Hi, I have a honeywell T40 thermostat which i want to replace with a programmable one. However on the new stat i only have 3 terminals:

COM
NO
NC

On my old one I have 6 and 4 of them are in use:

1= Red
2= Blue
3= Yellow
4= Earth

Any idea which wire i put in what terminal on the new stat? Unfortunately the old stat i numbered not letters?

Any help greatly appreciated.
Thanks

"DavidB" wrote in message
news

Hi, I have a honeywell T40 thermostat which i want to replace with a
programmable one. However on the new stat i only have 3 terminals:

COM
NO
NC

On my old one I have 6 and 4 of them are in use:

1= Red
2= Blue
3= Yellow
4= Earth

Any idea which wire i put in what terminal on the new stat?
Unfortunately the old stat i numbered not letters?

Any help greatly appreciated.
Thanks


--
DavidB

You have the same with the new one - but you are not counting the earth -
perhaps it is double insulated and doesn't need it - or is it a separate
tag?

If you examine the old one there may be an indication (maybe a diagram) of
the contact arrangement. Alternatively test it with a circuit tester. NO and
NC should never connect. COM to NO should make a circuit what the temp is
turned up to above ambient. You may find it worth looking at the boiler end
to deduce what is what. There may even be a diagram. Convention would be to
connect the red to the COM.

In article ,
DavidB wrote:
Hi, I have a honeywell T40 thermostat which i want to replace with a
programmable one. However on the new stat i only have 3 terminals:

COM
NO
NC

On my old one I have 6 and 4 of them are in use:

1= Red
2= Blue
3= Yellow
4= Earth

Any idea which wire i put in what terminal on the new stat?
Unfortunately the old stat i numbered not letters?

The old thermostat probably had an accelerator coil which needed a neutral.

The new one doesn't and is battery operated. It probably doesn't need an
earth either.

You'll also not need the NC connection since it didn't appear to have been
used on the old one.

So I'd *guess* at red to COM and yellow to NO, but there's no guarantee
who ever wired the original used the same colours as I would.

FWIW, with the old, I'd have used red as live feed, yellow as switch
return, blue as neutral and green as earth (if in a 6 core with those
colours available).

--
*Yes, I am an agent of Satan, but my duties are largely ceremonial

Dave Plowman London SW
To e-mail, change noise into sound.