NEST gen 3 fitting on Honeywell programmer and room stat

[Gary cox]

hi guys. Read a lot of other posts and can see some really good help.

I have a Honeywell programmer and room stat in hall. I want to change them over for the nest gen 3. Want to make sure I get it right and want some help on how todo it. Here are some pics of my wiring and system to help u guys know what I'm talking about. So reading from what Stem has put in other posts I know that the room stat is the tricky bit I think. I want the nest stat to go where the original room stat is if possible usinsing the power that is there to power it. I'm not a diyer as such as fit kitchens and do the wiring in there when needed so know a little bit but don't understand boiler wiring. Hope u guys can help

 

[ericmark]

Getting Nest to work, that's easy bit, getting some advantage is some thing else. It does depend on house, fitted a programmable thermostat to my house, and it was great, fitted one to mothers house, it was a flop.

Old central heating and central heating in open plan houses can work very well with a programmable thermostat, new condensate boilers and houses with doors on every room have problems. Nest however does use IFTTT so can be programmed to either follow eTRV heads, or the eTRV heads can follow nest so yes it can do a good job. However something like EvoHome is much better.

With mothers house in hind sight and hind sight is easy I should have used EvoHome. Now I have it working reasonable, but it took a lot of doing. So let me explain the changes. With the condensate boiler you got something very new, one the flame height can vary, and two the return water temperature matters. This has turned control of central heating on its head, now the thermostatic radiator valve (TRV) is the main control device, unlike the wall thermostat it does not turn heating on/off but gradually increases or reduces flow in the radiators and boiler flame height giving a hysteresis free individually room control.

However it has a problem, as summer arrives, the boiler needs to turn completely off, the best option is for the eTRV heads to tell a central hub when each room is satisfied and when all rooms are satisfied it turns the boiler off. However expensive, so cheap option is to fit a wall thermostat in the coolest room, which also has no alternative form of heating and no outside doors so when that room is warm enough the heating turns off.

The cheap option often has a problem, in that there is often no room which is always colder than the rest with no alternative heating and no outside door. This was the case with mothers house, so the way around problem was to fit the wall thermostat in the hall and also have a TRV in the hall and carefully match the two devices, so you get the hall to warm up rapid until nearly the set temperature, but slow down for last few degrees, so all other rooms can catch up.

OK it works, but the problem is matching the TRV with the wall thermostat, and once this is done you don't want to alter either. This is where the programmable thermostat falls down, although in theory you could use a eTRV head and nest thermostat matched to each other, the eTRV head will control the room to around +/- 1°C where the standard TRV is more like +/- 2°C and you need that extra to stand a chance of matching them up. So if you have to leave the Nest set to just one setting, then what is the point in having it?

As said that is not true for all systems, for boilers which can only switch on/off with no TRV fitted then Nest is likely a good move. But with such a simple system would it really be worth fitting Nest? If a simple system I would fit a wired programmable thermostat and set the existing programmer to 24/7 central heating running, unless thermosyphon hot water, in that case first move is fit a motorised valve to hot water system.

 

[Gary cox]

Thanks for your very detailed reply.

But I already have the nest. I just wanted help on wiring it up. Have attached pictures to show people what I have. Just wanted a hand with th wiring

 

[ericmark]

This is basic wiring diagram for nest, you have a three port valve likely Y Plan click here for PDF in essence the timer is replaced with the Nest, however very easy to show on a nice diagram but in real life there are often junction boxes which don't follow the wiring to the letter.

Page 25 shows Y plan

This is likely what you need, however I am only guessing you have a Y plan. However the W plan also uses a three port valve, but in that case it is not three position it is only two position, with the W plan hot water takes priority with the Y plan it can do hot water and room heating at the same time. Using motorised valves the basic idea is the thermostat works the valve and then the valve works the boiler, this ensures the valve is open before the boiler and pump starts.

These old systems are likely the hardest job a domestic electrician has to do, today they have been in the main replaced with the combo boiler which has all the divert valves and by-pass valves inside, the old system often did not have by-pass valves because it did not have thermostatic radiator valves (TRV) fitted.

However systems designed for use with solar panels still have all the motorised valves outside the boiler, some are very complex, with often three or four independent methods of heating water, all integrated, in the main they use very expensive cylinders like the mega flow which has very efficient heat exchangers, but the older cisterns can be used. I will guess you have not got a super complex system, but the point is I can only guess, it is down to you to work out what you have.

 

[stem]

Hi Gary

The wiring at the programmer should be moved over to the Nest Heatlink as follows:

Honeywell 1 (Hot Water off).......Nest 4 (Hot Water Satisfied)
Honeywell 3 (Hot Water on).......Nest 6 (Hot Water Call for Heat)
Honeywell 4 (Heating on)..........Nest 3 (Heating Call for Heat)

The Lives / Neutrals are a like for like swap.

Then you need to add some links, so that the Live terminal also connects to the two common terminals 2 and 5

Which brings us to decommissioning the original thermostat. There are 3 possibilities. 1 is the preferred solution, but 3 is the easiest.

1. Find the opposite end of the thermostat cable to the thermostat, and disconnect it. Then link the terminals where the yellow and red wires came from together.

2. Remove the existing thermostat and replace it with a junction box, connect the yellow and red wires together and isolate / insulate the neutral

3. Leave the existing room thermostat in place and put the yellow wire in terminal 1 along with the red.

There are two choices to power the Nest thermostat, either wire it to the heatlink T1 & T2 which you can do using the existing cable if you have decommissioned the thermostat using method 1 above. Or by using a separate plug in power supply.

EDIT
Just to clarify. When you say:

"I want the nest stat to go where the original room stat is if possible using the power that is there to power it."

You can't actually do that. The existing thermostat wiring is 230V, but the Nest thermostat is only 12V. So, it needs to get that 12V from either a 'plug in' power supply, or by Wiring terminals T1 and T2 at the Nest thermostat to the corresponding terminals at the heatlink. As I said above, you can use the existing thermostat cable for the 12V, but only if it has been completely disconnected from the existing mains wiring. However, even if you do this, you may find that the cable doesn't run to where the heatlink is located. Depending on the wiring layout of your installation, you may find that the other end goes to a junction box / wiring centre elsewhere, making it impractical to use.

 

[Gary cox]

Hi stem

I am going to go for option 1 as I want it to be done correctly

I have found the thermostat cable that goes to the junction box in airing cupboard. Linking the red and yellow together is all fine.

But what I want todo it use the 12v power from the the heat link. Is it possible to do this as didn't want to use the USB power thingy? Would there be a cable from the heat link to junction box that I could join to the one I have just disconnected?

 

[ericmark]

In theory you can reuse the old thermostat cables, but practice may be a bit harder, with my daughters old house it took me a long time to work out exactly which wires went where, it would seem you have identified which there are at the junction box, but you need a spare cable (not wires) from junction box to heat link, and simple maths shows you have more wires going to the heatlink than went to the old programmer so having a whole cable spare seems unlikely.

I did consider fitting Nest, and unlike most wireless thermostats nest needs a mains supply dropped down to either 12 or 5 volt and this to me was both a good thing in you do not need to change batteries and bad in that it needs that power. Since it has two way coms it uses more power than the cheap wireless thermostat normally used, what you need is some where that a power supply will not look bad, I would favour mounting a power supply near the junction box rather than taking it from the heat link and use old cable.

 

[stem]

Linking the red and yellow together is all fine.

That's not quite right. You don't link the thermostat red and yellow together, it's the terminals that they go to that need linking together.

So, if in your photo the red and yellow wires that come from the same cable and are in terminals 10 & 9 are the thermostat wires, then they should be removed from the terminals, and then the two remaining wires (white & red) would be linked together. You can do this by putting both of them into the same terminal (either 9, or 10, it doesn't matter which)

Then disconnect the blue neutral, so that the cable is completely disconnected. It can then be used to get 12v to the Nest Thermostat. But....

But what I want to do it use the 12v power from the the heat link...Would there be a cable from the heat link to junction box that I could join to the one I have just disconnected?

You can extend the original thermostat wires and take them to the Heat Link, but the connection should really be made outside of the Junction box (wiring centre shown in your photo) It's not good to have 12v and 230V wires in the same junction box, although you could argue that the Heat Link does, however, in the case of the Heat Link, its voltages are marked above the terminals

 

[Gary cox]

Ok cool got all of that.

Just 1 thing to clarify. When u say extend the old thermostat cable to be used as the 12v. Do u mean run a new cable from the junction box area(but connected outside) to the the heat link. Only ask this would be a pain as junction box it upstairs and the heat link is down stairs

 

[Gary cox]

Or could I power the nest thermostat using a 12v transformer powered from the junction box area and connect the old thermostat cable? As I have a socket in there for a back up heating element that is never plugged in

 

[winston1]

Or could I power the nest thermostat using a 12v transformer powered from the junction box area and connect the old thermostat cable? As I have a socket in there for a back up heating element that is never plugged in

I believe it requires 12v DC, so no you could not use a transformer (which are AC output devices). You could use a 12v DC power supply however.

 

[stem]

Just 1 thing to clarify. When u say extend the old thermostat cable to be used as the 12v. Do u mean run a new cable from the junction box area(but connected outside) to the the heat link. Only ask this would be a pain as junction box it upstairs and the heat link is down stairs

That's the idea. It is rarely practical though, for the exact reason you state. You were the one that said you wanted to use the existing thermostat cable, and I did warn you it might not be straightforward.

However, even if you do this, you may find that the cable doesn't run to where the Heat Link is located. Depending on the wiring layout of your installation, you may find that the other end goes to a junction box / wiring centre elsewhere, making it impractical to use.

There are really only two ways to get 12V to power the Nest Thermostat. Either by wiring it directly to the Heat Link T1 & T2 terminals, or by using the Nest power supply. ('USB thingy') provided.

Otherwise, unless you know the operating voltage tolerances that the thermostat has, how smooth the supply has to be, and whether the Nest Thermostat accepts ac or only DC, I would avoid using a non Nest power supply in case it causes damage.

 

[JC-DIYman]

I have followed these instructions but am not getting any heat or hot water. I believe I have Y plan and I removed the thermostat wires from the terminals on wiring block and wired the CH On from the programmer direct into connection 4 on the wiring centre that goes to the valve. My nest thermostat is connected and I set the temp so would call for heat but nothing. Set to Hot Water boost and although I can hear relay in boiler switching again nothing. Any help appreciated.

 

[stem]

There could be several reasons, those instructions were for the OP's installation, yours may have been wired differently. But at the moment we are working blind, if you could post some photo's it might help.

 

[JC-DIYman]

It appears that my boiler had tripped, once I pressed the reset all now appears to be working fine. However wouldn't mind you taking a look at my wiring setup for confirmation. Was expecting to find a link between 13 and 14 link the HWC to the pump but that connection must be made.