Nest + Drayton LP822/RTS1 & Potterton Promax

[tektonnic]

Hi, apologies in advance if you think this has been covered elsewhere, I promise I've searched here as well as more generally but I'm stumped...

Picked up a Nest v3 in sales, aim to replace a Drayton LP822 & RTS 1.

Understand how the heat link interacts with the boiler, was tossing up putting it in place of the existing LP822 but then thinking about the 12v requirement to the room stat I thought I'd put it by the boiler and re-use the existing stats wires...except I can't work out where the heck the existing stat is wired! I've attached photos of the boiler block + the back plates for the LP822 & RTS1...I cannot work out where that RTS wiring is going.

At the block you have power to the boiler, 3-core heading off to a frost protection stat in the garage and 5 core to the LP822. At the LP822 there are no wires heading away from it to anywhere but the boiler - anyone care to point out what is undoubtedly obvious?

 

[stem]

From the boiler wiring photo, it appears that you have the combi version of the Promax; yet the Drayton LP822 programmer is a dual channel programmer designed to control central heating and stored hot water (ie a system with a hot water cylinder) ...so a bit unusual to find a connection to the LP822 terminal (3) which is there to provide control of stored hot water, on a combi system.

If you don't have a hot water cylinder, probably the easiest solution would be to remove the existing wiring going to the boiler. (with the exception of the 230V supply and the frost stat) Mount the Heat link next to it and wire it straight in.

The top terminal (with the wire bk 1 in) goes to the Heatlink terminal 'Heating Call for Heat' (3)
The next terminal (with the wire bk 2 in) goes to the Heatlink terminal 'Common' (2)
The N & L terminals connect to the Heat link N & L respectively

The frost thermostat and 230V wires stay as they are.

You should only do this though if you can access your boilers wiring terminals without having to open a room sealed compartment. If it is necessary to open up a room sealed compartment to access the wiring terminals, this should only be done by a Gas Safe engineer who can ensure it is properly sealed up again afterwards.

If you do have a hot water cylinder there will be other controls and a motorised valve / valves elsewhere, in which case more work would be involved and you would need to provide details of them for more advice.

Where the existing thermostat cable goes is impossible to determine over a forum, there could be lots of scenarios. There may be a junction box somewhere, or a wiring centre, it may have been moved or reconfigured from an earlier boiler. I found one installation where the room thermostat wasn't even connected anymore and the householder hadn't noticed because all of their radiators had had TRV's fitted. In you can't locate the other end, you may have to physically trace the cable on site, so it may be easier to run a new cable for T1 & T2.

 

[tektonnic]

Oh jeez, I do have a hot water cylinder and it appears I might have been a little naive thinking I could do this myself...what do I need to work out from the water cylinder to integrate it? Thank you for taking the time to reply.

BTW, it's a brand new house in case that helps guide any schools of thought...

 

[stem]

Then that changes things. The thermostat(s) will not be wired to the boiler. they will be wired to motorised valve(s)

You say it is a "brand new house". New houses since 2010 should have more that one heating zone, usually one for upstairs and one for downstairs with separate controls. In which case, you would have a multi zone system and need two Nest thermostats and Heat links.

Without full details of all of the existing heating components (programmer, thermostats, motorised valves etc.,) and the system as a whole, it is impossible to advise further.

In a few multi zone cases I have worked on, almost complete rewiring of the heating has been required to fit Nest, as the existing wiring can be somewhat distributed. For example, the hot water control may be in the airing cupboard and the heating control in the kitchen. With Nest they need to be wired to the same location. These can be difficult job for a professional. So if you are not confident and have a good understanding of how the various components of your heating system work and relate to each other, then this will probably really be a job for a professional.

It's slightly unusual (but not unknown) for a combi boiler to supply hot water to just the kitchen perhaps, and the bathroom(s) are connected to the hot water cylinder.

 

[tektonnic]

OK we certainly don't have two sets of controls, however it's just nudged a thought that is the rads in the area where the control is have no controls, all other rads are TRV...I'm guessing that might count as multi-zone but without two distinct sets of controls? I'd not need multiple Nests in that case would I?

Agreed on the pro comment though, hoped I could do this myself but it's the wrong time of year to break your own heating system because you wanted to 'learn by doing'...

 

[stem]

Most systems from even before 2010 have TRV's on the radiators except in the rooms where a wired room thermostat is physically located.

However, For properties built since 2010 the building regulations require at least two separate room thermostats, each with a dedicated motorised valve. (This is in addition to TRV's on the other radiators).....and as your home is "brand new"........ Are you sure there isn't one behind a wardrobe or something?

The number of motorised valves you have will also provide a clue. There will be one for each heating zone, plus one for the hot water cylinder.

If you do only have one zone and hot water you will only need one Nest. The wires from the existing programmer will go to the terminals at the Nest Heat link that have the same function, and the room thermostat should be either set to, and left on its maximum setting, or decommissioned properly so that the integrity of the heating circuit is maintained and not left 'open circuit'

 

[tektonnic]

House was built in 2016, no obscured room thermostats and I've just investigated the motorised valve situation and there are just two - so my house wasn't built to code I guess...good news on the not having to invest in more Nests though I suppose, and also the relative simplicity of installing the heatlink where the existing programmer is.

I'm going to do my best to trace the room stat wires, I'd like to send 12v down those wires and stick the nest thermostat bit there.

 

[tektonnic]

Meant to include this photo, it's of one of the motorised valves and the cylinder stat...unless I've got it entirely wrong.

 

[stem]

That's indeed what they are. Looks like the motorised valve for the hot water control, there will be another one (or two) for the heating

 

[tektonnic]

Two sighted (only two!)! Unless one is hidden in that junction box!

Are there any issues with them potentially not doing the house to code?

 

[stem]

Looks like you do only have one heating zone and hot water. (Don't know how it got past the building inspector, or what the implications are though.) So, one Nest will suffice.

The boiler wiring won't change. No need to touch anything in there.

The Nest Heat Link replaces the Drayton LP822. The wires are moved from the terminals at the LP822 to the Nest Heat link terminals that have the same function.

Here are the Drayton terminals...

...and here is the Heat link

(N) & (L) are self explanatory
Drayton (3) 'HW ON' = Heat link (6) 'Hot Water Call For Heat'
Drayton (4) 'CH ON' = Heat link (3) 'Heating Call For Heat'
Then 'Common' terminals (2) and (5) need connecting to the (L) terminal with wire links
Earth wire goes in the earth terminal

Then trace the cable from the existing RTS room thermostat.....

.....back to it's origin. When you find the other end, note where the brown (L) and black (3) wires are connected. Then disconnect the entire cable and insert a link between the two terminals where you have just removed the brown and black wires from, take care not to disturb the other wires. The old thermostat can now be removed.

If you want to put the Nest Thermostat in this location, if you wish, you may reuse the old disconnected thermostat cable to connect the Nest Thermostat to the Heat link T1 & T2 terminals to power it. Otherwise the Thermostat can be powered by a separate plug in power supply.

 

[tektonnic]

Well that is super comprehensive and useful, thank you for all your time and help.

 

[Mikrosoftuk]

This helped me install my nest but one thing I noticed is the gas engineer came today to service boiler and asked me to put the heating on which I did and he came back saying it’s not on as the blue light isn’t on on the boiler. But the heating is working ? Any idea why, the hot water comes on with the blue light ok.

 

[stem]

Nest is just an on/off switch, so it's switching the heating on as it should. I've no idea why the blue light doesn't come on, but it sounds more like a fault with the boiler not the Nest.

Ideally you should have started your own new thread. Tagging your question on to someone else's is known as Hijacking and is against the 'forum rules,

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I suggest you start your own new thread in the plumbing and central heating system and ask the question there, you should give details about the make and model of your boiler so one of the guys familiar with it will hopefully be able to assist you.

Plus a new thread is likely to be seen more readily than an new post buried away at the bottom of an old thread.