Hi Guys,

You all seem to know everything, so I thought I'd ask a central heating question

We current have an old dial thermostat, and I want to change it for a digital stat that's programmable. On removing the cover from the existing one, there are four wires. From what I understand, most newer applications have two wires.

So what goes where? the diagrams these things seem to come with are border line hieroglyphics.......

If you need to know, its an oil fired boiler, serving normal rads, and hot water tank. Thats as much as I know!

Not long done the same thing...

Looked at the boiler manual and the unit had a feed out and a feed in and the stat just acted like a switch, even though there was 4 wires into the stat.

Yours may be the same?

No idea, its a new area for me. I must confess, I thought it'd be a pretty straight forward swap - if the thermostat is basically just a switch they why does it have four wires?

Nothing is ever quite as simple as I expect it to be!

My guess would be it is a 240v stat with a neutral, earth, live from the boiler and a return to the boiler to switch the heating on.

ours had 4 wires into the stat but the earth/neutral were capped off at the boiler end, I guess someone had changed the boiler to the newer type and there was no need to remover the 4 core and replace with 2 so just capped the unneeded 2 off

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Originally posted by Norfolkluegojnr
No idea, its a new area for me. I must confess, I thought it'd be a pretty straight forward swap - if the thermostat is basically just a switch they why does it have four wires?

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or get one of these Tado best thing I did for a heating system...

The extra wire in a 4 wire thermostat is the predictive element. It adds hysteresis (with a small heater) such that the bimetallic thermostat does not hunt damaging your boiler etc. An electronic thermostat does not need the extra wire as the hysteresis is done electronically. If your electronic thermostat is battery powered it will have 2 wires (live in live out). If it is mains powered it will require the neutral connection. The earth wire is for safety to any metal box etc.

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Originally posted by mookaloid
or get one of these Tado best thing I did for a heating system...

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Originally posted by theprisioner
The extra wire in a 4 wire thermostat is the predictive element. It adds hysteresis (with a small heater) such that the bimetallic thermostat does not hunt damaging your boiler etc. An electronic thermostat does not need the extra wire as the hysteresis is done electronically. If your electronic thermostat is battery powered it will have 2 wires (live in live out). If it is mains powered it will require the neutral connection. The earth wire is for safety to any metal box etc.

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quote:

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Originally posted by Norfolkluegojnr

quote:

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Originally posted by theprisioner
The extra wire in a 4 wire thermostat is the predictive element. It adds hysteresis (with a small heater) such that the bimetallic thermostat does not hunt damaging your boiler etc. An electronic thermostat does not need the extra wire as the hysteresis is done electronically. If your electronic thermostat is battery powered it will have 2 wires (live in live out). If it is mains powered it will require the neutral connection. The earth wire is for safety to any metal box etc.

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So how do I know which is which?

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Fairly easy.

You need to be careful as 240V can kill! My advice is put one hand in you pocket when measuring any voltages and wear good rubber soled shoes in a dry environment.

1) Isolate the cct

2) Loosen thermostat (usually two screws) and expose connections (make sure you don't short anything).

3) Switch on the circuit and start up the boiler with the thermostat turned down to minimum.

4) Measure voltage between earth (metal box if it has a ground connected) and the exposed wires one at a time. Warning if you short anything out you will get blinded by the flash (perhaps wear goggles). You said it had 4 wires then the box should be grounded (earth).

5) The Live feed will be the wire with 240V in the current state.

6) Turn the thermostat up fully, make sure the boiler is running.

7) The load will now be the terminal with 240V that did not have 240V to start with.

8) Neutral will be the wire with approx 0V on it.

Simples

You are supposed to be a trained operative to perform this procedure and if you undertake this procedure you are accepting full responsibility.

The stat must open a motorised valve.
Where is everything located in the system?
Boiler, motorised valve,programmer, room stat,cylinder stat etc and where does the room stat wire back to? Junction box? Valve?
In a combi boiler the stat is just a switch,on a system boiler you'll need to trace where the stat wire runs back to!
All you'll need on a modern stat is a live in from C/H on the time clock, and a switched live from the stat to open the motorised valve (usually brown on the valve) which in turn will send a feed to fire the boiler. ( usually orange on valve)

There will be no voltage at the stat unless Heating is turned on at the timeclock with a system boiler

[Edited on 4/10/16 by daniel mason]

Just pm me if easier.i can explain exactly what you need to do.
I cringe when reading these threads

I found fitting a programmer and rf digital stat easier when I started from scratch IE new wiring module very cheap and if you buy programmer stat and wiring module all from same manufacturer the job is really easy

It is on a combi!
Depends how it's wired on a system boiler.as there will already be a 2 channel time clock somewhere.itall depends how it's wired,where the stat wire runs back to and where the zone valve is.

Thanks Daniel , sent a u2u

Thanks to everyone for their input. As thought, far more complicated than I thought!