Replacing an old mechanical thermostat with a wireless one

3 Jan 2023
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United Kingdom
Hello! I have a problem I was hoping someone could help me with that.
I would like to replace this old mechanical Honeywell thermostat with a wireless Honeywell thermostat (T3R). I tried to install the base of the wireless thermostat in place of the old one.
The old thermostat has 3 wires: red, blue, and yellow. Based on the diagram on the back of the lid (shown in the picture) and the fact that the thermostat works, the wires should be: ( the ports are not in numerical order)
Blue: L
Red: N
Yellow: Switched Live

So I connected them to the wireless base like this: (based on the
Blue: N
Red: left L
Yello: B
Also connected the right L and A.

But it didn't work, the base didn't even go on. Does anybody know what might be the problem?
Thank you in advance!


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I'm not experienced with this type of mechanical stat, but to me something appears odd....
The circuit diagram, shows your (yellow) switched live, connected to terminal 3 as the 'normally closed' contact of the stat; I would usually expect it to be connected to terminal 4, the normally open contact.
What is the model of your old thermostat and your boiler?
...and do you have access to a multimeter?

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