Nest Learning Thermostat 3rd Gen - C Plan Heating System? Help!

[djsmith83]

Hi all,

Absolute and utter amateur here, pretty clueless when comes to plumbing and heating, need to learn! Moved in to my house about a year ago and at the moment we don't have a thermostat at all, we have a late 90s Glow Worm boiler and a immersion tank in the airing cupboard and only an old crude analog controller for turning on the heating and hot water - see attached image.

I have nest cameras in the house and have purchased a nest thermostat with a "nest pro" (a contractor from local plumbing company) to do the install. He turned up yesterday and said Nest Stat can't be installed because "Nest installation C Plan system with gravity hot water so nest thermostat is not compatible".

I've done some classic googling and read on forums on here that it is possible but way out of my area of knowledge. Had another plumber out and they've advised on replacing the whole heating system with a new combi system, for around 3K - not helpful as heating and hot water actually works fine at the moment. Plumber did say may be able to upgrade to a Y plan but needs to come back to investigate further and could be up to £500 to upgrade it. I understand that the issue is because we only have a 2 way valve in the airing cupboard and our heating cannot be on without the water being on also.

I basically want to know this: is the Nest thermostat really not compatible with a C Plan system with gravity hot water? If it isn't, where does that leave me with other smart stats like the Hive and even a standard cheap thermostat? I might end up giving in and getting a cheap battery operated stat and a new controller otherwise. Any help appreciated.

Thanks!

 

[stem]

First of all, there is a bit of a problem with your information.

Below is a wiring diagram for a C-Plan heating system. As you will see there are room and cylinder thermostats plus a motorised valve to control the hot water. With a C -Plan, the 2-port valve is controlled by a thermostat on the hot water cylinder to allow the facility to have the heating 'on' without the hot water being 'on'. The valve is not the cause of the restriction as you suggest, in fact just the opposite, it's there to allow independent control of the central heating, without the hot water operating at the same time.

But you say.

at the moment we don't have a thermostat at all, we have a late 90s Glow Worm boiler and a immersion tank in the airing cupboard and only an old crude analog controller for turning on the heating and hot water.......we only have a 2 way valve in the airing cupboard

So something doesn't add up. If you have a 2-port valve but no hot water cylinder thermostat, what is controlling it?

If you do have a genuine 'C-Plan' with a cylinder thermostat and a suitable 2-port motorised valve, wired as above, then Nest will work with it OK.

Even if you have a gravity system without any thermostats or motorised valve, whilst Nest doesn't have a gravity setting, it can still be used, but the wiring will be different from the instructions that come with it. And in this case, due to the limitations of the plumbing, the central heating won't work alone and the hot water will have to come 'on' with it.

IMHO your money would be better spent on upgrading it to a Y-Plan as was suggested. You will then have totally separate control of the heating and hot water times and temperatures.

New condensing boilers cannot be connected to gravity systems. So, the plumbing will have to be upgraded when you do replace your boiler anyway.

 

[djsmith83]

Thanks @stem, really useful information and I think I understand what you’re saying which is amazing considering I’m crap at this.

I think we do have have a hot water cylinder thermostat - this is it right next to the cylinder in the airing cupboard? See attached. If this is the case, why have I been told that the nest thermostat won’t work?

See pic of valve attached also (at least I think it’s the valve!)

 

[stem]

The photos shows a timeswitch that is connected to an electric immersion heating element fitted inside the hot water cylinder, these are not connected in anyway to the boiler.

As an illustration, in your lounge, there will likely be a radiator connected to the central heating to heat the room. If you were to bring in an electric fire and plug it in, it would also heat the room, but it is in no way connected to the central heating. It is totally separate.

The same is true with the hot water cylinder, it is connected to the boiler and the boiler will heat the hot water inside it, but the electric immersion heater, whilst it will also heat the hot water, like the electric fire, it is not connected to the boiler in anyway, it is totally separate.

A hot water cylinder thermostat of the type used in conjunction with a boiler looks like this. It is strapped to the cylinder about one third of the way up from the bottom. I don't see one in your photo.

Can you post a picture of the 2 way valve you refer to.

I understand that the issue is because we only have a 2 way valve in the airing cupboard and our heating cannot be on without the water being on also.

 

[djsmith83]

@stem yeah, there is no hot water cylinder thermostat like in the picture you posted.

I was told that our valve was only 2 way - I don’t know for sure myself. Is it one of these?

 

[stem]

A 2 port motorised zone valve is visible in the middle picture.

It's a Honeywell, like the one below.

Untitled

• stem
• 26 Mar 2010
• 6

But from the photos it's impossible for me to see what it's connected to, or indeed what is actually controlling it.

With a C Plan, it should be wired to a cylinder thermostat as per the picture in my previous post and normally fitted in the top (flow) pipe to the hot water cylinder like this. It is possible, but not usual to fit it in the bottom (return) pipe.

Unfortunately, I can't see enough details from your photos to see how the pipes are connected to the pump, boiler, cylinder and motorised valve to ascertain what's actually going on.

 

[Wildcat1961]

Certainly not a ‘C’ plan looking @ those photos.

 

[djsmith83]

@stem amazing info - thanks. From what I can see there definitely isn’t a cylinder thermostat anywhere unless it’s on the back of the cylinder out of view which wouldn’t make sense right?

I’ve attached some more photos in case they help - I doubt they will though. I can’t see where the connection from the valve is going to, it’s goes underneath the cylinder.

Just to be clear, at the moment I just want to use the Nest stat the same as we do the old controller, so heating and hot water on at same time or just hot water in its own. We just need a thermostat downstairs to turn heating on or off automatically depending on room temperature and we want to have an on/off schedule and option to turn on/off from phones remotely. That’s it.

Thanks!

 

[Wildcat1961]

The system you have is fully pumped.
The ‘C’ plan uses gravity fed primaries (HW) with pumped heating.

 

[Wildcat1961]

With a C Plan, it should be wired to a cylinder thermostat as per the picture in my previous post and normally fitted in the top (flow) pipe to the hot water cylinder like this. It is possible, but not usual to fit it in the bottom (return) pipe.

View attachment 138099

The motorised valve must also be installed in a position so as to leave an unrestricted path for the open safety vent.

 

[djsmith83]

@Wildcat1961 so ignoring all the technical stuff as I’m a complete plumbing amateur, will this work with a nest stat?

The “nest pro” said it was a gravity fed system

 

[Wildcat1961]

@Wildcat1961 so ignoring all the technical stuff as I’m a complete plumbing amateur, will this work with a nest stat?

An additional two port motorised valve should be fitted to the Primary flow to cylinder along with a cylinder stat.
The system will then be upgraded to an ‘S’ plan.

 

[djsmith83]

Thanks @Wildcat1961

What are your thoughts on this @stem ? Could it still work with nest as I described we want to use it or best to upgrade? I was told ‘Y plan by plumber that visited, not S.

 

[Wildcat1961]

What are your thoughts on this @stem

I’ll leave it to the professional (Stem) to answer any questions you may have, hopefully he may confirm the advice given.

 

[stem]

Yes if you really must. You will be able to use a Nest to control it. If it is gravity hot water system without any thermostatic control, you won't get all of the functions Nest can provide and you keep the inefficient system that you have. Personally I would convert it to a Y-Plan and forgo the Nest, but it's your choice.

As Wildcat1961 says it doesn't look like a C-Plan, it looks like a gravity hot water system with pumped central heating......except but the problem I have is what is the motorised valve doing? and what's operating it? Motorised valves are usually operated by a thermostat, so where is the thermostat? Perhaps it was connected to one in the past, who knows? This will still need to be bottomed before fitting the Nest.

As the motorised valve is in the pipe from the pump it may actually be in the central heating circuit. It seems to be connected to the junction box. djsmith83, can you take the top off the junction box and take a photo of the wiring inside? also can you check to see if the lever on the side of the motorised valve is in the 'Auto' or 'Manual' position. Maybe it's no longer being used and has been fixed manually open. Alternatively it may be wired to the pump and is just being used to stop gravity circulation of water around the radiators when heating is not required.