electrically competent to do this safely, moving the wire from terminal 3 (call for heat) and putting it in terminal 1 (common) along with the wire already in there will by-pass the receiver and put the heating permanently on. If it does, it's the SCR that is faulty. If it doesn't come on, it's the motorised valve or wiring to it.

It's generally best to do the test in isolation (IE with the other zone and hot water off) as they will still operate the boiler if their motorised valves are open.

Brilliant, thanks as did what you suggested and boiler fired up, motorised valve opened and radiators now warming up! The boiler and valves are only 18 months old so pleased that they are working. Not sure of the age of the digistat as was installed before we bought the house. I have a Nest Thermostat which came free with the new boiler so I can just replace the digistat with that. As a temporary measure can I just wire a light swtich to replace the digistat receiver, or is that risky?

 

[stem]

Either are possible. You can replace it with the Nest. There are two types of Nest, 3rd gen and Nest-e and they are wired differently. The light switch is also a viable alternative but of course you will loose thermostatic control. If you need any help with installing either just post back.

 

[baggielad]

I'll do the switch first as too much going on at the moment. It's a Nest Thermostat E which I have a look at after Christmas. I assume they are relatively straight forward to instal? Many thanks for your help.

 

[stem]

Nest-e very easy to install, just two connections. Same as a light switch.

The wire now in SCR Common = Nest-e 'C'
Wire now in SCR 3 Call for Heat = Nest-e 'NO'

Nest-e is battery operated so doesn't need N & L. But if you have more than one wire in either of the SCR N & L terminals, then they would need to remain connected together as they are already.

 

[baggielad]

Thanks for the advice. It does seem straightforward so hopefully I can do it without needing further help!