Heatmiser UH4 and Nest Gen2 wiring

5 Dec 2012
Reaction score
United Kingdom
We have a nest gen2 heatlink which only has 5 connectors N L 1(satisfied) 2(common) 3(Call). The boiler was connected to 2 and 3 but during some building works it was disconnected. We now have a Heatmiser UH4 and we are going to connect the heatlink to zone4 for radiator use.

The UH4 just has 4 terminals too, N,L,E and SL. Can anyone advise how to wire the two together? Obviously the L and N goto the relevant terminals, and i presume SL goes to the call for heat, but is there anything else? I found a diagram on here for the nest gen3 but that has a lot more terminals.

Sponsored Links
Do you have a wiring center for the boiler or is it a Combi boiler?
Hi it's a combi boiler and I've got the wiring all set up now and it seems to be working to a point.

When we had just a nest the boiler clock had to be set to always on but it only actually fired up when the nest heatlink called for heat.

Now the UH4 is installed the boiler is constantly firing if clock is left set to always on.

When one of the thermostats calls for heat, the relevant port valve opens, the lights come on the UH4 etc but the call for heat doesn't seem to be stopping the boiler when it goes off. It seems the clock/timer is overriding the boiler not the other way around.

One thing to note, the manual clock timer has to have all the pegs set to on so it's on timer mode as the switch between on, timer and off doesn't appear to work. Although I don't see the difference between timer always on and just always on. Hope that doesn't confuse things
  • Thanks
Reactions: CBW
Probably best showing photos of the timer, boiler make/model and wiring.
Sponsored Links
The problem with Nest is there are around 6 versions, Nest Gen 1 was only released in USA as far as I am aware, Nest Gen 2 and 3 the USA version and UK version are completely different, the USA has temperature sensors, which have not been released in UK, and the units look very different, plus we have Nest e which is a battery powered unit.

For Nest Gen 3 it was claimed Energenie Mihome TRV heads would work with it, that's not the case, I am considering ripping my Nest Gen 3 out and fitting a proper smart thermostat.

Having said that all it does with the early version is turn on/off, there was no OpenTherm, and not even sure if it had geofencing, the Gen 3 geofencing is rubbish there is no distance setting.

It seems Heatmiser UH4 has an option to fit a room thermostat,

The diagram show shows it uses the micro switch in the motorised valve to switch on boiler when using the radiators, the V3 micro switches often stick, so I would guess that's the problem, but I would not use Nest again, it's a load of rubbish.
Thanks guys.

To clarify we have had a best Gen2 (UK) running fine till about 2yrs ago when we disconnected to start on some home improvements, it's only now we are starting to get back to normal!

So we jus had UFH installed. That will be controlled with a neoair and RF switch into port one. Port 4 I have connected the old nest.

Port 1 and 4 out goto two port valves.

Mains power is via 5 core cable so the two spare cores are being used for the call for heat to the boiler. But it doesn't seem to want to switch on when called. I'm wondering if the boiler timer/clock is affecting it.

Boiler is a WB greenstar 30cdi.
And here's a pic of the overall layout and UF4 wiring.


  • PXL_20230419_211039574.jpg
    385.1 KB · Views: 17
  • PXL_20230419_211159808.jpg
    319.7 KB · Views: 17

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local