wiring for a room thermostat ???? Help and information on all topics relating to your central heating, air conditioning and ventilation issues.

[truepan]

I am trying to replace a manual thermostat control (acl TS142) with a digital model (Siemens Landis & Staefa RDE10.1)
The problem I have is that the original thermostat has 2 neutral wires (1 yellow on terminal 2 and 1 blue one on terminal 4) however the new thermostat has only one live and one neutral connection.
Is it ok to connect both the blue and the yellow to the neutral terminal of the new thermostat (terminal No. Q14)? Bit worried I may have an incompatible thermostat & I'll blow something - including myself!
Many thanks for any help &/or advice

Truepan

[plumbbob]

Is it ok to connect both the blue and the yellow to the neutral terminal of the new thermostat (terminal No. Q14)?

No, no, no! It'll go bang!

Old mechanical thermostat's needed a neutral connection to increase accuracy. New two wire digital stats do not require this neutral connection so leave the wire disconnected and tape it up for protection.

Only connect the Red to common and the yellow to one of the others. What options are there?

[truepan]

The old units terminals are labels 1 to 4. 1 being red, 2 yellow, 3 yellow/green,4 blue.
The new unit has 3 terminals Q11, Q14, Q12. I believe the live (red) connects to Q11, the neutral (yellow) connects to Q14, not sure what to do with the earth (yellow/green).
Thanks for the info about leaving the blue unconnected.
Cheers

[plumbbob]

The red connects to Q11. The yellow to Q12.

The thermostat backplate should have an earth screw. If it doesn't, just tape it up separate to the blue.

[truepan]

The wiring diagram I have implies the neutral should go to Q14 and nothing is connected to Q12 ??
why is nothing ever simple (rhetorical question)

Thanks for the help
Truepan

[plumbbob]

Old wiring.

Common 1 (Red) Q11
Demand 2 (Yellow) Q12 (or Q14 if you want to try it)
None 3 Not used
Neutral 4 (Blue) Not used

First of all, please forget the neutral blue wire on pin 4. It is not used, not required to make digital stats work.

You are only concerned with two wires. The red (live in) and yellow (switched live to the boiler). The yellow is NOT a neutral by the way!

Red goes to Q11 (Common)
According to the diagram, the N/C contact is Q12 which would normally be used.

Try the Q14 contact if you want. It is safe to do so. All that will happen if it is the wrong one is the stat will work in reverse. ie, as the temperature rises the heat will switch on and as the temperature falls it will turn off.