Understanding Heating Controls and Thermostat

23 Jan 2017
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United Kingdom

I'm currently having issues with my central heating system (oil boiler) following replacement of an old programmable controller and room thermostat. I have some electrical knowledge as an electrician but not specific to heating and controls.

So the basics; I have an oil boiler located in the garage, a cylinder in the airing cupboard, with a 2 port valve adjacent the cylinder. I have a two channel programmer located beneath the stair cupboard and a thermostat located in the hall located directly above it.

My old stat was malfunctioning on the display so I had to replace it, and none of the existing wiring was labelled or matched the new plate, as the old was original (1980's). the hot water comes on, although not sure it follows the set programme, but the central heating does not come on at all.

I want to know how to identify the cabling and to understand the system a bit more to enable me to re-wire it correctly.

i'll attach some images below to help diagnose my system and the problems.
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The junction box located next to the pump and valve setup as indicated, on the left; the black cable runs to the 2 port valve and the white runs to the pump motor. 2 white cables enter from the back, which I assume would run to the boiler and the controller? most of the diagrams I have looked at to match against only show 10 ways, not 12, so I cant find an existing schematic that explains any of this.

The programmable controller has 3 cables coming into it, one which is presumably a L/N/E supply, and then a L/N/E and a 4C R/Y/B/G&Y which go to somewhere else, I would assume the junction box and then landing thermostat.

At the thermostat I can see the R/Y/B/G&Y but have no idea where they connect to, what is clear is that the inputs on the controller don't have a connection in one of the 1-4 inputs which would be for CH and HW in and out.
It's hard to tell from the picture of the junction box, but I think the blue wire at the clock should be in terminal 4 and the red wire in terminal 3.
The CH off terminal 2 is not normally required.
It looks like you have a 3 port valve not a 2 port. This is known as a Y plan system. To help you look for wiring diagrams.
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If a Y plan CH off needs a supply. You cant trust cable colours in central heating circuits.
You need to identify your wiring and adapt to your new component diagrams.
CH off doesn't need a supply but HW off does which he has got on terminal 1
Thanks for the responses, Is there any pictures or further details i can provide to help identify any of the system?

You are right, it is 3 port, typo on my part, what i want to be able to do is identify what all of the conductors are doing, as i am aware that the colours used for systems are not always reliable, so a point-point check is the best method.

I am not convinced the room thermostat pictured is wired correctly (obviously i know ive removed them ;) ) but presumably the conductors will be dependant on what the timer terminal colouring is?
with a 2 port valve adjacent the cylinder.

You mention a '2 port valve'

But the black cable to the motorised valve here...

...has wires for a 3 port valve.

So, based on that, I will assume that you have a 3 port valve and the reference to 2 port valve is a mistake.

As has already been mentioned, you don't have a connection to your programmers terminal (4) this is for 'Central Heating on' and a connection to this will be needed for your heating to work. This wire should be connected to to the room thermostat Live terminal (1) [Usually the red wire*]

The wire from terminal (3) of the thermostat is the switched live [Usually the yellow wire*] which should be connected to the motorised valves white wire. When live, this wire puts the valve in the heating position.

The wire to terminal (2) of the thermostat [Usually the blue wire*] Should be a Neutral to operate the small heating device known as an 'accelerator'

Terminal (1) at the programmer for 'hot water off' should be connected to the motorised valve grey wire. [it should also be connected to the hot water cylinder thermostat at some point] when this wire is live it moves the valve to close off the hot water port feeding the hot water cylinder.

Terminal (2) at the programmer (Central Heating off) should not have any connection to it as has been said already.

*Usually in the vast majority of cases, but not always. Will depend upon the preference of the original installer.

To help you understand what should be happening, you will find a great graphic by clicking here. Then make selections from the Y-Plan menu and you will see what will be happening under different conditions and what should be 'live' and when.
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Thank you for the detailed response, and the link provided.

As you state it was an error on the 2/3 port valve. When im looking at the thermostat wiring, is there an electrical method to see which is which, i.e if the heating is on, will i see a voltage across the switch live, as i assume the red (live) into terminal 1, when the thermostat is calling, will make a contact to terminal 3?
With the thermostat wires disconnected from the thermostat, as they are in the photo, and I assume as an 'electrician' you can do this safely.

1) A continuity test between the each of the wires and a known neutral will allow identification of the Neutral.

2) When the heating is selected 'on' at the programmer (assuming that the thermostat is correctly wired to terminal 4 of the programmer of course, and there's nothing connected there now in your photo!) a multimeter will read 230V between the confirmed neutral and the live wire, and not between the confirmed neutral and the switched live wire.
Brilliant, yes i thought that i would be able to, hence why i had disconnected them to do a continuity test.

Thank you again, and for your speedy response, i'll try it later tonight and report back!
Thank you all for your help, it appears to now be working (fingers crossed) and I have managed to draw my system schematic so should be able to identify any issues in the future too!
So, now that I have it all working, I am considering completely mucking it up again by adding a hive/nest controller ;)

What do I need to consider to do this, is it as simple as connecting the cabling in the timer to the terminals on the hive backplate, and linking out the thermostat?
is it as simple as connecting the cabling in the timer to the terminals on the hive backplate, and linking out the thermostat?

Yes, pretty much. The Hive dual channel receiver will fit on to the existing backplate and the terminals have the same functions, so the wires don't need moving. You will have to link out the thermostat live and switched live as you have realised.

The Nest receiver (Heatlink) doesn't have a backplate, so the wires will need disconnecting from the existing backplate and wiring to the appropriate Nest Heatlink terminals which have the same function. Plus a couple of wire links will need adding.

Personally I wouldn't bother though, unless you feel the need to adjust your heating when you are down the pub.

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