Replacing a wired C/H thermostat with a wireless thermostat

[JasperW]

Hi,

We have a 4 story 4 bedroom house with a C/H system originally installed in the late 80s. Since moving in I have replaced the boiler (with a Worcester Bosch 40 CDi conventional) and hot water tank, I have also placed wireless radiator control valves on most of the radiators.

Still left over from the original installation is the main thermostat for the central heating, it is an old Honeywell model (see picture). What I would like to do is to replace it with a wireless thermostat so that I can move the control unit into the basement which is the coldest part of the house and the one room which does not have control valves on the radiators.

What I need is some advice on what wireless thermostat would work with my existing wiring. I have attached a picture of the thermostat. There are three wires coming in (I have marked them on the image); line 1 is the live wire, line 3 switches on if the room is cold and off when it has reach desired temperature. Line 2 appears to be off all time time (at least when measured with my electricians screwdriver)

Any help or suggestions gratefully received!

Jasper.


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• JasperW
• 29 Oct 2011
• 1

 

[ukrobj]

Your wireless room stat will usually comprise of two parts. The receiver and the sender. The sender is the room stat that you've taken a picture off and the receiver is the switching relay. The switching relay box is best placed near too the wiring center, depending on what type of system you have its *usually* between terminal block positions 4 and 5.

Check out a S or Y plan schematics.

Then your room stat will just sit over the old wiring (un-used)

 

[JasperW]

Your wireless room stat will usually comprise of two parts. The receiver and the sender. The sender is the room stat that you've taken a picture off and the receiver is the switching relay. The switching relay box is best placed near too the wiring center, depending on what type of system you have its *usually* between terminal block positions 4 and 5.

Check out a S or Y plan schematics.

Then your room stat will just sit over the old wiring (un-used)

Ideally what I would like to do is to replace the existing thermostat with the receiver of the wired unit and then place the sender in the basement which we need to control the temperature. The existing thermostat is in one floor down from the main controls which are right at the top of the house. I am a little worried that if I place the receiver at the top of the house and the sender in the basement the signal might not be strong enough to control it.

Since my original post it has occured to me that I would need to run a separate power line to the receiver since I am assuming that none of the lines running into the existing thermostat is a neutal.

What is puzzling me though is why are there three lines coming into the thermostat? Lines 1 and 3 make perfect sense to me, I don't understand if line 2 (which appears to be off all the time is likely to be doing anything.

 

[Dan Robinson]

2 Is neutral.

So,

Take off you old room stat.

Fit new receiver box.

2 goes to N,

1 Goes to L (left hand of the two)

Link second L to A

and 3 goes to B.

Of course I am assuming you are going to use a CM921/927 as your replacement and that the current room stat is CORRECTLY installed.

 

[D_Hailsham]

I am a little worried that if I place the receiver at the top of the house and the sender in the basement the signal might not be strong enough to control it.

Definitely a good idea to check the signal before fixing the units in place. Easiest way is to attack a long lead with a plug on one end to the receiver and then try various locations

What is puzzling me though is why are there three lines coming into the thermostat? Lines 1 and 3 make perfect sense to me, I don't understand if line 2 (which appears to be off all the time is likely to be doing anything.

Look at the wiring diagram - it's inside the stat. See the pic you posted!

 

[JasperW]

2 Is neutral.

So,

Take off you old room stat.

Fit new receiver box.

2 goes to N,

1 Goes to L (left hand of the two)

Link second L to A

and 3 goes to B.

Of course I am assuming you are going to use a CM921/927 as your replacement and that the current room stat is CORRECTLY installed.

Brilliant! Thanks a lot; I had not twigged that the box on the diagram represented the boiler, it now all makes perfect sense as does your wiring instructions. I was looking at CM921/927 as the replacement as people seem to rate it it and had already downloaded the installation guide.

Jasper.

 

[Dan Robinson]

Forgot to add - you will need to leave your timer on constant for Central Heating.

Remember to activate the CM92x's Optimisation and window Open features - which are turned off by default.

 

[D_Hailsham]

Remember to activate the CM92x's ... window Open features - which are turned off by default.

But that's only required if you are controlling Honeywell HR80 wireless rad valves. The OP has said he has wireless valves, but nothing about them.

 

[MJN]

Remember to activate the CM92x's Optimisation and window Open features - which are turned off by default.

What does the 'window open' function do?

At a guess, does it try and detect a window (or door) being open through a sudden drop in room temperature and either turn the heating off or not try and fight it through over-aggressive compensation?

Mathew

 

[Dan Robinson]

Yep - it switches off until the temperature plateaus.

Depending on your boiler there is a pump exercise program too. If the boiler does not have this feature, then the programmer will spin the heating controls once every 24 hours.

 

[MJN]

Thanks Dan.

 

[JasperW]

Thanks for all the advice, in the end I went for a DT92E (which has the same relay as the CM927 just fewer features and is about 1/2 the price(£77). Fitting it was very easy and the signal has no problems with our tall house.

I think this will easily have paid for itself by the end of the winter!

Thanks again.

2 Is neutral.

So,

Take off you old room stat.

Fit new receiver box.

2 goes to N,

1 Goes to L (left hand of the two)

Link second L to A

and 3 goes to B.

Of course I am assuming you are going to use a CM921/927 as your replacement and that the current room stat is CORRECTLY installed.

 

[Dan Robinson]

No worries mate.