Downstairs heating only working when upstairs is on

[rich19754]

I take it the (D/stairs) valve is opening but not firing the boiler?, if so then even though a new valve head I would confirm that its micro switch is closing by removing the valve head grey&orange wires and with the M.meter on resistance check for continuity between them, should read maybe 0.1/0.3 ohms or the same as when you touch the two probes together, this is with the valve energised and open.

Yeah that’s exactly it

 

[terryplumb]

WARNING.
Do not attempt to measure resistance on live conductors

I take it the (D/stairs) valve is opening but not firing the boiler?, if so then even though a new valve head I would confirm that its micro switch is closing by removing the valve head grey&orange wires and with the M.meter on resistance check for continuity between them, should read maybe 0.1/0.3 ohms or the same as when you touch the two probes together, this is with the valve energised and open.

 

[Johntheo5]

I did say to remove the valve head grey and orange wires before testing, maybe should have said "remove from the terminal block"

 

[terryplumb]

I did say to remove the valve head grey and orange wires before testing, maybe should have said "remove from the terminal block"

With the valve " energised" as per your instruction ,the orange wire could well carry 230/240 volts irrespective if it is in a terminal block.
John ,your instructions are not very clear and could possibly lead someone to try to measure resistance on conductors carrying mains voltage ,might be worth you amending the post ?