Central Heating Programmer

[H Bergeron]

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 are:

If I buy a new Honeywell unit like the one here

- 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!

[Set Square]

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

?

[H Bergeron]

Yes, it is electronic, not mechanical. It incorporates a thermostat and I have since realised that the product shown here

more appropriate than the one I mentioned before.

Thanks, that is good news.

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.

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

Many thanks for the response!

[Set Square]

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.

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

[H Bergeron]

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.

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

Thank you for your help. I mentioned

but I have not used them before. Do you know anything about them? Can you recommend an alternative supplier?

[Set Square]

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.

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.

I got mine from RKM Heating Controls

were the cheapest at the time, and provided very good service.

[H Bergeron]

Thanks for the recommendation. They still seem good on price, so I've ordered from there.

All your help is warmly appreciated!