Changing thermostat from Honeywell T6360B to Cm921

[gizzer]

Hi experts

I have just purchased a Honeywell CMT921 to replace my broken T6360B.

I found a thread here //www.diynot.com/forums/viewtopic.php?t=179838 with info on how to wire up the CM921.

However, when I opened up the T6360B I found mine only has 2 wires (instead of 3 - as per the mentioned thread).

The current wiring on the T6360B is as follows
1 - Coming from 4 at boiler (I assume live)
3 - Coming from 3 at boiler (I assume neutral)

(Boiler: Vaillant ecoMax 828 2e combi boiler)


Can anyone provide info on how I would wire up the CMT921?


Having looked at the manual, the obvious solution (to me) seems to be:

4 (from boiler - which I assume in Live) goes into B in the CMT921
3 (from boiler - which I assume in Neutral) goes into N in CMT921

A (on CMT921) connects to L (on CMT921)
L (on CMT921) connects to L (on CMT921) - there are two L sockets on the CMT921 - do I need to do this or does the CMT921 already have an internal connection between the two L sockets?

Does this look right to the experts?

Thanks

Giz

 

[D_Hailsham]

You need a three core heat resistant cable (not two core plus earth), though you may have to use a three core plus earth.

Connect Boiler L terminal to receiver L terminal (brown)
Connect Boiler N terminal to receiver N terminal (blue)
Connect Boiler 4 terminal to receiver B terminal (third colour)
Link receiver L to receiver A

(The two L terminals are linked)

 

[gizzer]

Thanks D_Hailsham for your prompt response.

Luckily for me there was a 3 core (+earth) already there.
There was an unused (grey) wire which was not connected at either end.
I have used that to provide a neutral connection between the boiler and the cmt921.

The connections I used (for my specific Vaillent ecoMax combi boiler) are:

3 (Boiler) --> L (cmt921) (I'm assuming live - doesn't say which is which in installation manual)
4 (Boiler) --> B (cmt921) (I'm assuming switched live)
5 (Boiler) --> N (cmt921) (refer to the boiler installation manual, bottom of page 29)

A (cmt921) --> L (cmt921)

All tested and working fine.

On a side note, I have noticed a delay in the response time between the thermostat switching on and the boiler switching on. Do you know if that is normal?

Thanks for your help.

Regards

 

[D_Hailsham]

There was an unused (grey) wire which was not connected at either end. I have used that to provide a neutral connection between the boiler and the cmt921.

If you have a blue in the cable, it would have been better to use that as the neutral, even if it means swapping some wires round, as anyone working on the system for the first time will automatically assume that any blue wire is a neutral.

The connections I used (for my specific Vaillant ecoMax combi boiler) are:

3 (Boiler) --> L (cmt921) (I'm assuming live - doesn't say which is which in installation manual)
4 (Boiler) --> B (cmt921) (I'm assuming switched live)
5 (Boiler) --> N (cmt921) (refer to the boiler installation manual, bottom of page 29)

Correct. 3 is the live and 4 the switched live (see Fig 4.22 on page 30). However terminals 3 is only live when the boiler is switched ON at the front control panel. This means that the receiver will be turned off if the boiler ON/OFF switch is OFF.

The receiver works best if it has a permanent supply; which is why I said that the receivers L and N terminals should connect to the boiler's L and N terminals. These are permanently live, unless the boiler's wall switch/fused spur is turned off.

Its similar to the clock wiring shown in fig 4.22

On a side note, I have noticed a delay in the response time between the thermostat switching on and the boiler switching on. Do you know if that is normal?

This could be due to the time it takes for the motorized valve(s) to move.

 

[gizzer]

If you have a blue in the cable, it would have been better to use that as the neutral, even if it means swapping some wires round, as anyone working on the system for the first time will automatically assume that any blue wire is a neutral.

I assume the cable must have been installed prior to the regulation change to standardise wire colouring (it was already here when I moved in). The cable has 3 wires - brown, black & grey. Grey was the only one free that's why I used it. I guess I should label each end with the appropriate info.

Correct. 3 is the live and 4 the switched live (see Fig 4.22 on page 30). However terminals 3 is only live when the boiler is switched ON at the front control panel. This means that the receiver will be turned off if the boiler ON/OFF switch is OFF.

The receiver works best if it has a permanent supply; which is why I said that the receivers L and N terminals should connect to the boiler's L and N terminals. These are permanently live, unless the boiler's wall switch/fused spur is turned off.

I was planning on leaving the boiler ON all the time and using the cmt921 to control heating on/off based on the ambient temperature. Is this inadvisable?

Also, the reason I took from L from terminal 3 is because according to the installation notes if there is no room stat installed then terminals 3 & 4 need to be linked with a jumper cable, which I took to mean that terminal 4 needs a Live supply from terminal 3 to close the loop (hope that make sense).

If I take the L & N from the boiler's L and N terminals then terminals 3 (live) and 5 (neutral) would become redundant. Would it still be okay to have the switched live going back to the boiler into terminal 4? I have no idea what how the boiler routes Live into terminal 3 (ie. direct from the boiler clock or via some other circuitry).

Once again, thanks very much for you help and advice.

 

[D_Hailsham]

The cable has 3 wires - brown, black & grey. Grey was the only one free that's why I used it. I guess I should label each end with the appropriate info.

I assumed a brown/blue/grey or a red/black/grey. Labelling would be a good idea.

I was planning on leaving the boiler ON all the time and using the cmt921 to control heating on/off based on the ambient temperature. Is this inadvisable?

I was thinking more of when you went on holiday. Many people will turn the boiler off, knowing it will not be required for a week or two. If you take the supply to the receiver from 3 and 5 it will be turned off. The control unit will spend the whole of the holiday trying to make contact with the receiver.

the reason I took from L from terminal 3 is because according to the installation notes if there is no room stat installed then terminals 3 & 4 need to be linked with a jumper cable, which I took to mean that terminal 4 needs a Live supply from terminal 3 to close the loop (hope that make sense).

Your reasoning is correct.Terminal 3 supplies the live to the thermostat which closes the circuit and supplies 230v to terminal 4; but that assumes a standard mechanical stat. Wireless ones are different as they require a permanent supply to run the receiver. You can use this supply to mimic the connection from terminal 3; the link from L to A does this.

If I take the L & N from the boiler's L and N terminals then terminals 3 (live) and 5 (neutral) would become redundant. Would it still be okay to have the switched live going back to the boiler into terminal 4? I have no idea what how the boiler routes Live into terminal 3 (ie. direct from the boiler clock or via some other circuitry).

Yes it will be OK to ignore 3 and 5.

 

[gizzer]

Yes it will be OK to ignore 3 and 5.

D_Hailsham, thanks for all the advice, it has been extremely helpful.

I have taken your advice and changed the L & N supplies over so that they come directly from the boiler's L & N.

Cheers