Electric underfloor heating (wired) thermostat - help

[greyridersalso]

We've an old, electric, E7, u/f heating system (30+ years). The living room thermostat packed in so I tossed it out - should have kept it for referenc e, but there you go. It had only two connections and my wiring is a single brown and a twisted p air of reds which were made, one to each. I took a working Honeywell 'stat from the hallway to use as a replacement - we don't heat that area. But....it had three wired connections. So, I mimicked them, as I thought correct, with the brown to N and split th e pair to connect one to L and the other to 3 (see below). No good. I swapped brown to 4 and remade the twist and connected to 1. No joy. Asking before I blow the lot.

From the label in the Honeywell;

TERM 3.: 20(4) @ 240v TERM 4.: 6(4) @ 240v

N [2] L [1]---| [4]-v | [3]---|

(the loop between 1 and 3 has a break with two little triangles, looks like a switch)

Help appreciated, it's a bit cold in the room and SWMBO insists she needs t he convector heater on all day........ Out 'till tomorrow afternoon so, apologies if I don't answer any replies im mediately, and thanks in advance for any advice - Oh, I also tried a new, m echanical thermostat from Screwfix which turned out to have the same choice of connections and with the same, non working result.

[greyridersalso]

And, because it's E7, I either have to stay up 'till gone 00.30 or wait to see if it's worked the next morning!

[John Rumm]

OK the way this works is you connect the N from the supply (if you have one) to 2, and the Live from the supply to 1. 3 is then the "call for heat" and will go live whenever the stat decides its not warm enough.

(You can ignore 4 since that is the "satisfied" signal - normally used for bringing in cooling when the temperature gets too high)

If you have access to a multimeter, then you should be able to check it all out without needing to wait for midnight!

Its slightly unclear from your description exactly what wires you have... one would expect there to be a L & N from the supply, and a L & N to the UFH - this assumes that the UFH has just one mat and one set of connections. Since you have 3, that suggests L & N in, and just a single L out, with the N for the UFH being permanently connected elsewhere. The N in is not really "required" although is used by old mechanical stats to run a small heating resister in the stat to keep it warm and this speed up its response.

[Roger Mills]

If the stat has a neutral connection, it sounds as if it has a built-in heater/resistor which gets a bit warm when the load is switched on and helps to reduce the stat's hysteresis. This type of stat is usually used on 'wet' CH systems to switch the boiler and pump on and off - so the first thing to check is whether it has sufficient current switching capacity to switch your UFH on and off.

Assuming it *does* have sufficient capacity, you only need to use *two* connections. Do not connect anything to N. If you're not sure *which* ones to use, you need a continuity tester to determine what is connected to what when the stat is calling for heat. Typically you may have contacts labelled COM (Common), NO (normally open) and NC (normally closed). Typically, when the room temperature is below the set temperature - so that heat is required - COM will be connected internally to NO (but it could be the other way round!) changing to NC when the room warms up to the set temperature.

Anyway, I'm assuming that the brown wire is the live feed - and should be connected to COM. I'm assuming that the two reds are the load, and go to two segments of under-floor heating elements. These should both be connected to the same terminal - NO or NC as appropriate (or however else the terminals are labelled).

[greyridersalso]

Thanks for the responses.

I have three wires, one brown and two reds which are twisted together. On the old thermostat there were only two terminals and the single brown went to one with the two twisted together reds going to the other.

I've connections now of N[2], L[1], [4] and [3], know nothing about Coms or NO or NC, sorry.

Have to go, check back tomorrow and, again, many thanks for the help.

[Robin]

Was it possibly was an old 2-wire thermostat - ie "live" in on one and when cold enough out on t'other. I'd guess in on brown (newer colour as that side rewired in past?) and out on the reds (2 as there are 2 heating circuits?) But a meter on them after the witching hour needed?.

If so time to shop for

or similar?

Been there, done that, for n/bour in December :(

[Robin]

Apologies. I should have read your post before replying to John Rumm's as you had already said what I wanted to and more.

[Roger Mills]

OK, it's likely that L = COM, 3 = NO and 4 = NC (where "=" in this context means "is equivalent to")

So ignore the N terminal, connect Brown to 1 and the two reds to whichever of 3 and 4 is internally connected to 1 when heat is required.

So, you'll need your continuity tester, as I said before - and don't forget to check that the stat's current handling capacity is adequate - unless, of course, the stat switches a relay (or two) which does the high current switching.

[greyridersalso]

Terrific help guys! Phoned home (stuck here a bit longer) and they've gone for 1 & 3, fingers crossed and I'll let you know how it goes

[greyridersalso]

Woohoo, toasty toes this morning! Profuse thanks to one and all contributors. (Now all I have to do is get "someone" to crank the setting down to a sensible level........small chance) Thanks again one and all.