I’m replacing a wired digital thermostat on the wall, taken the faceplate off and see N L 1 4 & and Earth

On the v3 wiring diagram it shows 1 and 4 are both “satisfied (optional)” and 1 goes to the heating part of nest and 4 to the hot water part…

I’ve got a combi boiler though that does the whole shebang.. doesn’t sound right to me – I’d have expected it only to control the heating system (I think 1,2 & 3 are the ones)

The current thermostat doesn’t control hot water so I’m not sure why it’s wired into 4

What am I missing!?

Thanks

Need photos! But the numbers on your old back plate will not correspond in any way to the numbers on the Nest Heatlink. Why on earth didn’t you take the discounted installation on offer with the Nest? The heat link is a fiddly bastard to install at the best of times with form over function very much on show.

Your combi may accept a hot water input to regulate pre-heating the water circuit. But since you’ve not named the manufacturer of either your boiler or the old programmer, it’s anyone’s guess. Bear in mind that if you get it wrong you will probably zap the heat link, Nest base, and thermostat unit simultaneously and if your boiler uses 24v switching you’re at risk of nuking the PCB too.

Really glad I didn’t read your post before I cracked on and had a fettle… I went back to the boiler and took the frontage off and followed the wires around to see where they had been linked up.

I was happy with the feed being used so I wired that up and figured the worst that can happen if I wire “4” into “3” was that it wouldn’t control the boiler… so I gave it a shot and hey presto we are in business!

The boiler is from around 96 so has nothing fancy on it for water

I got the nest for £140 posted from someone who had bought it from their supplier and not wanted to install it hence not getting a discounted install!

The house hasn’t melted yet so fingers crossed we are all good!

phew (I hope)

😀