Control question: New CH System; Large house, S-Plan Plus, 1 or 2 boilers.

[willrook]

Hi.
Wondered if you guys with a lot of knowledge could help. recently moved into a 3 storey (plus large basement) victorian house. installing a new CH/DHW system, and working out the best way to do it. Most heating/plumbing companies seem to know even less than me about evohome so could anybody offer advice on this.

Specifications are
CH and DHW demand equates to roughly 59-60kw - Can be met with either a single Light Commercial boiler (eg. Worcester GB162) or 2 smaller Domestic system boilers.
Pipework; too much for all on one circuit, so it is going to be arranged as S-Plan-Plus (I've attached a schematic diagram of what I think it'll look like).
Number of rads; 34 in total which can be nicely grouped into 11 zones.

My main questions are;
1. If we use a single boiler and S-Plan-Plus configuration, is this the correct layout (ie. the relay switching the motorised valves for both CH circuits on if any of the rads call for heat).
2. How well will this work if we install 2 smaller system boilers with the same S-Plan-Plus layout? Anybody got experience of this and have any advice?
3. Is there a better way of approaching this? Would it be better to totally separate the 2 CH circuits and have an evohome system on each one? (And perhaps even have 2 smaller separate DHW cylinders?).

Thanks in advance, and any questions or things I've not made clear, please ask. I'm not an engineer, I work as an anaesthetist with a fair understanding of how stuff at home works! I can understand stuff that I read about, but I'm not as versed in the technical stuff as some, so sorry if anything I've said is stupid!

Thanks,

Will

Heating Diagram.jpg

[dty]

If you’re having HR92s on all rads (or deliberately leaving some open such as towel rails to act as a bypass), then you don’t need the zone valves at all. I don’t have one - just a DHW valve.

You might want separate circuits to meet demand, or to limit heat losses in the pipe, etc. Personally, I’d be tempted to go for a circuit per floor. I’d specify a low-loss header. You’ll need a separate pump per circuit and each one will need a bypass.

I’d also question the size of that boiler! We have a 4,000 sqft 1950s house which has 25 rads, a few droughts and a mixed bag of insulation, and we have a 25kW boiler. Yes, it struggles a bit to heat the house from cold, but how often do you do that? We turn the heating on the day before we come back from holiday to cope. For efficiency, you care about the bottom of the modulation range. A boiler that size won’t modulate down nearly as far as a smaller boiler. Mine goes down to 6kW.

[willrook]

Thanks dty.

Low loss header certinaly.

re. Separate circuits; do you mean this so that the flow out of the boiler splits relatively early, and then has a shorter circulation just so that it can avoid going through as many pipes to get ti the rad that's calling for demand.

re. Boiler size; two separate heating companies have arrived at approximately 58-60kw. One company wants a 65kw Light Commercial, and the other 2 30KW domestics in a row! However; modulating down is something I'd not considered. I'll ask them about that.

Thanks a lot.

Will

[Edinburgh2000]

Thanks dty.

re. Separate circuits; ........................

re. Boiler size; two separate heating companies have arrived at approximately 58-60kw.

I have Evohome but I don't use HR92s, so my experience is not directly relevant to you. However, I do have a GB162 65kW boiler in a three storey Victorian house with 32 radiators. I have no regrets. I use Loop to monitor my consumption and, even on this relatively mild morning, my boiler is running close to 65kW to get the house up to temperature first thing in the morning. The boiler modulates down well and I would not want a smaller boiler. I guess dty's 1950s house is much better insulated than our Victorian houses.
Consumption.jpg

And I would suggest that having separate systems for each floor, with isolating valves, is extremely useful. I have found that, when decorating rooms and wanting to move or replace radiators, being able to drain down just one floor while the rest of the house can carry on as normal is extremely helpful. (It also helps with diagnosis, if you are losing system pressure, to be able to isolate each floor, to find the leak.) I have a zone valve, isolating valves and drain points for each floor. FWIW, here is the schematic of my heating system. This works for me.
CH schematic v7.jpg

[willrook]

I have Evohome but I don't use HR92s, so my experience is not directly relevant to you. However, I do have a GB162 65kW boiler in a three storey Victorian house with 32 radiators. I have no regrets. I use Loop to monitor my consumption and, even on this relatively mild morning, my boiler is running close to 65kW to get the house up to temperature first thing in the morning. The boiler modulates down well and I would not want a smaller boiler. I guess dty's 1950s house is much better insulated than our Victorian houses.
Consumption.jpg

And I would suggest that having separate systems for each floor, with isolating valves, is extremely useful. I have found that, when decorating rooms and wanting to move or replace radiators, being able to drain down just one floor while the rest of the house can carry on as normal is extremely helpful. (It also helps with diagnosis, if you are losing system pressure, to be able to isolate each floor, to find the leak.) I have a zone valve for each floor. FWIW, here is the schematic of my heating system. This works for me.
CH schematic v7.jpg

This is great - thanks for letting me know your experience.

At the moment it's a complete toss up between going 'full Evohome' (HR92 on every rad but make the circuitary suboptimal for anything else if it doesn't work or I change to something else in 5-10 yrs) or do a botch of HR92 on every rad, but also zone valve per floor, but have all zones open together on one Evohome relay, or just sod the whole lot, and put a basic stat on every floor and make it fairly 'dumb'.

I don't know!

[Edinburgh2000]

At the moment it's a complete toss up between going 'full Evohome' (HR92 on every rad but make the circuitary suboptimal for anything else if it doesn't work or I change to something else in 5-10 yrs) or do a botch of HR92 on every rad, but also zone valve per floor, but have all zones open together on one Evohome relay, or just sod the whole lot, and put a basic stat on every floor and make it fairly 'dumb'.

I have stuck with the 'one zone per floor' approach, controlled by Evohome, rather than universal HR92s because:
(a) 32 HR92s would cost me about £1.5k, which I don't want to spend;
(b) Several of my rooms are used rarely (guest bedrooms; drawing room) and I am very happy to use manual TRVs to control them;
(c) The physical structure of my house, with thick solid walls and floors with the traditional Scottish clinker fill for sound insulation, makes the range of Evohome unreliable. I have five WiFi Access Points around the house because a standard router cannot cope. Evohome would not be reliable for HR92s at the extremities of the house.

If you start with the "dumb" one zone per floor, you can always add HR92s later and then just wire the zone valve on the relevant floor to be permanently open. (I use and recommend motor-open / motor-close valves, not spring return zone valves, which means you can leave the valves open without them consuming any power, and they facilitate pump overrun (i.e. water still flows after the boiler has shut down but the pump is still running).)

[willrook]

32 HR92s would cost me about £1.5k, which I don't want to spend

I don't mind a big expense now, if it is going to work well for the next 10 years. I'm beginning to wonder if it will

(b) Several of my rooms are used rarely (guest bedrooms; drawing room) and I am very happy to use manual TRVs to control them;

Same; but I want to be able to heat the non-occupied rooms for an hour or so each morning just to keep them dry. Single glazed sashes throughout.

The physical structure of my house, with thick solid walls and floors with the traditional Scottish clinker fill for sound insulation, makes the range of Evohome unreliable. I have five WiFi Access Points around the house because a standard router cannot cope. Evohome would not be reliable for HR92s at the extremities of the house.

This is probably the most key point. I have to have 6 Google Wifi points to get coverage. Thus, need to test throughout to ensure I can make it work with evohome before any sort of commitment. From what you're saying, it sounds like it might be a non-starter anyway.

Frustrating, as I really want to use Evohome, but I'm not sure they've got themselves set up for these big projects difficulties with wireless communication yet...

[willrook]

If money not an issue - why not try a system that supports extenders to resolve connectivity challanges?

I'm guessing you are talking Heat Genius? Don't think it'll work out much more expensive, and has repeater function, which is very attractive.

Alternatively Tado.

What a shame Evohome won't sort their range issues.

[someuser08]

I accidentally deleted my message when trying to edit...

Anyway, there is also Wiser that has extenders and is cheaper than even Evohome.

[GreenGuy]

With a 60Kw system I would go for 2*30Kw and load sharing. Apart from deep winter you will find that the demand is below 30Kw so only need 1 boiler firing. Viessmann have a load sharing system and other manufacturers will also do this. I think you will need an industrial heating company rather than a BG or other bodger.