New Room Thermostat Help

[Bread]

Hi, I have an old room thermostat that needs replacing. It's an old 70's style one which is no longer working. Inside the case it says that it's an Honeywell 4200 3879-01.

I perhaps looking to replace with either a basic digital model or another honeywell dial type. From my research I see that there are generally 2 types, a 2-wire and a 3-wire?

My problem is:

I've undone the cover on the old thermostat and i've got 5 wires! 2 red, 2 black and 1yellow/green that come from3 cables.

There is a single black wire going to term 2
A single red going to term3

Then the large cable (mains?) is split:

red - term 1
black - term 2 (along with single black)
Yell/Green - Earthed to back plate

So to sum up only 3 teminals are used on the old therm.

How would i arrange the wires into a 2 or 3 wire thermostat? If it was a 3 wire thermostat such as http://www.screwfix.com/prods/26159...ing-Controls/Honeywell-T6360B-Room-Thermostat would i just wire it up as in the old one, ie. the same wires to terms 1,2,3 and earth left as it is? http://www.screwfix.com/sfd/i/cat/pdfs/36/p4734536.pdf

I have just a conventional boiler and with a separte potterton timer which controls HW and CH. There is also a pump next to hot water tank.

Any help appreciated.

 

[ericmark]

Old three wire units required a neutral wire the new digital units don't require the neutral wire however they are designed for cooling as well as heating so still have three connections.

Likely you will need a junction block to terminate the wires into and from that you will have just two wires into thermostat.

Not sure why you want an old three wire unit the differential between on and off temperature is a lot higher than new digital units These units will allow time as well as temperature control at very little extra cost and is what I have replaced my old thermostat with which then allowed me to dispense with frost stat as well. You need to change the battery once a year (Will last two but I change every year) but it controls to half a degree not 2 degrees of old type.

The neutral connection on old type was to a small heater in unit which was to try to reduce the difference between on and off temperatures but all a bit hit and miss just try moving the dial and note difference between on and off you can normally hear the click.

 

[Bread]

Thanks for the reply. I do like the look of the digital ones.

If I were to go down the digitial (2 wire) route would it be a case of wiring up the 2 red wires? i.e. 1 Live and the other switched live to the new thermo. Then putting the 2 black wires into blocks and leaving the earth wired as it is to the recessed back plate?

 

[ericmark]

Most likely yes. Of course without testing all wires I can't be 100%.

I find set at 14 degs rather than off at night is great in this weather. I find all thermostats over shoot so I step up temperature 6am up to 17 deg then 7am to 19 degs and this reduces over shoot then 20 degs in evening.

On weekend only two temperatures but still a lot better than old control. I don't use same boiler for hot water so in my case the time clock has also gone.

 

[Bread]

Thanks

So I would assume then that the red wire from the large cable is the live and that the single red is the switced live?

I've got a multimeter so will test with that.