Evohome BDR91 relay wiring to Worcester Bosch boiler

[Bill M]

Hi

I have bought a Evohome WiFi Connected Thermostat Pack (ATP921R3100) and the HR92 radiator valves.

I am trying to understand the wiring for my Worcester Bosch (highflow 400) combi boiler. I don't have a hot water tank, the hot water is on demand from this boiler.

I have 4 wires on the control panel
L
N
DHW (assume Domestic Hot Water)
CH (assume Central Heating)

IMG_0661.jpg

On the Honeywell BDR91 relay, I have connected the :
N wire -> N
L wire -> L and a jumper wire to the second L
CH wire -> A
DHW -> B

The hot water is still working on demand, but I can't get the heating to come on when I turn the temperature up in any of the zones.

Please can someone tell me what I am doing wrong? I have been searching the internet for hours!

thanks
Bill

[Fursty Ferret]

Hi Bill,

You shouldn't use a G/Y insulated conductor without adding brown sleeving to both ends (or some other way of identifying it as live). This is common sense (and also a legal requirement), and while I'd run new four core cable as an alternative to doing this it's not explicitly forbidden by regulations.

Looking at your picture - if it's a combi boiler, why is there even a connection to the DHW terminal in the first place? Disconnect and terminate the green/yellow wire safely in a chock-block (you don't need it).

Basically, you've got it wired wrong. On the BDR91, the two "L"s are linked internally. Connect a link of wire from the second L to A, connect the wire for CH to B, and leave C empty.

If it still doesn't work, temporarily link the live and CH terminals - the boiler should fire. The reason it didn't work is because the CH terminal never received a call for heat from the BDR91.

Cheers,

Dave.

Edit: Though if it still doesn't work after that, call a heating engineer.

[electronicsuk]

Dave's instructions above should get you going, but I would also add that using the earth conductor for anything other than earth is poor practice, even when sleeved as a live conductor at either end. There's nothing to stop someone splicing into it and assuming, as one would, that it's connected to earth at the other end. Probably not an issue for the short length of cable between boiler and relay box, but still bad practice.

With regards your boiler, was there a connection to the DHW terminal before you fitted evohome? I ask because I believe the Highflow 400 uses a heat store for the hot water, therefore depending on how the system was operating before, the heat store may eventually cool down without any call for heat on the DHW terminal.

[Bill M]

Hi Bill,

You shouldn't use a G/Y insulated conductor without adding brown sleeving to both ends (or some other way of identifying it as live). This is common sense (and also a legal requirement), and while I'd run new four core cable as an alternative to doing this it's not explicitly forbidden by regulations.

Looking at your picture - if it's a combi boiler, why is there even a connection to the DHW terminal in the first place? Disconnect and terminate the green/yellow wire safely in a chock-block (you don't need it).

Basically, you've got it wired wrong. On the BDR91, the two "L"s are linked internally. Connect a link of wire from the second L to A, connect the wire for CH to B, and leave C empty.

If it still doesn't work, temporarily link the live and CH terminals - the boiler should fire. The reason it didn't work is because the CH terminal never received a call for heat from the BDR91.

Cheers,

Dave.

Edit: Though if it still doesn't work after that, call a heating engineer.

Thanks for the reply Dave. I was just starting to realise and your post has confirmed it. There is the possibility on this boiler of turning the water off (even had a timer). However the water was always 'on' as it was demand so not a lot of point really. I realise now that this BDR91 relay is only for one connection!

Regarding the wires, yes I agree and I will replace it. Bit lazy yesterday because I wanted to test the new system. In fact I only need the 3 wires and leave the water on the original controls.

thanks again
Bill

[Bill M]

Dave's instructions above should get you going, but I would also add that using the earth conductor for anything other than earth is poor practice, even when sleeved as a live conductor at either end. There's nothing to stop someone splicing into it and assuming, as one would, that it's connected to earth at the other end. Probably not an issue for the short length of cable between boiler and relay box, but still bad practice.

With regards your boiler, was there a connection to the DHW terminal before you fitted evohome? I ask because I believe the Highflow 400 uses a heat store for the hot water, therefore depending on how the system was operating before, the heat store may eventually cool down without any call for heat on the DHW terminal.

Yes the DHW connection photo is inside the boiler. The boiler controls are always set to 'on' for the water and there is a very small tank in the boiler which must be the heat store. When I turn the hot tap on I hear the boiler fire up and the demand light comes on. I realise now that I need to leave all this alone and just connect the evohome system to my CH, which I did last night and all is working now I am pleased to say

thanks
Bill