Replacing T6360 with CM900

[Ukpmel]

I decided to replace my fully functional T6360 room thermostat with a more efficient CM900 automatic digital thermostat. After several hours and a couple of blown fuses, I'm a bit stuck. I have an Ideal boiler, which has its standard controller. That controller is set to an auto program for the hot water. The heat is set to constant, and controlled by the T6360 in my entry hall. The T 6360 had the following connections: Live to connector 1, switched neutral in to connector 2, switched neutral out to connector 3, earth to earth. As I said, everything worked fine, I just wanted something automated. The CM900 is hooked up as follows: Line to A, switched line out to b, switched line in to C. Now, my heat won't turn off. if I switch the neutrals around, I blow a fuse. I have it programmed for 20 degrees at 5 am, and 18 degrees at 9 pm. It does the same thing in manual mode. Can anyone help me out?

Thanks in advance.

 

[muggles]

I have an Ideal boiler

I have a Ford car. Can you tell which model it is with your crystal ball?

You're attempting to replace a non-programmable room thermostat with a programmable one, which is almost certainly not going to work. Do you have a time switch elsewhere in the house? If so, what is it?

 

[Dan Robinson]

Terminal 1 to A.

Terminal 3 to B

Terminal 2 made safe.

Leave the timer on constant for heating.

Terminal 2 was the only neutral. The fact that you thought you had 2 switched neutrals tells us that you are not competent to be working on electrics safely.

 

[Ukpmel]

I have an Ideal boiler

I have a Ford car. Can you tell which model it is with your crystal ball?

You're attempting to replace a non-programmable room thermostat with a programmable one, which is almost certainly not going to work. Do you have a time switch elsewhere in the house? If so, what is it?

I will have to assume when you say time switch you are talking about what I referred to as the controller. That is a Honeywell ST6400C. Obviously I'm new to this and don't know all the requirements and terminology, otherwise I would have just plugged my crystal ball into my ford ideal boiler and used that as the thermostat.

 

[muggles]

Terminal 1 to A.

Terminal 3 to B

Terminal 2 made safe.

Leave the timer on constant for heating.

Terminal 2 was the only neutral. The fact that you thought you had 2 switched neutrals tells us that you are not competent to be working on electrics safely.

As you've got an ST6400C, you need to do as Dan has said above. I was trying to establish whether you had a combi boiler or a system/heat only, it seems you have the latter.

 

[Ukpmel]

Terminal 1 to A.

Terminal 3 to B

Terminal 2 made safe.

Leave the timer on constant for heating.

Terminal 2 was the only neutral. The fact that you thought you had 2 switched neutrals tells us that you are not competent to be working on electrics safely.

As you've got an ST6400C, you need to do as Dan has said above. I was trying to establish whether you had a combi boiler or a system/heat only, it seems you have the latter.

Thanks for the help, guys. I probably am in over my head. I'll ask my boiler guy when he comes to service it.

 

[D_Hailsham]

Terminal 1 to A.

Terminal 3 to B

Terminal 2 made safe.

Leave the timer on constant for heating.

I'll ask my boiler guy when he comes to service it.

No need to wait for the boiler guy! Just wire as Dan's instructions above.

Terminal 1 mean the wire connected to T1 on the old thermostat, and so on.

The safest way of making the wire from T2 safe is to use a small terminal block behind the backing plate.

PS Terminal C is used when controlling a cooling system.

 

[Ukpmel]

Terminal 1 to A.

Terminal 3 to B

Terminal 2 made safe.

Leave the timer on constant for heating.

I'll ask my boiler guy when he comes to service it.

No need to wait for the boiler guy! Just wire as Dan's instructions above.

Terminal 1 mean the wire connected to T1 on the old thermostat, and so on.

The safest way of making the wire from T2 safe is to use a small terminal block behind the backing plate.

PS Terminal C is used when controlling a cooling system.

Thanks - Of course, the instructions show examples for every type of system you could want, except the one you have...

The "switched neutral" confused me too, but that's what I found on another forum on a diagram that looked a lot like my situation. It made sense, the neutral is broken by a switch (in this case, the thermostat).

 

[D_Hailsham]

The "switched neutral" confused me too, but that's what I found on another forum on a diagram that looked a lot like my situation. It made sense, the neutral is broken by a switch (in this case, the thermostat).

A switched neutral makes no sense at all. Can you provide a link to the diagram?

 

[Ukpmel]

The "switched neutral" confused me too, but that's what I found on another forum on a diagram that looked a lot like my situation. It made sense, the neutral is broken by a switch (in this case, the thermostat).

A switched neutral makes no sense at all. Can you provide a link to the diagram?

I'll have to find it again. I hooked it up as you said, but it has the same problem as before - the heating comes on and stays on. I tried manually setting the temp, even turning the new thermostat to off. The display shows that it isn't calling for heat (no flame icon), but my boiler Is running, and my rads are getting hot. If I put the old thermostat back, everything works as normal.

 

[Ukpmel]

A switched neutral makes no sense at all. Can you provide a link to the diagram?[/quote]

Tell me about it... It made no sense to me either, but since I'm not an electrician or plumber, I figured somebody must know. I must have been hallucinating, though, because now I can't find anything that mentions it.

 

[EFLImpudence]

A switched neutral is not allowed.
Edit - unless a double pole switch.

A neutral is the return path from the load back to the supply.

What you are describing is a live to the thermostat, a switched live to the boiler, the load, and only then a neutral return.

 

[D_Hailsham]

I hooked it up as you said, but it has the same problem as before - the heating comes on and stays on. I tried manually setting the temp, even turning the new thermostat to off. The display shows that it isn't calling for heat (no flame icon), but my boiler Is running, and my rads are getting hot. If I put the old thermostat back, everything works as normal.

I have a horrible feeling that you have knackered the stat when you wired it incorrectly. Connecting Live to A and neutral to B or C would cause a short, blowing the fuse, and possibly fusing the contacts of the switch together.

You can check the stat (Not connected) with a multimeter set to measure resistance. You should get

A-B
Temp low = open circuit
Temp high = low resistance (1-2 ohms)

A-C
Temp low = low resistance (1-2 ohms)
Temp high = open circuit

 

[D_Hailsham]

Just a thought.

Are you talking about the wired or wireless version of the CM9XX?

 

[Ukpmel]

I hooked it up as you said, but it has the same problem as before - the heating comes on and stays on. I tried manually setting the temp, even turning the new thermostat to off. The display shows that it isn't calling for heat (no flame icon), but my boiler Is running, and my rads are getting hot. If I put the old thermostat back, everything works as normal.

I have a horrible feeling that you have knackered the stat when you wired it incorrectly. Connecting Live to A and neutral to B or C would cause a short, blowing the fuse, and possibly fusing the contacts of the switch together.

You can check the stat (Not connected) with a multimeter set to measure resistance. You should get

A-B
Temp low = open circuit
Temp high = low resistance (1-2 ohms)

A-C
Temp low = low resistance (1-2 ohms)
Temp high = open circuit

I tried that a little while ago, A to B is always low resistance. Looks like I buggered it when I hooked it up wrong the first time. Oh well, moderately priced lesson learned...