Thermostat Wiring

[Pagey]

I have a very simple question that I'm sure is obvious to most. I am replacing both my room & cylinder thermostats and don't quite understanding the wiring diagram on how to connect them up.

i.e. the cylinder stat has a brown, blue, and white wire though the wiring diagram only shows a picture with connections 1,2 & 3 (with what I presume is a switch symbol between connectors 1 & 3)

How do I know which wire refers to which connection? Also, is there a basic rule to follow with wiring diagrams?

Any help would be greatly appreciated. Thanks[/img]

 

[oilman]

How do I know which wire refers to which connection?

I don't know, but I do know we don't know because you haven't told us what make and model thermostat it is. You could probably find the information from the manufacturers website.

 

[croydoncorgi]

Cutting to the chase.....

If you've got three wires on numbered terminals on a cylinder stat, they are probably Common (ie. feed from programmer hot water on connection) 'HW on', and 'HW off' . A Honeywell stat would be marked C, 1, 2 but yours obviously differs! If you have a 3-port motorised valve ('Honeywell Y Plan') on your system, all 3 wires need to be connected to make the valve work properly.

A room stat MAY have 2 connections plus Earth (common, heating on) but many stats also have a Neutral connection needed to make the stat itself work properly. Be EXTREMELY careful if you're replacing stats. Some boilers REQUIRE 2-wire 'dry contact' thermostats because they have low-voltage controls, and will be toasted if you connect them to a circuit including a mains-operated stat. Alternatively if you (incorrectly) wire a mains-type 4-wire stat into a low-voltage system just using 2 wires then you will probably get wild swings of temperature in the room.

 

[Pagey]

many thanks croydoncorgi. I've wired up the 2 stats in the honeywell Y Plan and everything is working fine again.

 

[20pictures]

Just a word of warning from a non-plumber but someone who has made mistakes with cross-wiring stats....

MAKE SURE YOUR JUNCTION BOX IS PROTECTED BY A FUSED CONNECTION UNIT.

Easier to replace the fuse than all the burnt out components!