Not another nest thread!

[Earl_Cranberry]

Hi all, I’ve been poking around here for a while recently as I’ve just bought my first house and looking to learn the ins and outs of DIY!

I currently have a Worcester Greenstar 28i junior boiler in the garage, which is controlled using a Honeywell BDR91 relay to the CMS927 panel. Sadly, the panel is goosed, the display is so far gone I can barely make it out. This seems to be a common fault?

Anyway, as Amazon had their prime day, I grabbed a nest 3 thermostat as this suits my needs better (largish house, I’m out a lot with random work times so app control is a massive plus!)

I opened up the BDR91 and took a look, obviously the N and first L go to the respective inputs on the heat link, but then the second L is bridged to A and then a new wire for B

my understanding is
L + N = power
The bridge from L to A is for common
B = Call for heat

so, would the following be correct

BRD91 Nest
N N
L (first) L
L (second) 2 (common)
B 3

any help much appreciated as the heating is all over the shop and I can’t use the Honeywell to change it



PS the thermostat itself will be USB powered so no need to worry about the T wires

 

[CBW]

Yes, sort of, I think you’ve complicated it a bit:

L in BDR to L in Nest
N in BDR to N in Nest
Copy link wire from L to Common (2) in Nest
Grey wire in BDR (B) to Normally Open (3) in Nest

 

[Earl_Cranberry]

Wonderful, thank you! All works perfect