Help me understand my boiler wiring so I can install Evohome

24 Oct 2018
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United Kingdom
I've had my current home for 2 years, and have decided to install Evohome in it. There are a couple of oddities I've lived with for that time. Now that I need to wire Evohome in, I need to try and understand those oddities better.

I have my house (with a Centaur C27 CH+HW), and an outbuilding (converted garage) (with a Centaur C17 CH). Physically the house and outbuilding have a 5m gap between them, and the boiler is in a boiler house that leans against the external wall of the outbuilding. My hot water cylinder is in my house, is about 7m from my Centaur controller, and has no wiring around it. (There is not Cylinder Stat)

My oddities are:
1) To have heating on in the outbuilding, I need to have the main house central heating on. (My explanation for this is that the motorised value for the outbuilding is controlled by the C17, but the boiler pump is controlled by the C27)

2) To have hot water on, I need to have central heating in the house on. (I assumed this was bad wiring of the C27, and that only the CH circuit of the C27 would operate the pump).

3) The HW on the C27 actually does nothing, having just tested putting the CH on, but not the HW, it turns out that my Cylinder is still filled with hot water after an hour. I suspect that both the CH + HW values are always open, but that the C27 is controlling the pump.

I've put together a photo-diagram of the different parts of my system.
Attached, and google link (for zoom etc.)

From looking at the wiring, I have the following issues/questions:
1) The house controller doesn't have enough wiring to be able to control more than just ON/OFF for boiler.
2) I can't work out if anything is currently wired to control my HW. (But there is a motorised value for it, so I assume this must be possible?)
3) I'm not sure what the extra wiring on the C17 for the outbuilding is. (e.g. 2 cables into the N terminal)
4) For Evohome, I'd planned to wire 2 BDR91's into my house. I don't see how I can do this with the current wiring?

Thanks for your help!


  • Boiler Wiring(1).png
    Boiler Wiring(1).png
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20 Jul 2005
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United Kingdom
Unfortunately that doesn't really provide enough information and I don't think you will be able to provide it over a forum.

We would need to know where every wire from each piece of equipment goes, so as to understand what is actually controlling what. For example, what is actually controlling the boiler? What is the pump wired to? How are the motorised valves controlled? Are their internal switching contacts used for anything, if so what? The Centaurs are just time switches, no mention of any room thermostats, (odd not to have them when there are motorised valves in the system). If there are room thermostats, what are they wired to? There are three motorised valves. If two are for the heating, the third would normally be for the hot water cylinder, yet there isn't a cylinder thermostat to control it!

From the photos of the wiring that you have posted, I would be staring again from scratch, rather than trying to modify what's already there. It would probably be quicker and easier in the long run.

Before upgrading the controls though, the plumbing must be suitable for it. Maybe you can't have the heating 'on' in the outbuilding because the pipework is connected to the main house radiator pipework, and doesn't have its own independent supply from the boiler, in which case that would need changing. That would also explain the lack of room thermostats; main house thermostat goes off, outbuilding heating goes off too.

The system should be 'fully pumped' including the hot water cylinder. Older hot water systems (without a thermostat) were usually gravity fed, but, if one of the motorised valves is in the hot water circuit that suggests that it is pumped, but the lack of a thermostat usually suggests gravity. So all that also would need sorting first.

To be honest, it sounds like the system has been messed about with and extended in an ad-hoc way. I think it needs a professional to carry out a full survey of the system and make recommendations based on the results.
27 Jan 2008
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Llanfair Caereinion, Nr Welshpool
United Kingdom
As stem says more info required, including how boiler is controlled, it seems two completely different methods can be used, on/off and modulating, I will guess your using on/off as think motorised valves are either open or closed, however the TRV which is a motorised valve with EvoHome heads can it seems gradually open/close, so it could be to gain all what EvoHome offers you need to remove some motorised valves and basic start from scratch.

I wish you luck, I am also trying to increase control, although not quite as much as you are doing, and I sit their scratching may head at results wondering if that a fault in my method of installation, or is a part faulty.

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