Upstairs thermostat not triggering boiler despite showing as 'on'...

[akbmcf1]

Quick overview of system: New build house (2018) with dual-zone central heating (downstairs and upstairs) with a Danfoss TP4000/5000 series programmable thermostat in each. Boiler is an Ideal Logic Combi ESP1 35, serviced annually.

Currently having fun and games with the upstairs thermostat - it's showing as 'on' (target temp is higher than current room temp) but it's not triggering the boiler to fire - boiler is showing '00' on the display which I understand to indicate its standby mode.

Thermostat itself has been a bit awkward of late ever since it had a change of batteries (to brand new alkaline AAs, fwiw...) e.g. button presses don't always immediately register on the display.

Any ideas on what to check first - does the DIYnot hive mind think the thermostat's on the blink, and is there a way I can test it (for instance, is it possible to remove the thermostat from the equation and manually trigger the boiler to fire)?

Any suggestions/guidance is appreciated before I incur a call-out fee from a plumber (if there's even a plumber available at this time of year!).

Thanks in anticipation!

 

[RandomGrinch]

Behind the Danfoss, there should.be a couple.of wires, something like this...

With the power safely isolated (these are likely at mains voltage), the two wires can be placed together into the COM terminal.
If when the power is restored, the heating comes on, the stat is at fault.
If the heating doesn't come back, a zone valve may be at fault.

If the wiring doesn't appear as above, post a pic back here.

 

[akbmcf1]

Behind the Danfoss, there should.be a couple.of wires, something like this...

View attachment 363294
With the power safely isolated (these are likely at mains voltage), the two wires can be placed together into the COM terminal.
If when the power is restored, the heating comes on, the stat is at fault.
If the heating doesn't come back, a zone valve may be at fault.

If the wiring doesn't appear as above, post a pic back here.

Thanks, I'll take a look at the next available opportunity and report back. Appreciate your quick response!!