Nest Learning Thermostat 3rd Gen paired with Glowworm Ultracom2 18sxi - Hot water issue

12 Nov 2022
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United Kingdom
Hello every one,

I recently got a Nest Learning Thermostat 3rd Gen on sale as I wanted the ability to switch on and off the Central Heating & Hot Water remotely.

I have a Glowworm Ultracom2 18sxi boiler (I believe its a system boiler?) with a seperate hot water tank to store hot water as well. It was controlled using a HORSTMANN CentaurPlus C27 programmer.

I was able to install the Nest thermostat & heat link and I am able to control the central heating using the "nest thermostat" & "nest APP" in my mobile. However I am able to switch ON the hot water only by manually pressing the center round button in the "nest heat link". The question I have is this: Is it possible to control the hot water as well using the nest thermostat?

The attached picture shows the wiring that I had before I installed the Nest thermostat.

Using the picture I basically did the following to connect to the Nest Heat Link:

N -> Nest N
L -> Nest L, Nest 2 (common) & Nest 5 (common)

3 -> Nest 6 (call for heat HW)
4 -> Nest 3 (call for heat CH)

Can anyone please help by sharing whether it is possible and if so how?


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If I have the right installation manual 1669940939561.pngIt seems there are multi-ways to control the boiler, extra low voltage, connection to BUS, or low voltage, I don't know which you are using, but likely the low voltage (230 VAC) and likely this is controlled by some motorised valve, normally the Nest Gen three feeds the motorised valve direct for CH and via a tank thermostat for the DHW, but is general we use the Honeywell systems or Plans we have S, Y, and C there are others but not used as much.

I use C Plan and have two motorised valves, so I can select main house or flat, the DHW does not have a motorised valve or tank thermostat, in the main two motorised valves means S Plan, and one three port valve means Y plan.

I have DHW off in the Winter 1669941559089.png as it always heats with central heating, no way to stop it, and I only set a schedule in the summer. 1669941700480.png 1669941776219.png only the central heating has a schedule set in winter. But I will admit when I first fitted it I did have some problems setting the software for my system, had to turn off legionnaires protection, and geofencing and auto learning, or it did some daft things, I am not impressed with Nest Gen 3, but not really worth dumping it and getting a better system, I was told it would work with Energenie MiHome TRV's, it seems this is not the case.

But to advise how to set up, need to know what you have, and not sure what I can remember, many items need setting at the dial.

The book shows S and Y plans, but not C Plan, it will work with C Plan, but there are really three types of C Plan, no tank thermostat or motorised valve (mine) tank thermostat for summer use, and tank thermostat and motorised valve for winter use.

This is the unpublished C Plan.
Thanks for the quick response ERICMARK.

I am using the 230V and I think in my house its a S plan (can you please confirm my assumption is correct) as seen from the 2 pictures below (1st picture shows the hot water tank & valves and 2nd picture shows the valves & pipes).

Also in the Nest Learning thermostat I have already set the "Bacteria protection" to OFF and the "Home/Away Assist" to OFF (I have set most of the settings to OFF so that its just a remote switch to ON / OFF the central heating and hot water). I have also set the "Heating System type" in the NEST thermostat i.e. boiler type to OTHER BOILER as shown in the 3rd & 4th picture below.

I did try to set the "Heating system type" to SYSTEM BOILER, but when I do that the boiler constantly switches ON and switches OFF continously.

Can anyone please help?



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Your photograph shows 3 motorised valves which is known as an 'S-Plan Plus' and as the one you have circled is connected to the hot water cylinder would be the one controlling the hot water. System boiler would also be correct.

Nest only has two channels, one for central heating and one for hot water.... however as you have 3 motorised valves, something doesn't quite add up.

If you have two heating zones and a hot water cylinder, you would normally have two Nests. One for one zone of the central heating (ground floor?) and the hot water, and another Nest for the second heating zone (first floor?)
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Thanks for the quick reponse.

Both the ground floor & 1st floor valves are linked in the junction box and so when I switch ON the "Heating" in the NEST thermostat, both the ground floor and 1st floor radiators work (i.e. central heating is working well in the house).

The only issue I have is with the HOT WATER as I am unable to control it using the NEST thermostat. I have to switch ON hot water manually using the central button in the HEAT LINK.

I have wired the NEST thermostat as mentioned in my first post above.

Can anyone please help with this?
The wiring looks correct from your photographs, but is easy to test to be sure. If you isolate the power to the Heat link, remove the wire from terminal 6 (hot water call for heat) and put it in the L terminal. Then restore power and switch CH and HW off with Nest. The hot water should continue to run, provided the cylinder requires heat, you may need to turn up the hot water cylinder thermostat if required.

If the hot water valve remains opens and the boiler runs. The wiring is OK. If not, there may be a fault with the motorised valve, in that the valve is opening but not starting the boiler. When you press the manual button, all the valves will open so one of the others could start the boiler causing water to flow to the hot water cylinder because it is open as well.
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Thank you very much for the suggestion.

As suggested I connected the wire from terminal 6 (of nest thermostat) to the L terminal of nest thermostat and then switched OFF both the CH and HW in the NEST thermostat. I also increased the temperature of the HW cylinder thermostat.

When I do that the boiler (Ultracom2 18sxi) switches ON, but it immediately switches OFF. This is happening continously and I tested it for more than 30 mins. This is similar to what happened (mentioned above) when I set the "heating system type" to "SYSTEM BOILER" in the NEST thermostat i.e. the boiler fires up and then shuts down and this is happening continously.

So does this mean the HW valve has an issue?
A motorised valve has a motor that winds it open. When it's open it operates a small microswitch that starts the boiler. However, I can't think of a problem that would generate the fault you describe.

At least you know now the problem isn't with the Nest, because the test took it out of the circuit, and nothing changed.

Was it all working correctly before the Nest was installed?
Thanks for the response.

I remember the HW working correctly when using the HORSTMANN CentaurPlus C27 programmer. But just to confirm it, I today again disconnected the NEST thermostat and connected back the HORSTMANN CentaurPlus C27 programmer. When I tested by programing a time to start and stop the HW heating it worked normally (i.e. it started the boiler and shutdown the boiler at the correct time and it did heat the water). Its the same, when CH was programmed to start and stop and when CH & HW was programmed to start and stop together. It did heat the water & radiators.

The only thing I noticed was that when the HW and/or CH were switched OFF, the valve LEVER position were as shown in the 1st picture below. Is that correct? i.e. they dont move to the the very end on the Right, they are kind of in the middle.

When the HW was switched ON, as shown in the 2nd picture the valve LEVER did not move from the OFF position but the HW valve did make a winding noise and the bolier switched on and the water became hot. I think HW motor was changed only 5 months ago. Also I was able to move the lever without much resistance when it was running.

Also the 1st floor CH valve LEVER some times move to the Left i.e. ON POSITION, but some times the LEVER is stuck in the same position as the OFF position picture. But the radiators were hot in both the ground floor & 1st floor, so the CH is working. Also I was able to move the lever without much resistance.

But the Ground floor valve LEVER did move to the left when CH was switched ON as shown in the 2nd picture.

So I am not sure. But the question I have is, when the CH/HW is switched ON, should the valve LEVER always move to the right?


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The lever is for manual opening of the valve when the system is initially being filled with water to stop airlocks forming, it slides across and is lifted up to lock it into the manual position. Once the system has been filled the lever should remain in the auto position, although it may move about during use but won't latch in the manual position.

With everything switched off, if you slide the lever across from Auto to Man, but don't latch, it you will feel the resistance of the valve mechanism as you open the valve.

When the valve has been opened by the controls if you move the lever, it will feel loose and floppy because the motor has wound the valve open, and the resistance has gone.

Once the valve has been wound fully open by the motor, it operates a small internal switch that controls the boiler (Grey and Orange motorised valve wires) This means that if any one of the valves is open the boiler starts.

The wire in Nest 6 should be connected to the brown wire of the hot water motorised valve. When Nest makes it live, the valve should wind open.

The same when the Horstmann switches the hot water on, terminal 3 of the Horstmann which has the same wire in is made live.

With the test I asked you to do, putting the aforesaid wire directly in with a permanent live, by-passed the controls, but created exactly the same electrical circuit but with the Nest out of the equation.

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