C Plan Upgrade

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Hi there,

I've been lurking about these forums for years but don't often post.

I'm about to embark upon an upgrade to my heating system to make it "smart" and a bit more efficient.

Currently I have a gravity hot water pumped central heating system with TRV's on all radiators except one (in the hall where the room stat is mounted).

Now I'm going to go full HR92 Honeywell TRV's on all 11 rads with wireless wifi controller and hot water kit.

I'm basically looking to fully zone my large house and create a C plan system at the same time.

My questions are.

1: Is the system "smart" enough not to require a bypass valve i.e. ensuring at least one valve is always open (and would I want this?). I'm pretty sure I can get decent access to the pipes coming off the boiler if I cut an access into a cupboard back wall to fit an automatic bypass valve before the first radiator.

2: I think I need a 28mm 2 port valve (Honeywell V4043H1106) to control the hot water side using the hot water wireless kit that will come with my Evohome system. But does it go in the flow or return pipework? The best access I have is to the upper pipe on the cylinder which tee's up to an overflow into the F&E tank and down to the boiler. The lower pipe which Tee's into the bottom of the F&E tank and the boiler would be very difficult to fit the valve into.

I might have more questions but I think that's the main two I have for now before I go order all my kit.
 
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1. Depends on boiler but if on gravity primaries No.

2. Valve on return between c/f connection and cylinder.
 
OK Cheers. I've ordered the kit with one wall mounted zone thermostat instead of a TRV for the first radiator in the circuit and I'll leave it with the two lock shield valves. This will do for now until I get round to cutting a hatch into the back of the cupboard and installing a bypass loop and automatic valve. Then I'll maybe go full TRV.
 
The room stat prevents the pump running when not needed. Leave as it is.
 
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You will need a 2 Port valve for the hot water, and although it will work in the flow or the return, it is customary to fit it to the flow anyway. You should make sure though that where it is sited that it is not in the vent pipe anywhere between the boiler and the open vent. If it's teed off to the side, that's fine. The Honeywell V4043H1106 will do the job, but whatever valve you choose, it should have the additional white wire from the internal microswitch that a C Plan requires to provide electrical isolation between the boiler and pump, so that the pump doesn't run when only hot water is required. Not all two port valves have this extra wire, but it is essential for a C Plan.

The Evohome will have two relay boxes, one for the heating and one for the hot water. They will carry out the functions of the 'Room Thermostat' and 'Cylinder Thermostat' shown in the diagram attached below.

However, I would recommend that you convert it to a fully pumped S Plan or Y Plan now. Modern boilers are not suitable for use with gravity hot water systems as they have to maintain a certain flow rate through their heat exchanger, which doesn't happen with gravity hot water circulation. So, in the future, when you have to replace your boiler, you will have also to rip out the C Plan. However, if you convert it to Y or S Plan, it will be OK to keep.
 

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  • C Plan.pdf
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Thanks for the good advice and diagram that'll come in handy I'm sure. I'm not going S or Y plan at the moment as the whole house needs redecorated in the next couple of years and I'm going to give it a major overhaul then with new boiler and un-vented Megaflow then mixer showers instead of the electric ones. I'll also be taking up a lot of the flooring to replace the squeeky nails with screws so good time to replace pipework. The system at the moment is incredibly poor as it is a flat roofed house so no loft and the hot water tank is in an upstairs cupboard with the F&E tanks on a shelf above it. Not much head!
 
OK so I've now got all the kit, 11 TRV's a wall stat, a 28mm 2 port valve (with 6 wires including white) and the hot water kit. But I can't find a wiring diagram for C plan using the evohome kit. My main concern is what do with the white wire from the 2 port valve. My hot water tank is in a cupboard up the stairs and the boiler is down in the utility room, with all this kit being wireless I really don't want to have to get that white wire back to the boiler by wire.
 
I doubt you will find a drawing for a C Plan and a Evohome kit. Most 'C Plans' have long gone now and Evohome is newish technology.

However, it's not difficult. I sent you a C Plan wiring diagram in an earlier post, but I have now modified it for you showing how to connect the Evohome components. All you have to do is simply substitute the Cylinder Thermostat and Room Thermostat shown on the C Plan diagram with the appropriate Receiver. You will need to add Lives & Neutrals to the receivers to power them.

The numbers below refer to the main terminal strip, which follows the conventional Sundial Plan format and as used in the Evohome installation manual for S and Y Plans.

Central Heating Receiver
L = Terminal 1
N = Terminal 2
A = Link to L
B = Terminal 5
Now link L & A

Hot Water Receiver
L = Terminal 1
N = Terminal 2
A = Link to L
B = Terminal 8
You don't need a connection to the receiver C, that's only for a Y Plan.

As you won't have the programmer shown in the standard 'C Plan' diagram, just ignore it. The live supplies that would come from the programmer are now made via the L & A links at the Receivers.

C Plan with Evohome.jpg


with all this kit being wireless I really don't want to have to get that white wire back to the boiler by wire.
The white wire from the motorised valve needs to be connected to the pump [not boiler] live, as it is where it gets its switched supply from. You could possibly get around it with a relay at the wiring centre energised by connecting the coil to the motorised valve L & N and using a set of changeover contacts instead of the internal microswitch inside the motorised valve, as shown below.


C Plan with Relay.jpg
 
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OK great advice once again cheers. I'm handy with Autocad so I've drawn up what I think I should wire up to help me understand what is going on. I think I can run a cable from the motorised valve position at the hot water cylinder to the boiler without completely ripping the house to bits so that's good! I'll also draw up my piping setup. Will come in handy for when I upgrade the system to a more modern plan later when we rip the whole house apart.
Evohome%20C-Plan%20Wiring%20Diagram_zpsjg45x5w0.jpg
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Evohome C-Plan Wiring Diagram.jpg
 
This is where I plan on putting the motorised valve, is this right? Between the boiler feed/vent and the cylinder? Looks like it will be
pretty tight but I should be able to get it in there.

2-port valve.jpg
 
OK not sure why I couldn't see your diagrams earlier perhaps you edited the post later? But using a relay looks like a much better plan than starting to rip into my walls as I've already cut some small holes and the pipework is not where I thought it was. So I'll go down the relay route. I think I have an AC relay somewhere........(n)
 
OK not sure why I couldn't see your diagrams earlier perhaps you edited the post later?
Yep. I hadn't posted the diagrams originally, but as found myself hanging around waiting for a delivery, knocked them up whilst I was waiting.
 
Ah I've hit problems again whilst adding the relay to my wiring diagram. If I wire it with the relay I still need a switched live to go from the terminal box pin 8 to the motor so again need a wire from utility room to the cylinder upstairs. Or if I put the DHW reciever up the stairs near the cylinder I'd need a wire going down to pin 8 for the relay. Ah well it looks like I'd better start ripping holes in things and finding a route. Cheers for the help though, but I think it's the only way forward with my C-plan.
 
Probably the best solution really. The relay is complicating it a bit. Also if you can fit the motorised valve where you show it, that will be fine.
 
Well everything is going good. Everything is wired up and running this morning. I let the house get cold overnight to see what happened when I switched it all on this morning. The rooms are coming up to temperature, but the boiler is cycling on and off maybe every 5 minutes. The heating relay is still calling for heat tho. Is this just because the boiler is up to temp and doesn't need to be on? And when it drops below a certain temp it is cutting out till required again? Just that all the rooms are still heating up so it should still be heating all radiators. Also is there a way I can check the CH pump is running when required? It's so quiet I don't know how to tell if its working?
 

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