Electric underfloor heating thermostat wiring - HELP

greyrider · 2 Feb 2015

The old thermostat stopped working so I pulled it, and binned the thing (ooops) instead of keeping it for reference. The wiring was a single black and a twisted pair of reds to two, separate terminals. It's an old system, some 30+ years, on E7 and works fine.
I took a known working, Honeywell, thermostat from the hallway, but it had three wired terminals.

Choices are:

Term 3 20(4) @ 240v
Term 4 6(4) @ 240v

N:2
L:1
E:4
Switch:3

I've tried the black to N and the twisted pair to L.
The black to 3.
Split the pair and ran one to L and the other to 3.
The black to 3 and the pair to 1.

No joy and our living room is COLD.
Any help much appreciated.

deadshort · 2 Feb 2015

Hi,

sorry you're information is unclear. If you post pictures that may help.

Regards,

DS

greyrider · 2 Feb 2015

Cables I have, one black and twisted together, two red.

Label from thermostat case,

stem · 2 Feb 2015

greyrider said:
The wiring was a single black and a twisted pair of reds to two, separate terminals.

Do you mean

A. The twisted reds went to two separate terminals, or

B. the black went to one terminal and the two twisted reds to the second?

Wiring colours don't confirm anything, it all depends on how the original installer used them. We would need to know what is connected at the other end of each wire.

The thermostat cover looks to be from a Honeywell T6060, if it is

N = Neutral
L = Live
3 = Switched live for heating load

With this thermostat, it is possible for it work without the neutral connection, which is used to operate a small internal heater known as an 'accelerator' to improve the accuracy of mechanical thermostats. If so, and there are only two connections, terminals L and 3 would be used.

However, it is important to check that you do not have a neutral wire to the thermostat, if you inadvertently wire it so that they are connected together when the thermostat operates, you will create a short circuit, blow a fuse and possibly cause damage.

deadshort · 2 Feb 2015

Hi,

L = two reds plus a loop wire to terminal 3 (it must be minimum 1.5mm)
terminal 4 =black

Your thermostat may not be compatible if it requires a neutral.

DS

winston1 · 2 Feb 2015

Perhaps your old thermostat was not faulty and the fault is somewhere else.

greyrider · 2 Feb 2015

B. two reds twisted together went to one terminal and the single black to the other.

I'll be out from 12.30 'till around the same time tomorrow so apologise in advance if I can't reply - it's not that I'm ignoring anyone's help.

Thanks all.

stem · 2 Feb 2015

If you are sure that there were only two connections made at the original thermostat, then they will be the live and a switched live. If so, terminals 1 and 3 on the thermostat shown in your photograph are the ones required.

It doesn't really matter which way around they go, but we'll assume the two reds are the live (1) and the black the switched live (3)

However, as you say you have tried this combination and it didn't work, it would suggest that something else may be at fault, which may require test equipment to check. Does the thermostat still click as you turn it up & down? Has a fuse blown or MCB tripped? Is the timeswitch on?

greyrider · 3 Feb 2015

Yep, thermostat is a proven working item and dutifully makes "click, click" sound when turned.
Thanks all for the help, phoned home and they've gone for 1 & 3. Find out tonight/tomorrow if it's a goer.

greyrider · 4 Feb 2015

Well, I doff my, virtual, cap to you and raise a, virtual pint, it only works! Reports from home are toasty toes this morning. A thousand thanks for all answers and help.

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Electric underfloor heating thermostat wiring - HELP

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