In an earlier contribution to this discussion,
H Bergeron wrote:

Dear All,

Our central heating controller is on the blink.

How straightforward is it to replace this myself?

The programmer is a Honeywell unit, no model number shown, and I don't
know how old it is. The front section hinges up and separates from a
smaller piece screwed to the wall to allow access to batteries and a
reset button.

The wall section has terminals labelled T3, T4, T7, T8 which the front
snaps onto. The unit is roughly 8 by 13 cm.

My questions a

If I buy a new Honeywell unit like the one here
http://www.plumbworld.co.uk/760-2963 - which looks a similar size to
what I have but is clearly a different design - then will it:

(a) Simply snap on the the existing wallmounted section (this would be
great)

(b) Require the same four wires as I have in the wall now, but require
me to connect them anew to the new unit (this would be fine)

(c) Require different wires from what I have now (this would be a
nightmare!)

Any and all advice gratefully received!

It sounds as if your existing programmer is an electronic one, rather than
using a mechanical clock - in which case, a new one will probably fit
straight onto the existing backplate without disturbing the wiring.

Even if you have to rewire it, it won't require any more wires than the
additional programmer.

I am slightly surprised that your existing programmer appears to have
provision for only 4 wires. I would expect there to be more than this, even
if they're not all used.

What type of heating system do you have? Does it correspond to any of the
"Plans" defined in
http://content.honeywell.com/uk/homes/systems.htm ?
--
Cheers,
Set Square
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