Electricians: help with wiring a new thermostat?

Factual Questions
1

Hi,

I’ve got an old manual room thermostat that I want to replace with a digital programmable one.

I have taken photographs of the wiring diagrams, but can’t really figure them out - the new one looks pretty straightforward, but the old one doesn’t.

Old diagram.

New diagram.

There are three wires and a disconnected earth coming out of the wall. In the old thermostat, the wires went thus:

  1. Red
  3. Yellow
  4. Blue

I hooked up the new thermostat as follows:

A. Red
B. Blue
C. Yellow

…and it blew a 3 amp fuse down at the boiler master circuit.

Note that I am in the UK, so we are dealing with a 240V system, and there is no a/c - it just controls the boiler.

Any idea what might be going wrong? Any ideas what these wires might actually be? (I’m guessing Red is live…) They seem to be coming out of the programmable timer unit that turns the boiler on and off. Another point that might be pertinent: I can have the boiler heating the water but not the central heating.

2

Unfortunately nothing factual to offer, but I did want to pop in and wish you luck!

At least here in the US, thermostats are notorious for having non-standard wiring and for being hooked up in varying ways depending on the contractor. Your best bet is to get a really good tone probe and trace the wires at each end of the connection, figure out what each is actually doing (rather than what it’s supposed to be doing) and hook it up that way. I’ve changed out three thermostats over the years and two of those times, I ended up having to do that.

3

Try using A & B and simply disconnecting the C (Y) terminal and see if all is well

4

Refitted the old thermostat and ran a voltage meter against it:
ON OFF

  1. Live Live
  2. 0V 240V
  3. 240V 240V
5

OK, now I’ve taken the old one off and tested thus:



   OLD            NEW

   ON   | OFF  |    ON   | OFF
1. Live | Live | A. Live | Live
3. 0Ω   | 0Ω   | B. 0Ω   | ∞
4. 100Ω | ∞    | C. ∞    | 0Ω
6

I do HVAC work, but in the US, and so unfortunately these are foreign to me. That said, on the old stat the Y circuit is a parallel circuit with R and has a load in series.

When you wired it up, it appears that you created a direct short R to Y. (iow, no load in between)

That parallel circuit may be a heating anticipator. At any rate, it appears that A & B will close a circuit (with the required load in between).

My swag? Try it with A & B and see what happens. (Don’t replace the fuse with anything but a 3 amp fuse)

7

OK, sorted it.

  1. got a pack of 3V fuses as recommended
  2. connected Red and Blue to A and B, taped up Yellow
  3. blew the fuse
  4. replaced the fuse
  5. connected Red and Yellow to A and B, taped up Blue
  6. success!

Thanks raindog! The house is starting to warm up again…

8

Well, I don’t know I helped you any, but I’m glad you got it whipped!