Replacing Honeywell Thermostat - Help!

[Space cat]

If your old thermostat is a mechanical one it probably has an accelerator heater. That's a fancy name for a tiny heater that connects from the output (load) terminal to neutral. Its purpose is to make the thing cycle on and off and thus provide a form of pulse width modulation to your temperature control.

To make the heater do its stuff, one of those three wires must be neutral so the last thing you what to do is connect all three to your new thermostat. You have a two-in-three chance of frying it!

Have a good look at the old stat. You should be able to see a moving contact on the end of a bi-metal strip and one or two fixed contacts. The heater, if you have one, will be somewhere near the bi-metal strip. It will have one end internally connected to a contact and the other going to its own terminal. If you can find the heater terminal, take note of which wire is connected there and DO NOT connect it anywhere on the new stat!

The remaining two terminals on your old stat will go to contacts which open when room temperature rises above set temperature. Check this with an ohm-meter. When you turn the stat down they should close, and vice-versa. One will also have the internal connection to the heater. This is the load side so put this wire into B on the new stat. The remaining wire goes to A. Terminal C is not used.

PS: If, by some chance, your old stat has all three terminals connected to contacts and there's no heater, post the information. I'll be watching.

 

[Space cat]

the last thing you what to do is connect all three to your new thermostat. You have a two-in-three chance of frying it!

Unfortunately, while I was writing that ---

To elaborate on the above I connected it up (before) as follows:

Red - Terminal A
Blue - Terminal B
Black - Terminal C

OOPS!!! It looks like you've fried it. Not only that, there's a good chance you took out the controller too!

 

[D_Hailsham]

I'm not sure which wires go where as the instructions with the new stat are pretty dire

You admit you don't know what you are doing, so you post a question which gets a reply explaining what to do. But you couldn't be bothered to wait for instructions, so you have a go and eventually post:

Then you write

I connected it up (before) as follows:

Red - Terminal A
Blue - Terminal B
Black - Terminal C

Which,surprise, surprise, is all wrong and blows your new stat and probably your programmer.

STOP WASTING OUR TIME