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If the radiator in the cold room gets hot (all of it from top to bottom) reasonably quickly and the room is still not warming up as you'd like you need a 'bigger' radiator.

You do need to understand what a room thermostat in it's simplest form actually is, ie it operates as a switch and in order to fire up your boiler it would have to switch together terminals 3 & 4 in your photos. As you don't have a room thermostat at the moment these two terminals are linked...

Bear in mind there may come a time when you'll need reasonably easy access to the receiver if the system starts playing up. Either just to override the thermostat and get the heating going or to check for faults.

I'd say that receiver unit has to come out. If you don't have the original blanking plate it may be possible to remove the pins (3 pins in 1 that stick out at the back of the receiver to make the connection), then push the receiver back into place on the boiler.

Pretty sure the 24V link also has be disconnected for 230V thermostat operation and connected for ebus controllers.

It might just be plugged into the front of the boiler, under the pull-down panel.

Looks like with newer boilers (after 2012) the receiver unit has been incorporated into the main pcb. Maybe someone here can shed some light on how to check if it's been decommissioned.

Was the wireless receiver unit removed from the boiler. TBH I don't know enough about these systems to be sure but I'd be wary of leaving in a unit which could still be sending ebus control signals to the boiler.

is the VRT350f still connected to the boiler?

Then your boiler is probably working normally as stem described above. Some modern boilers can modulate their output power down to as low as 5kW and in most systems they would happily keep burning as long as the thermostat was calling for heat.

Agreed, you didn’t quote my second sentence where I said as much. I’d be interested to know which boiler this is and what it can modulate down to.

If the thermostat is calling for heat the boiler should ideally be firing constantly until the house is up to temperature. Depending on the make and model of the boiler, number of radiators, whether the system is a bit clogged or the pump is not working too well, this may not be possible.

The boiler will certainly cope unless your house is huge with a large number of radiators. Is the pipework into the upstairs radiators 15mm and do the existing radiators get nice and hot reasonably quickly when the heating is running?

You don't say what the heating flow temperature is set to on the boiler. Is there some wiggle room for increasing this so the system can sink a bit more heat and get the house up to temp a bit faster with fewer burn cycles?

The indicator lamp on the room thermostat? That's a tricky one I'm afraid. The lamp could just be very sensitive to some slight current leakage in the 3 port valve although I wouldn't have expected it, at least not in connection with the DHW OFF signal.

You are right about Yellow being Live to the Thermostat. Overall the colours of wiring in use are confusing and the wires could do with some coloured insulating tapes to identify each end. You wouldn't normally expect blue and yellow/green wires to be at 230V unless clearly identified as...

Can you post photos of the wiring centre?

ok it’s just possible the new thermostat is faulty but try moving yellow back to terminal 3 and give it another go. Do you have a multimeter?

Looks good. With the power off... move the yellow wire in 3 over to join the red one in Honeywell terminal 1. Power up and check that the heating comes on permanently.

Can you post a picture of the new thermostat wiring just to make sure? Plus confirm that no other connections in the system have been changed.