Replacing complex Horstmann C21 & Drayton Digistat+ with Nest V3

[will3h]

Hi,

I have what seems to be a complex system with a Worcester Greenstar RI condensing boiler, a hot water tank with two pumps for HW and CH, a Horstmann C21 programmer, linked to a Drayton Digistat+ wireless thermostat.
I'd like to replace them with a Nest V3, but I discovered a nightmare of cable when opening the backplate behind the programmer.

First, the regular backplate of the C21 programmer seemed simple, but I was puzzled by the fact it was thin (12v like) wires and not thick, 220V ones :


Then I opened the plate behind this backplate, and found this :

Which lost me a bit.
The white cable above the rest links to the Drayton SCR thermostat receiver, but I don't get what is the PCB circuit board doing inside?

Basically, I thought I would just link the two small red wires together on the left hand side to get rid of the thermostat, and use the backplate wiring to connect to the nest? Am I right or am I missing something?
Does this system talks to anyone?

Thanks a lot for your help, if anyone can shine a light on what does this spaghetti of internal wiring?

Will

 

[Echo the husky]

That sounds fine, the PCB appears to be an overrun timer for a pump, but it looks as if it may not be in use.

 

[MeldrewsMate]

The white cable will carry 240vac to the SCR receiver, this can be used to power the Nest receiver. The Nest receiver will replace the SCR.
The heating output of the Nest will go to the terminal that the SCR output previously went to, and it's HW output to the terminal that the old timer's HW went to.
Thus the SCR and old programmer are discarded.
If you can, run a SEPARATE 2 core cable fron the Nest (T1 & T2) terminals to the Nest thermostat, then you won't need to use that unsightly USB lead to power it.

PS The main drawback IMO of the Nest are the fiddly and small terminals. You'll be glad of those 0.5mm2 cores when you try to get 2 cores into one terminal!
PPS Mains cable does not have to be larger than 12v cable, eg the battery cables on a car are 12v, n'est pas? It's the current carried that mostly determines cable size, so check (and replace as necessary) there's a 3A fuse in the mains supply to the heating system.

MM

 

[will3h]

Thanks for confirming all this! Seems it’s all good to proceed then.

Just a thought however: currently, the way the system is designed, I need to have HW programmed at the same time as CH for the radiators to be hot. (Turning CH on only will not trigger the boiler, only the pump that will just flow the water around from the tank to the radiators).
Will the Nest be able to have a different behaviour and trigger HW when CH is needed?

Thanks a lot

 

[D_Hailsham]

Just a thought however: currently, the way the system is designed, I need to have HW programmed at the same time as CH for the radiators to be hot. (Turning CH on only will not trigger the boiler, only the pump that will just flow the water around from the tank to the radiators).

It's a pity you didn't mention this at the start.

You have a semi-pumped system, aka pumped heating/gravity hot water system.

Will the Nest be able to have a different behaviour and trigger HW when CH is needed?

Unfortunately the Nest cannot do what you require.

The solution is to convert the system to fully pumped, which involves piping changes and the installation of one (Y plan) or two (S Plan) motorised valves.

 

[will3h]

You’re right I should have led with that. Sorry.

My understanding would be that, just like the current system, in the winter I should put the hot water on always on and the tankstat will regulate the need for the boiler to kick in. It’s just that I find it not energy efficient (nor money-saving).

I guess doing the same with nest could work, even if it’s not optimal, but will best be smart enough not to start the HW if no one’s home? (i.e: if CH is not triggered as well?)

 

[will3h]

After careful review, what I actually have and mistook for a HW tank is actually a Flowmax Thermal Store with a 2 pump system.
So basically the pump controlling hot water is a closed circuit with the boiler, and the CH pump circulates it to the radiators (hence, taking it from the mains).

I couldn't find any reference to having a smart thermostat such as Nest with such a setup... Is it possible? Advisable? And what would be the best option to overcome the limitation of having to fire up the boiler all day to maintain the Thermal Store to temperature? (in the summer, firing up the boiler an hour in the morning and one hour in the evening is more than enough to provide us with hot water on taps and showers all day). But now that winter's here, clearly not enough for bringing the radiators to the thermostat level without leaving the HW taps cold. And fiddling with the timer to progressively extend the boiler hours of operation depending on the outside temperature every week hardly seems a convenient solution.

 

[D_Hailsham]

My understanding would be that, just like the current system, in the winter I should put the hot water on always on and the tankstat will regulate the need for the boiler to kick in.

If you do that the house will only heat up when the HW cylinder needs to be heated

After careful review, what I actually have and mistook for a HW tank is actually a Flowmax Thermal Store with a 2 pump system.
So basically the pump controlling hot water is a closed circuit with the boiler, and the CH pump circulates it to the radiators (hence, taking it from the mains).

The hot tap water comes from mains, not the rad water.

Here is a pdf for the Flowmaster, which, hopefully, explains everything.

 

[will3h]

Thanks! I had found the notice but doesn't really help to figure out if I can improve the current controls with a Nest?

If you do that the house will only heat up when the HW cylinder needs to be heated

That is what I have been doing so far, and actually what it does is this: the boiler heats up the tank until the optimum temperature is reached and the tankstat makes it stop. But, meanwhile, the radiators are hot and working. Obviously, if it's cold outside the tank temperature decreases and the boilers kicks in again.. But when the tank is warm and if I disable the HW/boiler, the radiators will still get warm until they've used up all of the tank water (usually one hour depending on outside temp).

As for the summer, as I said, I just heat the tank a couple hours a day and it keeps it warm enough to use the mains/taps for our usage.

It just that it seems a waste to keep the tank fully heated at all times depending on when the room thermostat will decide it's time for the radiators to heat up?

So what's the best alternative?

 

[D_Hailsham]

So what's the best alternative?

Get rid of the thermal store and replace it with an unvented cylinder. It's expensive to run - as you have found out.

The TS requires the boiler to run at maximum temperature which was OK if you had an old, non condensing boiler, designed to run at 80C. Modern condensing boilers, however, are designed to run at temperatures of 70C or lower. This is so the boiler runs in condensing mode, which only happens if the return temperature is below 55C.

 

[will3h]

Thanks D_Hailsham.

That's probably what I should do in the long run. But it's a large job.
In the meantime, I wanted to try to mitigate the headache I have with this system, and replacing the antiquated controls with a smart thermostat seemed like a place to start, first to avoid going back and forth to the boiler room to launch a HW boost every time the radiators are mildly hot because the HW timer is off, and/or, on the opposite, be able to remotely turn the CH or HW off if I forgot to switch them off before leaving the house. Hence the use of the Nest or similar.

Looking at the v3 doc I could probably achieve what is done today: having a HW timer that I will extend progressively as we enter deeper into the winter, and CH determined by the thermostat, I guess it's not as ideal but it would be a start.
Or is there a wiring scheme that would always trigger HW when CH is requested by the thermostat? An additional relay in parallel for instance, because the way I see it, having CH running by itself without the boiler (if of course Tank temp < threshold) is useless.

I've made a drawing of what it would look like if I was replacing the current system like for like. Does it looks right to you?


Thanks a lot for your pieces of advice!

 

[will3h]

Also, it looks like people with a similar setup to mine are using Hive, which supports gravity fed systems. Is it preferable, or only due to the fact that Nest didn't support HW until the v3 earlier this year?

 

[Echo the husky]

Nest can support gravity hot water but the connections need to be wired a bit arse about face with extra links and so on. One of the problems with nest is that it is an American product and doesn't fully take into account how heating systems are wired here. Hive is more UK based, hence using the 'universal' backplate.

 

[will3h]

mmm... so I have the choice between Hive that is almost guaranteed to work with my system, but not as widely used and recognised as Nest, that will be more complicated to fit on my gravity system... what to do, what to do...

 

[will3h]

Nest can support gravity hot water but the connections need to be wired a bit arse about face with extra links and so on.

I couldn’t find any wiring plan or details about installing a nest on a gravity/semi pumped. Would you have any docs on this by any chance?