DIY Doctor

Wiring Diagram Advice for Installing Nest

Postby Adam13 » Mon Jan 20, 2020 10:00 pm

Hi all, I’ve purchased a nest and will be fitting next week. Just wanted to check comparability and if anyone could shed any light onto which wiring diagram I should follow and anything to look out for.

I have a glow work micron 50ff boiler and will be fitting nest gen 3

Thanks in NB advance.
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Postby ericmark » Tue Jan 21, 2020 11:24 pm

In real terms the Nest Gen 3 connects to valves or pumps, not direct to boiler. There are what we call plans, C, S, W and Y most popular, and the C plan has a number of updates.

The Nest Gen 3 is so versatile it is unlikely there is any system it will not fit on, but as to how you first need to work out what you have.

Have you got any motorised valves, and if so are they two port or three port, I would assume you have a hot water tank, does it have a thermostat on the side of the tank, do you have a programmer, is so what design.

I looked at the instructions for boiler https://www.tvha.co.uk/wp-content/uploa ... n-50FF.pdf and it seems very basic, which in some ways is bad as it means all the control gear is outside the boiler.

In the main Nest replaces the programmer using the heat link, or it fits by a junction box (often called a wiring centre) of some type, the terminals in the heat link are small, so I put a wiring centre right next to the Nest heat link so I have some where to connect all the cables.

The basic way any central heating works, is the programmer (Nest Heat Link) has the thermostats connected to it and it connects in some way to the motorised valves which have micro switches inside that in turn connect to boiler and pump.

Some really basic systems have no motorised valve, and there is just a pump, the domestic hot water is heated with thermo syphon (gravity) and central heating with a pump, so switching pump on/off decides if just DHW or DHW and CH.

Nest in the instructions have how to wire S plan and Y plan, it does not show C plan however this house is C plan and it does work.

Once I have an idea what you have, I can advise further.
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Postby ericmark » Thu Jan 23, 2020 11:43 am

[attachment=0]C-Plan_basic_Nest.jpg[/attachment]
This is the basic C Plan for Nest, you will note for DHW com feeds boiler and N/C contact comes from CH so the boiler is both run with or without the pump depending on if CH + DHW or just DHW required.

If a tank thermostat is added then the thermostat uses N/O and N/C in same way but the heat link is wired as with all other systems. The tank thermostat was added to C Plan so that in summer the boiler would only run as required, and not simply on time.

Still latter a motorised valve was added to C Plan so that the temperature of DHW could be regulated both summer or winter, however the motorised valve also stops the boiler cooling through the thermo syphon connection to the cistern so it depends on boiler, it seems it is not recommended for some makes and model as boiler can over heat, since I am not a heating engineer but just an electrician I can't say which boilers are suitable for adding motorised valves.

In real terms Nest is rather tight for wiring, although in theory it can replace the wiring centre in practice often you need the space to work in, so easier to retain the wiring centre and mount nest next to it.

Again although the Nest heat link does same job as the old programmer, one does not need access to heat link in the same way, so where as the programmer would be placed 1200 mm high on a wall allowing both visual and manual manipulation of the controls as laid out in Part M building regulations, since the heat link requires neither it can be mounted anywhere and does not even need to be in the house it controls, mine is in the flat below the house, near the boiler.
Attachments
C-Plan_basic_Nest.jpg
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Postby Adam13 » Thu Jan 23, 2020 8:26 pm

Thank you both for your replies. I’ll be taking a closer look at the boiler this weekend to identify what valves and how it’s wired etc so I can try to identify what diagram I should be following.

I’m fully competent in following a wiring diagram but not too far Iliad with boilers. Can you offer any advise on what to look for in terms of identifying what type of wiring it is?

Thanks again
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Postby ericmark » Fri Jan 24, 2020 12:07 pm

The heating system is made up of boiler, programmer, valves, thermostats, pumps, and tanks.

The valves if fitted come as two or three port, the three port means Y or W plan and 2 port S or C plan. The Y and W are very similar don't often see a W plan, Y can heat CH and DHW at same time, W gives DHW priority. The Y plan uses both N/O and N/C contacts on the tank thermostat.

With a S Plan normally two motorised valves one for DHW and one for CH and always the hot water tank will have a thermostat.

The C plan has three versions, they all use thermo syphon (gravity) for DHW only the CH is pumped, the change is how the DHW is controlled or not controlled.

Early C plan DHW could not be turned off, no real problem in winter, but hard to use in summer as only control was time boiler runs for, no allowance made for how much DHW is used.
The first improvement was to add a thermostat to the hot water tank. As with Y plan both N/O and N/C contacts on the tank thermostat are used, and the thermostat means the system can be left on DHW all summer and boiler only fires up when required, and the tank can be kept cooler. It made no difference in winter, only in summer was there an advantage.
Last improvement was adding a motorised valve to the DHW, this allows the DHW temperature to be reduced in the winter, and you could even turn it off, however many boilers relied on being cooled after being turned off by transferring heat to DHW, both the Y Plan and first two C plans allow this (Default on three port valve is DHW and some thermo syphon will pass through a pump even when turned off) but the last C plan and the S plan does not allow boiler to cool into DHW so it needs a run on fan to send excess heat out of flue when the boiler turns off.

C plan is used a lot with oil, and old gas boilers, Y plan was popular before the economy drive, there seems to be a move to the S plan which is easy to add zones with so you can heat upper and lower floors independently, seems to me an odd method, as we all tend to use our homes different, so I for example use two upper floor rooms as bedrooms and one as an office and one as a craft room, and entrance level dinning room only used in the evening, and after 8 pm kitchen hardly used, pop in to make coffee, so I really want each room independently controlled, just 2 zones does not really work, so I have wifi programmable TRV heads on entrance level except for kitchen, which like the 4 room upstairs has a blue tooth programmable TRV head. So I have 8 independent zones, (living room has two radiators so 9 programmable TRV heads).

I got the wifi heads with the whole idea of latter getting Nest, but by time I had got Nest, Nest had withdrawn support for the Energenie MiHome TRV heads, so in real terms the £45 (plus cost of hub) wifi head does very little more than the £15 eQ-3 bluetooth heads, and since the eQ-3 has the ability to manually control, you don't NEED a phone to swap Eco to Comfort and they have a window open function.

Also I have found Nest mounted closer to centre of house and higher than the TRV often shows 2°C higher than the TRV, so the follow command does not really work, I set the TRV in the hall 2°C lower than Nest also in the hall.

So today Nest is the odd one out, Hive, EvoHome, and Tado all have dedicated TRV heads, Nest does not, Hive is also an odd one out, that does not work with OpenTherm rest do, but your boiler not opentherm anyway.

I have used a TRV and a wall thermostat in the hall in two houses, this one and mothers, in mothers house the TRV and wall thermostat was closer and once set worked well, this house two doors between then and a stair case next to hall radiator means a big delay between radiator getting hot and the heat reaching the wall thermostat, so even with Nest it tends to over shoot, so Nest is set to rise 0.5°C per hour in the morning to reduce the over shoot, the TRV's in the rooms ensure the rooms at target temperature but hall heats up over 4 hours. This is the advantage of Nest I can set it to raise 0.5°C per hour.
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Postby Adam13 » Fri Jan 24, 2020 6:28 pm

Thank you all so much for the detailed replies it’s much appreciated. I’ll be looking at the wiring tonight but I suspect it is S or Y plan (the boiler was installed early 2000’s) and from looking at the wiring diagrams for the neat, the way they are wired to the actual nest receiver looks to be the same can anyone confirm?
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Postby Adam13 » Sat Jan 25, 2020 9:26 am

Looking into my set up a little more I have a Lifestyle LP522 programmer. Am I right in thinking it is possible to remove this, use the existing wiring to wire to the google heat link and wire t1 / t2 to the existing thermostat wires so I can replace the wall thermostat with nest. Or am I being too simplistic in my thinking?

Thanks once again for your detailed replies. Radiator valves will be next on the list, currently finishing my Hue set up too
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Postby Adam13 » Sat Jan 25, 2020 5:55 pm

Hi everyone so tonight I’ve managed to take a closer look and taken some pictures

https://m.imgur.com/a/w9GsoIb

Is anyone able to advise me what system I have and if I am able to remove the programer and simply replace with the nest heat link?

Also I can’t see a junction box in the airing cupboard, should this mean the thermostat is wired directly to the programmer and I can put in t1 and t2 to replace the stat on the wall currently?

Many thanks in advance
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Postby ericmark » Sun Jan 26, 2020 2:49 am

I am sure Nest can be wired for your system, having volt free contacts it can be configured for nearly any system.
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Postby Adam13 » Sun Jan 26, 2020 8:52 pm

I decided to get a qualified electrician to come and install so I didn’t blow anything up.
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Postby ericmark » Tue Jan 28, 2020 12:49 pm

I think you have made the right call, could not view pictures on tablet, now on PC I look at the plumbing and it seems a little more complex to what I expected.

Nest heat link will likely replace the programmer, and the installer will identify the wires going to the thermostat and re-use them. But Nest heat link to thermostat is 12 volt, so any mistake with 230 instead of 12 would be expensive.
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