Replacing my old central heating controller

[craker]

It's an old Honeywell controller with a dead battery I can't find a replacement for (Varta V110R the ones on the net just aren't the same).

Thing is, all the controllers I've seen have a two cable input for c/h and hot water, whereas my Honeywell has 3 cables for both of these (marked COM, NC, NO).

Can someone enlighten me?

Ta.

 

[D_Hailsham]

What is the model no of the old controller?

 

[craker]

Honeywell St699b

Having just googled it there are a few on the 'bay (not too cheap though)

 

[D_Hailsham]

Honeywell St699b

my Honeywell has 3 cables for both of these (marked COM, NC, NO)

Don't understand this. The ST699 terminals are labelled HW On, HW Off, CH On and CH Off.

Thermostats are sometimes labelled Com, NC and NO.

 

[craker]

Yes, my apologies, the terminals marked NC aren't connected to anything. I wonder if it's just a common backplate?

That gives me

1 - Mains Live
2 - Mains Neutral
3 - C/H NO
4 - (No connection)
5 - C/H COM - Connected to Live
6 - H/W NO - Connected To Live
7 - (No Connection)
8 (H/W Com) - Connected to Earth

Which doesn't make an awful lot of sense. How can COM for the C/H be Live and COM for the H/W be earthed?

Pin 3 (C/H NO) is connected to a cable that disappears into the walls. On the face of it that's the only cable that the controller can apply a signal to. How on earth do I get separate hot water / central heating control?

Thanks for any advice.

ps we're currently without heating What can I connect to what to turn the heating on?

 

[D_Hailsham]

6 - H/W NO - Connected To Live
7 - (No Connection)
8 (H/W Com) - Connected to Earth

Which doesn't make an awful lot of sense. How can COM for the C/H be Live and COM for the H/W be earthed?

Because the wire connected to 8 may look like an earth wire (green/yellow), but it is not being used as an earth wire. I assume it disappears into the wall, so you can't see where it ends up. It will connect, at the wiring centre/junction box, to the C terminal of the cylinder thermostat.

There is an error in the wiring of the HW, though it does not prevent it working:

Terminal 8 should be connected to L or terminal 5
Terminal 6 should have the " earth wire" connected.

Pin 3 (C/H NO) is connected to a cable that disappears into the walls. On the face of it that's the only cable that the controller can apply a signal to.

This wire will go to the room thermostat, via wiring centre.

How on earth do I get separate hot water / central heating control? What can I connect to what to turn the heating on?

Link T3 to T5, which bypasses the HW switch in the programmer. The heating should then work, controlled by the room thermostat. You can do the same for the hot water (link 8 to 6). HW will then be on continuously with the temperature controlled by the cylinder stat. The water will only get heated if it is used.

If HW or heating do not work there is a problem elsewhere, possibly a thermostat or one of the motorized valves.

If both work, it means the ST699 is faulty. Any two channel programmer can be used. You will have to change the backplate. The Honeywell equivalent is the ST9400A. There is also a seven day version, the ST9400C.

 

[craker]

Big thanks for your responses - really appreciated

I accept that the wiring has been bodged slightly - but to use an earth cable to carry live mains voltage is just bonkers. I was going to attach the metal box of my rigged up c/h switch to that to make it safer!

 

[D_Hailsham]

to use an earth cable to carry live mains voltage is just bonkers.

You would be shocked if you knew how often this is done by heating installers; it saves having to buy a different cable.