Terminating unused blue wire at the thermostat

[Jodes]

Apologies if I'm posting in the wrong section!

I hope to replace an old broken analog central heating thermostat with a digital one.

Background: There are 4 wires coming into the box: yellow/green, blue, red and yellow. The yellow/green is tucked away, physically floating about. The other three are wired into the thermostat. The problem is, the new digital thermostat only has two terminals, which AFAIK should take the yellow and red wires. This leaves the blue...

How should I terminate the blue wire? I don't know if it's been disconnected on the other side. AFAIK, electrical insulating tape is bad, so how should I do it?

Thanks

 

[Octopus]

How do you know the blue one isn't wanted?

 

[Lucien Nunes]

If the spare core is the neutral to the accelerator resistor, then it won't be needed and can be parked in a Wago or similar. If it's the N/C from the stat to a motor-closed valve then it will still be needed. You need to confirm this first, but in either case don't make any assumptions based on colour!

 

[Jodes]

Thanks for both your input. I was lead to believe that the blue wire was only needed for older systems, but I'm just going to get a sparky in to do it now that I know I'm in over my head. Basically I don't know the blue isn't wanted. (Although now I know a Wago would be the tool for the job).

Just curious though...
1. If the blue is N/C, then by definition doesn't that mean it's not connected so not used?!
2. What is the motor closed valve?
3. And the accelerator resistor? Would that heat up the old thermostat? The new thermostat has features like prediction for steadier temperature regulation, based on previous cycles, which I thought would mean it doesn't have any accelerator resistor?

 

[Lucien Nunes]

1: Sorry, ambiguous terminology! In context of switching, N/C = normally closed, opposite of N/O = normally open. Can also mean no connection, as you say.

2: Some types of motorised valve require a feed to close them, as well as to open, in which case each stat must return both N/O and N/C.

3: Yes, it would heat the bimetal thermostat at something less than the average rate of rise of room temp to reduce the lag in its response.

 

[driverman]

What ever you do ...don't cut it off, even if you don't need it. Like above make it safe using wago (similar) and tuck it away.