Nest learning thermostat gen 3 query

[Dorr69]

Hi folks
I’ve just purchased the above thermostat and decided to go all modern
I have a two year old oil fired boiler with separate hot water tank.
There is a standard Drayton 7 day programmer on the wall.
The boiler has two wall plugs and sockets on the wall one 3 pin std , one 3 pin round . One is pump one is boiler. The boiler was controlled by the programmer until 6 years ago when a thermostat was installed interrupting the wire from the controller to the plug on the boiler.
I intended to install the nest heat link in the boiler room , and replicate the wiring in the programmer but use a junction box to make the job easier and tidier.
I take it all I do is run a n & l to the heat link from the old timer and join live link 2 and 5 then use 2 and 4 from the hw and ch ?
There is no zone valve , seems that to heat the water tank the pump is not switched on so itself turns into a zone valve and water is heated by gravity. Not great I know but I intend getting a zone valve fitted so I can heat the hot water pumped.
To turn my system into a full s plan, all I need to do is add a zone valve and a tank stat ?
Sorry for the long thread but my head is buzzing trying to work this out

 

[ericmark]

It would seem you have an old C plan as shown, there were 10 options with the time controller rather than normal 16 and there were mechanical and electrical switches inside the old time controller so you could not select central heating without hot water.

You can add a motorised valve and cylinder stat so you can get the full 16 options, as to how this works with Nest I don't know? There are 2 motorised valves with S plan, there is also the W plan which gives a priority, the latter can be a two twin port valves or a single three port valve, and the Y plan uses a three position three port valve.

I note Nest instructions give S and Y plan, I see no reason why it would not work with a C plan, once motorised valve and cylinder stat added, however there was a problem with C plan with some houses, once the pump ran, upstairs would continue to heat up using thermal syphon, I seem to remember there was a way to stop it, but I am an electrician not a plumber.

 

[Dorr69]

My controller is a Drayton LP522
Two wires in, L&N and two red wires on terminals 3&4
That’s it. Before the thermostat was wired in that was the only way to control the boiler.
How ever I have noticed that when the ch is selected on the controller, the pump is on wether the thermostat is turned up or down
Must’ve been the easiest option when they wired the old boiler. When they installed the boiler 2 years ago they just installed it like for like

 

[ericmark]

Step one is to find instructions for old timer the instructions say

The programmer must be set to the type of system it is to control.
The link on the back of the unit has two settings – P and G.
P = a fully pumped and controlled system – allowing heating and hot water to be set separately.
G = a gravity-fed hot water system – which does not allow independent setting of heating without hot water, though hot water can be controlled alone.

I will guess yours is set to G? I don't see any similar instructions for Nest, that does not mean Nest would not work, however the program may let you select central heating only but you will not get central heating only.
I would hope some one else would step in here as I don't know all the dos and don'ts, as with an always open connection to the domestic water heating hot coil the boiler can naturally cool on switch off, however once you fit a motorised valve then it may need wiring so it can cool before switch off, much depends on boiler and how wired.

Years ago one fitted the thermostat into boiler supply, but today there is normally a set of connections inside the boiler going to time clock or thermostat so it does not remove all power to boiler it just tells it to switch off and allows it to cool first.

I am an electrician not a heating engineer and will be first to admit I don't know what will and what will not damage a boiler, and I have done very little with oil fired, but boilers have moved on since the early days, with most now being more efficient, and to do that they use the latent heat of evaporation, they allow the gases from combustion to cool to a point below boiling point, for this to work the return water from central heating must be cool enough, so the boiler has to be able to modulate its output, so instead of a fixed 28 kW boiler, today we have a 6 kW to 28 kW boiler and the flame height reduces as the return water gets hotter.

This change means having thermostatic radiator valves (TRV) is essential, or something similar, and we have things like by-pass valves to ensure water can still circulate even if all valves are closed.

From what I believe you can replace an old inefficient boiler with another inefficient boiler, however finding one is some thing else. So you need to find out how your boiler can be wired, and used, to just jump in without checking could cause problems.

I looked at what floor boards etc needed lifting to fit better controls and took the lazy way out, I simply fitted electronic TRV heads so each room is more accurately controlled but I did not have to install any new wiring, I got MiHome heads which will work with Nest with idea of adding Nest latter, however once set up they work well enough so never bothered fitting Nest. The problem to my mind with Nest is getting a 5 volt supply to the thermostat, and originally I thought I could connect Nest to boiler eBUS but found boiler not suitable.

Normally some one jumps in to tell me I have got it all wrong, so maybe you will get some better advice, so may be worth waiting a little longer.

 

[Dorr69]

Thanks for your reply Eric
I’ll have a look tonight at the back of the timer but I’d say it’ll be on g for gravity. As far as I can make out it for either hw or ch it turns the boiler on and for hw only it emits the pump
The old boiler and set up is way over 25-30 years old. I think I’ll get it uprated next year to a full s type system with pumped hw system, tank stat and zone valves. Apparently it’s much more efficient.
Thanks for your help