Nest Installation Advice to replace Honeywell CM927 / BDR91

Sibley14 · Sep 15, 2017

Hi there,

i have zero electrical experience, but am looking to replace my wireless honeywell CM927 / BDR91 Control unit with the Nest 3. This is to control a Ferroli combi boiler.

I could pay a professional, but It doesn't look to complicated to fit.....but then what do i know

Looking at the existing wiring to the honeywell system, there are 4 wires, L & N wires plus 2 additional white wires from the boiler going into to A & B

Looking at the Nest installation manual im confused as to where these wires need to go. Am i right to assume that L > L, N > N and A > 2, B>3? which would suggest the low voltage / dry contact combi boiler system?

Your advice and assistance will be much appreciated.

Thank you in advance.

[ElectriciansForums.net] Nest Installation Advice to replace Honeywell CM927 / BDR91

davesparks · Sep 15, 2017

Yes you have correctly identified which conductor goes where on the nest, however it does not necessarily mean that the control circuit is operating at low voltage, it may well be 240V controls.

Sibley14 · Sep 15, 2017

Thanks @davesparks nice to see I'm on the right track, you mention control circuit and by this you mean the A B wires as I refer to them. Is that right?

Next Q. How do I know if they are 240V?

Is it worth me paying for a pro?

Cheers

Leesparkykent · Sep 15, 2017

Sibley14 said:
Thanks @davesparks nice to see I'm on the right track, you mention control circuit and by this you mean the A B wires as I refer to them. Is that right?

Next Q. How do I know if they are 240V?

Is it worth me paying for a pro?

Cheers

why do you want to know if they are Mains voltage or not? It's still going to work the same if you connect A to 2 and B to 3.

buzzlightyear · Sep 15, 2017

Sibley14 said:
Is it worth me paying for a pro?

no part L on this one then.

Leesparkykent · Sep 15, 2017

buzzlightyear said:
no part L on this one then.

Conservation of fuel and energy?

Sibley14 · Sep 15, 2017

Leesparkykent said:
why do you want to know if they are Mains voltage or not? It's still going to work the same if you connect A to 2 and B to 3.

Ok thanks @Leesparkykent & @buzzlightyear - sorry if this seems really dumb....

If i just replace the Honeywell BDR91 and wire up the nest Heatlink box as indicated on the low voltage page instructions, so:
L>L,
N>N
A Wire (from honeywell) to 2 on nest heatlink box
B wire (from honeywell) to 3 on nest heatlink box
Then in theory it should work ok?

I'm not wiring up the Nest wireless thermostat as this will be plugged in to the mains plug elsewhere via USB, only the Nest heatlink box.

Again, sorry if my questions/responses are a bit dumb, but as you can obviously tell - Electrics are not my strong point...(even though i'm sure this is basic stuff).

Strima · Sep 15, 2017

If electrics aren't your strong point then get someone in who knows what they're doing before you either electrocute yourself or burn your house down.

Sibley14 · Sep 15, 2017

Thanks @Strima, agree with you there.

Before i ventured down the pro route i thought i'd explore the self install option first. It doesn't look that complicated, and i would of course turn off the power. I already have a wireless system installed and thought i'd check with the experts here as to whether it was as straightforward as it seemed.

telectrix · Sep 15, 2017

rule of 3'''''''

1. it works.

2. it don't work.

3. it goes bang.

Antimus · Jun 1, 2021

Sibley14 said:
Thanks @Strima, agree with you there.

Before i ventured down the pro route i thought i'd explore the self install option first. It doesn't look that complicated, and i would of course turn off the power. I already have a wireless system installed and thought i'd check with the experts here as to whether it was as straightforward as it seemed.

So did it work? I'm in the same situation

Nest Installation Advice to replace Honeywell CM927 / BDR91

Sibley14

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davesparks

Sibley14

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Leesparkykent

buzzlightyear

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Leesparkykent

Sibley14

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Strima

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Sibley14

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telectrix

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Antimus

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