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!

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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On Sat, 16 Oct 2004 15:39:28 +0100, "Set Square"
wrote:

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.

Yes, it is electronic, not mechanical. It incorporates a thermostat
and I have since realised that the product shown here
http://www.plumbworld.co.uk/761-2966
is more appropriate than the one I mentioned before.

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

Thanks, that is good news.

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.

There are two more terminals on the front section (labelled T5 and T6)
but they are not fitted with clips for connecting to the back plate,
and the back plate itself has no sign of more than the four
connectors.

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 ?

I should also have mentioned it is a combi boiler, so there is no need
to programme hot water.

Many thanks for the response!

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


Yes, it is electronic, not mechanical. It incorporates a thermostat
and I have since realised that the product shown here
http://www.plumbworld.co.uk/761-2966
is more appropriate than the one I mentioned before.

In that case, you *will* need to rewire it - but it's quite simple. The CM61
uses only 2 wires to switch the heating on - because the timing and
switching functions are entirely battery operated - so there's no need for a
neutral to drive a clock.

If you are likely to want different programmes for different days of the
week, you might consider using a CM67 instead. It looks the same, but
provides more programming flexibility.


I should also have mentioned it is a combi boiler, so there is no need
to programme hot water.

Fine. The CM61/7 is ideal for this - but wouldn't be any use if you had
needed to programme CH and HW independently.
--
Cheers,
Set Square
______
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On Sat, 16 Oct 2004 17:52:27 +0100, "Set Square"
wrote:

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


Yes, it is electronic, not mechanical. It incorporates a thermostat
and I have since realised that the product shown here
http://www.plumbworld.co.uk/761-2966
is more appropriate than the one I mentioned before.

In that case, you *will* need to rewire it - but it's quite simple. The CM61
uses only 2 wires to switch the heating on - because the timing and
switching functions are entirely battery operated - so there's no need for a
neutral to drive a clock.

Right, and now I've taken a peep behand the back plate I see there are
only 2 wires connected. In fact it looks just like a mains cable with
the live and neutral used but not the earth. I found installation
intructions at the Honeywell site that indicate that the live connects
to terminal A and neutral to B. This is consistent with the labels
used on the existing unit.

Many thanks. I now feel confident to fit one of these.

If you are likely to want different programmes for different days of the
week, you might consider using a CM67 instead. It looks the same, but
provides more programming flexibility.

Yes, I saw that. We don't need the flexibility now, but might when my
missus goes back to work.


I should also have mentioned it is a combi boiler, so there is no need
to programme hot water.

Fine. The CM61/7 is ideal for this - but wouldn't be any use if you had
needed to programme CH and HW independently.

Thank you for your help. I mentioned www.plumbworld.co.uk but I have
not used them before. Do you know anything about them? Can you
recommend an alternative supplier?

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


Right, and now I've taken a peep behand the back plate I see there are
only 2 wires connected. In fact it looks just like a mains cable with
the live and neutral used but not the earth. I found installation
intructions at the Honeywell site that indicate that the live connects
to terminal A and neutral to B. This is consistent with the labels
used on the existing unit.

Actually, they're not live and neutral - they're both live when the switch
in the stat connects them together - but red and black twin cable is often
used because it's plentiful.


If you are likely to want different programmes for different days of
the week, you might consider using a CM67 instead. It looks the
same, but provides more programming flexibility.

Yes, I saw that. We don't need the flexibility now, but might when my
missus goes back to work.

Then I'd go for a CM67. It's very easy to programme one day and copy it into
the other 6 days. If you find later that you want different programmes on
different days of the week, you can do it. [It actually gives you 7
independent programmes - rather than weekday/weekend settings like some
programmers. For each day you can specify 6 temperature change events
(including off, in effect, if you specify a low temperature)] If you go for
the version which includes optimisation, it will decide when to turn on the
heating in order to achieve the desired temperature by the specified time.


Thank you for your help. I mentioned www.plumbworld.co.uk but I have
not used them before. Do you know anything about them? Can you
recommend an alternative supplier?

I got mine from RKM Heating Controls
http://www.rkm-heating-controls.co.uk/shop/index.html
who were the cheapest at the time, and provided very good service.

--
Cheers,
Set Square
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