Replacing Honeywell cm921 with Hive

11 Jan 2020
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United Kingdom
Looking for some help / guidance to replace our existing Honeywell cm921 thermostat with a Hive. I've removed the faceplate and there is a red wire connected to A a yellow wire connected to B and a blue wire going to one end of a terminal. I can also see a green/yellow wire behind this not connected to anything.

Would I be correct in wiring the hive as follows?

Blue to N
Red to L + a loop from L into 1
Yellow to 3
Green/yellow to earth

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You need to establish whether they’re connected to 230vac or 24vac. Is it a combi boiler you have?
So it could be 24v or 230v, connect it wrong and you’re looking at around £150 for a new pcb. Do you have a multimeter or a voltage tester?
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It does seem likely it is 230 volt control with that voltage, the coil of a relay or other item on PCB will likely account for volt drop, however you don't know if neutral is connected would expect two other voltage readings yellow to blue and red to blue would expect one with show full 230 volt showing that is line feed. (Expect it will be red)

Personally I would not trust meter readings I would want to look in boiler, however I have been told with some boilers to get to terminals means breaking a gas seal, so can see why some rely on readings.

So very likely blue to N red to L with link to 1 and yellow to 3 assuming a single channel Hive. And the green/yellow to earth tether.
So to confirm red is live feed should I take a reading from red to the blue that's in the terminal bloc and confirm it's reading the full 230v?
So here are the readings:

Red to Blue 4v
Yellow to Blue 244v

I get the same when testing Red to Earth wire and Yellow to Earth wire.

I am now thinking Yellow is the live feed and should go to L with link to 1. The Red should go to 3. Does this seem correct?

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