ES3247 to Wiser Thermostat 3 - HW Off Wire

15 Dec 2022
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United Kingdom
Hi all

I'm looking to replace my ES3247 with a Wiser Thermostat 3 and have checked out the wiring diagrams to figure out the swap.

All was looking well until I realised HW off is wired up which doesn't correspond to anything I can wire into the new programmer.

The house was built in 2014 and we have two heating zones. One for upstairs and one for downstairs.

Are these two just incompatible, and if so are there any smart programmers which I can use in their place?

Here is the current wiring

And the new wiring diagram for the new programmer

I've also attached a photo of my hot water cylinder if that's useful


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So there is something odd here:

The 'hot water off' connection is used with a Y-Plan system that has a 3-port motorised valve. These are installed with systems only having a single heating zone and hot water cylinder. 3-port valves don't work with systems that have hot water and 2 heating zones, because 3-port valves are never closed, one port is always open. Which means if the 3rd destination wants heat, one of the other two destinations connected to the 3-port valve would also heat up whether it needed to or not.

So here's the thing. The wire in hot water off says Y-Plan, but the fact that there is hot water plus 2 heating zones say S-Plan Plus.

It would require investigation to find out what the hot water off wire is connected to and doing. It might not be connected to anything at the other end maybe. Also, it would help to have details of the system and the motorised valves controlling the hot water and 2 heating zones.

Because systems with HW and 2 CH zones aren't fitted with a 3-port valve the hot water off is omitted as it isn't used hence why it's missing from the Wiser.

A Nest 3rg Gen has a HW off, but you would need to buy two Nests. One for CH1 and HW and one for CH2. But I suggest you find out first what the HW off connection is doing and if you really need it.

I see you've edited your post to add the photo of the cylinder and motorised valves. So, it is an S-Plan plus where each circuit HW, CH1 and CH2 have a dedicated 2-port valve. So in theory it shouldn't need a HW Off connection. So if it were me, I would try and find out what is on the other end of the wire in HW off.
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I'm glad I'm not going mental. I've been trying to figure out why there might be the HW Off with the two zones all morning. Everything I can find basically says "Y-Plan doesn't have more than one zone" so what the HW-Off is doing there is beyond me.

For finding out what its used for, would it be enough to unplug it and then try each of my heat zones and hot water up and down stairs? I guess if everything basically works then I can ignore it going forwards.

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