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

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!

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.

RandomGrinch said:

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.

Click to expand...

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

If it’s the motorised valve (sounds likely), the head can usually be removed, turn the spindle to have heat all over using the one thermostat for control until you can get a plumber in.

I have started to experience this same issue at a new build with the same Danfoss Thermostat and Logic Combi boiler showing 00. The only way to get the heating to work is by turning the thermostat to 23-25 C on the ground floor and same temperature for 1st floor to get the heating working on the 1st floor. Before I could independently turn the heating on and off for each floor via the Danfoss thermostat. I have changed batteries, switched the thermostat around from both floors, check the actuator and both are working. Any help would be great before I contact my plumber.

Should really start a new thread, but it is likely that you have motorised valves which have a micro switch in them, and the micro switch has failed.

Normally it is the reverse, where the micro switch gets stuck in, and the boiler will not turn off.

But it is not the boiler or thermostat with matters, it is the system being used. In the main, boilers are modulating type, that is they can turn up or down, rather than on/off. So the control also needs to be the same, so the valves used can vary the flow, one on each radiator called a TRV (thermostatic radiator valve) these can have a mechanical head, or an electronic head, the latter can also link to a hub to tell the boiler when to fire.

However, for I assume to reduce installation cost, often we find on/off zone valves, completely unsuitable for a modulation system, unless needed to turn off large sections of the heating, as when we have a granny flat, which may not be used for weeks on end, but it seems they comply with government requirements, and are cheaper to fit than proper linked TRV heads.

Z Tariq said:

I have started to experience this same issue at a new build with the same Danfoss Thermostat and Logic Combi boiler showing 00. The only way to get the heating to work is by turning the thermostat to 23-25 C on the ground floor and same temperature for 1st floor to get the heating working on the 1st floor. Before I could independently turn the heating on and off for each floor via the Danfoss thermostat. I have changed batteries, switched the thermostat around from both floors, check the actuator and both are working. Any help would be great before I contact my plumber.

Click to expand...

Is it not that they’re already at temperature and you just have to set desired temperature higher?