Nest thermostat w/hw control install help please

[timmysd88]

Hi,

I am looking to install the nest 3rd gen thermostat as a replacement to my current setup. I currently have a control unit near the boiler with BDR91 transmitter situated in cupboard above which i believe communicates with the thermostat in the hallway. I am pretty sure I can wire in the nest heatlink but I am unsure what to do with the BDR91? I can only upload the one picture of the transmitter, I hope this helps.

Can I put a terminal box in its place and wire these into it? do any of these wires need to be joined? or should i terminate them separately?

Thanks

To add. I am in the UK and have a heatlink

BDR91 wiring

• timmysd88
• 9 Nov 2020

 

[flameport]

Grey and black need to be connected together. The others must be separately terminated in insulated connectors, and a suitable enclosure installed
However it's more usual to locate the other end of that wiring so it and the BDR91 can be removed and the link installed in the wiring centre, junction box or similar.

The Nest replaces the timer/programmer.

 

[timmysd88]

thankyou for the reply. So by joining the two wires A and B am i essentially telling the current system to keep heat permanently on? then once the nest is connected it sorts this out?

 

[ianmcd]

Hi,

I am looking to install the nest 3rd gen thermostat as a replacement to my current setup. I currently have a control unit near the boiler with BDR91 transmitter situated in cupboard above which i believe communicates with the thermostat in the hallway. I am pretty sure I can wire in the nest heatlink but I am unsure what to do with the BDR91? I can only upload the one picture of the transmitter, I hope this helps.

Can I put a terminal box in its place and wire these into it? do any of these wires need to be joined? or should i terminate them separately?

Thanks

To add. I am in the UK and have a heatlink

BDR91 wiring

• timmysd88
• 9 Nov 2020

You have got that the wrong way round, that pic is the receiver not the transmitter, the thermost is the transmitter, just fit the heat link in place of the receiver, very simple no wires need joining together

 

[stem]

@timmysd88 your initial post is lacking in some very basic important detail. How you install the Nest depends upon whether:

1. It is fitted to a combi boiler without an attached stored hot water system (aka a hot water cylinder), in which case the Nest replaces the existing room thermostat receiver as per @ianmcd

or;

2. It is fitted to a system that has an attached stored hot water system (aka a hot water cylinder) If you do have a stored hot water system with a dual channel programmer then the Nest replaces the programmer and not the thermostat as was pointed out earlier by @flameport.

I'm not sure what the title of your thread "w / hw control' means. Only you can tell us for sure

 

[timmysd88]

thanks, I appreciate your reply. with hot water control, so a conventional boiler

I think @flameport makes sense, Join A and B together in a junction box and terminate L and N. This way I dont have remove any cables.

Im assuming that if i didnt join A and B then heating would not come on?
thanks

 

[stem]

Correct. There were originally two devices, the programmer and room thermostat with their switches wired in series. Both had to be 'on' for the heating to operate.

The Nest Heat link replaces the programmer but has the thermostat functions built in also. If the old thermostat was just removed it would leave the heating wiring open circuit. Joining the wires in A and B together completes the circuit.