Stuck on Nest install.

[MB200]

Hi all. any help is gratefully received!

I installed a Nest (DIY) back in October but I have since realised that it was not working as expected.

It does not seem to be using the tank stat and hence I suspect that I am using a lot of gas. Not good at the moment.

It was also not firing the boiler for calls for heat, instead just pumping any hot water that was present due to a previous call for hot water.

I believe that I have a 1990's style S-Plan with no zone valves. Instead I have seperate pumps for H and HW. I also have a non-powered valve that diverts water to H or HW. Not quite sure how that works.

The tank is (I believe) a thermal store.

Pictures attached are all before I made any changes. I will then post pics of where I am at.

Don't reply yet - part 2 to follow.....

 

[ianmcd]

You dont have an S plan ,doubt if you will find a diagram for what you have

 

[MB200]

So, I have:

1) Used the old thermostat wires to power the thermostat via the HeatLink 12V outputs.
2) I could not find any permanent live in the boiler cupboard and hence used the redundant wires from the programmer (call for H and HW) to get live to get live and neutral to the boiler cupboard.
3) All other programmer wires are joined together using Weygos, in the same way that they were joined up previously.

I can't recall why I still have a blue wire from the room stat going in (over the S in Roomstat). If I remove this wire then I can hear the boiler clicking when there is a call for heat, but it does not fire.

I have for now run a wire from where the HeatLink call for H is wired, to the (empty in this pic) plug above where the HeatLink call for HW is.

I did that because calls for HW fire the boiler and this was the only way I could find to make a call for H fire the boiler. No doubt it is should not be required though.

I am not convinced that the tank stat is being powered properly, but not sure what to do about that.

I have clearly got something wrong

(The unused red wire (was never in use) and the now unused blue wire are both safely capped off with seperate Weygoes).

Happy to be called a numpty but I am stuck and would welcome your advice, please.

 

[MB200]

@Ian - I was told by somebody that I have an S-plan. Happy to be wrong.

As you say though - I can see videos for almost every combination but not mine.

I have seen a post saying that this config was common in the 1990's and is "really good". The boiler is great and has never failed (apart from the fan) and hence I am reluctant (and too poor) to get a new boiler etc.

 

[ianmcd]

I was told by somebody that I have an S-plan. Happy to be wrong.

No you have a thermal store, this is how it works when switched on the water from the boiler is circulated around the cylinder via 1 pump, when a demand for heating is requested the CH pump circulates this water around the radiators, you do not have a HW pump, when hot water is demanded there is a high heat gain inside the cylinder runs through the inside of this coil and runs from the tap or outlet that has opened, I have never wired a nest to your set up, give me ten mins and I will try and work out a way to do it

 

[MB200]

Thanks Ian - but see the attached (respectfully). The red pump is labelled "heating" and the blue pump definitely activates when HW is requested. I believe this is why we have such a powerful shower (which was one of the selling points of the house when we bought it all those years ago).

 

[MB200]

That's why I was treating it as an S-Plan.

 

[ianmcd]

believe this is why we have such a powerful shower (

you have a powerfull shower because it is mains fed and blends, the pump comes on because the stat in the cylinder detects a temp drop and pumps HW from the primary water from the cylinder back to the boiler keep the primary water hot the pump is not pumping the water that comes out of your hw shower or tap, you should wire the room the existing room thermostat to the nest as a single channel ane leave the rest of the wiring as is, the Cylinder thermostat on that does not directly control the temp of the water inthe taps but in the core of the cylinder , the temp of the HW coming out of the taps is controlled by the blending valve on the pipework

 

[MB200]

Thanks Ian.

I sort of knew that about the water pump. The power shower worked because the hot water works so well, rather than pumping the actual water...

So are you saying that I should have both the HeatLink call for HW and H go to the wiring centre tank stat connection? That's what I have effectively had (I think) but the thermostat was being ignored and also. Calling hot water caused the radiators to heat up. That never used to be the case.

Also the boiler fires for the duration of the call for water- must be costing me a lot.

The call for HW and call for H used to be connected separately, pre-Nest. i.e. yellow and the black wires on original diagram?

(Looking to understand, rather than challenge your clearly superior knowledge ).

 

[ianmcd]

when you have CH calling the CH pump runs to circulate the water from the cylinder to the radiators, when you run the hw the CH pump stops, when the hw demand stops, the ch pump doesnt start again until the store is back to temp (Cyl stat) then the ch pump will kick in again, the cyl stat acts as a changeover, it doesnt work like a standard cyl stat does

 

[ianmcd]

Had a think about it and should be quite easy

Horstman terminal 3 to nest 3
Horstman terminal 4 to nest 6
N&L to nest and L&2&5 linked
the wires in centaurplus 1&2 join together isolate or totally remove all the other wires
let me know how you get on and dont mess about with the wires inside the wiring centre

 

[ericmark]

I would think some where it needs relays. You need to power pump and central heating boiler independently can't use same contacts or since the boiler will otherwise connect both pumps together.

I have same with my system. Motorised valves also act as relays.

 

[ianmcd]

I would think some where it needs relays. You need to power pump and central heating boiler independently can't use same contacts or since the boiler will otherwise connect both pumps together.

I have same with my system. Motorised valves also act as relays.

no relay not required, this is not a normal system and cant use motorised valves

 

[ericmark]

no relay not required, this is not a normal system and cant use motorised valves

Oh I wish it was that easy, we in the main relate to our own systems, mine is a C Plan, which had two pumps, and no motorised valves, some one clearly thought if they select a pump, only circulation will be the pipes and radiators connected to that pump, wrong, the other system will also have flow in the reverse direction.

OK non return valves could do the same as the motorised valves, if the pipes are small enough, even 15 mm pipes can have circulation inside the pipe, a single 22 mm pipe can heat up the DHW even when the stop cock is closed on the other pipe, depends on distance, but thermo syphon is hard to predict.

As an electrician I have been called to so many systems where clearly the installer had not expected some thing to happen. Or has not configured the system correctly, the "I have turned off DHW but it is still getting hot." when the installer has failed to turn the selector or micro switch, so user can select CH only, but that was never an option.

As to rooms always cold, where the installer has not set the lock shield valves? But to be fair, as the electrician we have no idea if the installer really left it like that, or the home owner fiddled after he left.

I would say in the main a miss understanding as to what the installer considered his job was. I know many jobs where I followed a plan, and the commissioning engineer set it up. So the plumber installs the pipes, the electrician installs the wiring, and the commissioning engineer sets it all up, but there was no commissioning engineer, it was left to the owner to work it all out.