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PostPosted: Tue Apr 11, 2017 7:52 pm 
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Hi folks,

I'm new to this board and have already tried using the search function - lots of variations of my query but want to be 100% sure I'm doing the right thing!

I have a Vaillant ecotec plus 837 boiler to which I am trying to attach a new honeywell evohome BDR91 wireless relay switch. Some pictures leading to the help I need:

connection diagram of the circuit board:
Image

What the wiring looks like now for AC (bottom of the connection diagram):
Image

However there's also a manual room thermostat in the 24V port (just above the 24V boundary in the connection diagram):
Image

The evohome instructions show:
Image

So, questions:

1) What should I do with the 24V room thermostat connected in - the grey and black cables, should I just leave them intact and turn the thermostat all the way up? Or remove them entirely and leave the ports empty, clamping away the cables?
2) to wire this correctly, it appears i need to:
- take the brown L wire currently in the L port, 2nd picture above, and put it into the left L port of the BDR91
- Get another brown wire to connect to the 2nd L port of the BDR91 and plug it into where I just removed the L wire from in the step above
- Run another blue N cable (split the original?) into the N port of the BDR91.

Is that correct?

3) What's the point of ports A, B, C on the BDR91? they don't "connect" to anything. Looks to be an internal switch to send a signal to turn on the boiler, but why does it also have cabling entry points?


Any help much appreciated, thank you!


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PostPosted: Wed Apr 12, 2017 2:40 pm 
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Need to know more about your Evohome installation.

1. What is controlling the BDR91?

2. How is the BDR91 it configured in the Evohome system?

3. Won't the thermostat connected to the white 24V=RT terminals be redundant?

The BDR91 needs a permanent 240V supply to L and N, which can be taken from the L and N terminals of connector X1.

Terminals A and B (and sometimes C) are what turns the boiler on! They are connected internally to a relay controlled by the wireless receiver in the BDR91 which, is controlled by a transmitter in the evohome system.


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PostPosted: Wed Apr 12, 2017 2:56 pm 
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D Hailsham wrote:
Need to know more about your Evohome installation.

1. What is controlling the BDR91?

2. How is the BDR91 it configured in the Evohome system?

3. Won't the thermostat connected to the white 24V=RT terminals be redundant?

The BDR91 needs a permanent 240V supply to L and N, which can be taken from the L and N terminals of connector X1.

Terminals A and B (and sometimes C) are what turns the boiler on! They are connected internally to a relay controlled by the wireless receiver in the BDR91 which, is controlled by a transmitter in the evohome system.


1: I have a evohome wifi that will control it
2: Not sure what you mean?
3: Yep - it will be redundant! I'm guessing that I'll need to run a lead from the boiler's 24V room thermostat entry point to terminals A and B of the BDR91...


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PostPosted: Wed Apr 12, 2017 3:56 pm 
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chamsters wrote:
1: I have a evohome wifi that will control it
2: Not sure what you mean?
3: Yep - it will be redundant! I'm guessing that I'll need to run a lead from the boiler's 24V room thermostat entry point to terminals A and B of the BDR91...

2. If you have a multizone system, a BDR can be used to control zone valves etc. The Evohome master unit has told what each BDR91 does. Not relevant in your case.

3. Disconnect existing thermostat then connect A and B to the 24V terminals - doesn't matter which way round.


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PostPosted: Wed Apr 12, 2017 10:34 pm 
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D Hailsham wrote:
chamsters wrote:
1: I have a evohome wifi that will control it
2: Not sure what you mean?
3: Yep - it will be redundant! I'm guessing that I'll need to run a lead from the boiler's 24V room thermostat entry point to terminals A and B of the BDR91...

2. If you have a multizone system, a BDR can be used to control zone valves etc. The Evohome master unit has told what each BDR91 does. Not relevant in your case.

3. Disconnect existing thermostat then connect A and B to the 24V terminals - doesn't matter which way round.


amazing, thanks :)

Now here's an odd one. is there actually any reason to have the live going through the BDR91 into the boiler? Why can't i wire it to the mains, and then just wire up the 24V room thermostat which would turn on/off the boiler? The BDR91 would never 'cut' the mains supply! It's essentially setting up a series connection to the boiler as a power source, so when you disconnect the boiler the bdr91 also turns off?


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PostPosted: Thu Apr 13, 2017 7:17 pm 
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chamsters wrote:
is there actually any reason to have the live going through the BDR91 into the boiler? Why can't i wire it to the mains, and then just wire up the 24V room thermostat which would turn on/off the boiler? The BDR91 would never 'cut' the mains supply! It's essentially setting up a series connection to the boiler as a power source, so when you disconnect the boiler the bdr91 also turns off?

That's OK, so long as the boiler and all related components (BDR91, etc) are controlled by the same switch/fused spur.

However it's usually easier to locate the BDR91 near the boiler and run a 4-core cable (brown, blue, etc - possibly + earth) between the BDR91 and the boiler. A heat resisting cable must be used for the cable going to the boiler, e.g Tower Brand.


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