Nest Thermostat and my ageing system

[Tom Waller]

Hi all.

I was hoping someone might be able to point me in the right direction with regard to using a Nest Learning Thermostat v3 on my current old boiler system.

I currently have an old Potterton Profile 80e boiler which touch wood is running perfectly. I have lived in my house for 4 years and it has not failed once. Seems to be very reliable.

The system type I have is gravity fed hot water with pumped central heating. It is currently controlled by an even older Danfoss Randall 103 timer and an even older still room stat.

I understand from a plumber that inspected my system that I have some valve (can't remember the name - something like a non-return valve?) which prevents the rads from getting hot when the hot water is called. This has worked fine in the past. The timer has two times of day I can call hot water (I do morning and evening for a couple of hours which seems to heat the water enough for my use). I leave the setting on 'continuous' normally with the stat on my desired temperature which seems to run nicely.

Now - my question is - can I swap this out for a Nest v3? I purchased one with the stand (so I can stick it somewhere else off the wall). I understand from my research here that I may be able to do so with limited support for fine control of the hot water due to no motorised valves being present and no cylinder stat. Not too much of a problem for now, I intend on replacing the system with a new setup within the next 1-2 years. This is more about me being able to remotely control and tune my heating when I suddenly have to travel for work or I'm coming home from somewhere and want the house heated on my return.

Any advice would be much appreciated.

 

[ianmcd]

Yes the nest has a setting for gravity HW, easy to install on your system and alter when you upgrade

 

[ericmark]

Since Nest is volt free, just down to how wired.

 

[Tom Waller]

Thank you both for the responses so far. This is definitely seeming positive. I guess my main concern was whether the fact I didn't have a motorised valve would be a problem but it seems like I can still make use of the Nest.

I've got a couple of spanners to throw in...

I'm assuming I won't get granular control of hot water? For instance, since I have no cylinder stat, if the heating is on, it will be constantly heating the water. If this is true, apart from inefficiency, is there anything I need to worry about?

And also, when the builders moved the timer and stat, something failed. The boiler seems to be working as when I have the timer on constant and flick the power on the boiler heats the water and the rads. I think either the timer or stat has died in the move as the electrician seems to have wired everything as it was previously. Everything is hanging off a 3amp fused box, the wiring center. Given your experiences, is replacing the timer with the Nest Heat Link something an enthusiast could attempt or should I get a boiler engineer to get the system up and running with a dumb replacement timer and stat before attempting the switch?

Thanks again. Tom.

 

[picasso]

Now would be a good time to upgrade your heating and hot water , convert your system to fully pumped with a cylinder stat and the nest would be a good start,

 

[Tom Waller]

Thanks @picasso - I'm awaiting a couple of quotes to upgrade (and re-site) to a fully pumped system but I will likely have to wait till the end of the year to be able to budget for the upgrade. This is more of a stop gap until I can get the upgrade done, and since I already own the Nest and have a defunct timer and stat, figured it could get me out of a bind.

 

[ianmcd]

It is very simple to do but the diagram above is wrong so dont try and wire it like that

 

[picasso]

It is very simple to do but the diagram above is wrong so dont try and wire it like that

Whats wrong with it ? looks ok to me .

 

[ianmcd]

Whats wrong with it ? looks ok to me .

should be 6 to SWL on boiler and 3 to pump, no link between 3&4 link needed between L 2 &5 it is Gravity HW

 

[picasso]

Have to agree to disagree on this one

and wouldn't your way run the pump when ch is on but not turn on the boiler ?

 

[ianmcd]

Have to agree to disagree on this one

and wouldn't your way run the pump when ch is on but not turn on the boiler ?

No that is why you have to tell the nest that it is Gravity HW, the CH can not be selected on its own the HW turns the boiler on and the CH operates the pump, the HW is always on even if only CH is selected, and the temp of the HW is controlled by the boiler stat, of course this can be changed but will need additional controls fitted

 

[picasso]

So how do you tell the nest its gravity hot water ?

 

[Tom Waller]

It is very simple to do but the diagram above is wrong so dont try and wire it like that

What's wrong with the diagram? Is there a corrected version you could share maybe?

 

[ianmcd]

So how do you tell the nest its gravity hot water ?

It is in the settings menu when you set the nest up, can be changed if you upgrade the controls at a later date, gravity HW is exactly that, the boiler comes on and the HW circulates through a large coil in the HW cylinder through gravity, no pump is used , when the CH is asked for, the pump comes on and circulates water through the heating system, but the gravity HW will still be on, you can not get CH on its own with a gravity HW system, many people mistake what a grvity HW system is, as opposed to an open vent system, they are not the same thing, @picasso , this is for others reading this , I know you will already know this but it is the open forum, and is advice to those that mistake open vented systems from Gravity HW systems

 

[ericmark]

For me with an oil fired boiler, to alter I am told I need a new boiler as there is no cool down sequence, it relies on thermo syphon to cool the boiler down, but the C Plan does have three versions.
1) Only control for DHW in winter is how hot circulating water is, in summer how long you heat the water for.
2) Adding a cistern thermostat and in summer the boiler only fires up when required, and water temperature is controlled, but winter no change.
3) Adding a motorised valve you can control DHW temperature summer and winter, but boiler loses the ability to cool down.
Changing to the Y Plan means default is DHW so boiler can still cool down with thermo syphon, enough water will travel through the pump, and the increased flow of water through the hot coil means the DHW heats up faster.

The S Plan means no cooling of boiler.

If you change boiler to one which has a post burn cool down stage built in, then the S Plan is good, and with gas boilers condensing boilers CAN save you money, however to work the radiators need to be larger as the return water temperature becomes important, and there is a debate on how many modulating gas boilers are installed in a way where they actually gain the latent heat.

There does not seem to be any single manufacturer who makes a complete system that will work efficiently, you end up with a hopscotch of parts which often don't really work together.

Nest and Energenie both have the wall thermostat telling the TRV what to do, well Nest has withdrawn support, so as it stands Nest does not have any TRV which is designed to work with it, and in my old open plan house Nest would work fine, as a single thermostat can control whole house. This house however has internal doors on the rooms, many houses do, and so each room is heated independently, all the Nest thermostat does is control when the boiler fires, it does not really control room temperatures, I do not class the hall as a room. With oil fired boiler which does not modulate that is near enough, not perfect, but it will do, but if I went for LPG, then likely I would need to change all my radiators and TRV heads as well, to gain the advantage of the latent heat and modulating boiler.