Replacing old analog Room Stat with Digital Wireless Stat

[sseager]

Evening all,

I am looking to replace my old Honeywell room stat with a newer, digital model.

I've gone for an RF reciever and transmitter, and am looking to wire the receiver into the old slot where the Honeywell came off.

I have a brown, blue and yellow wires in the existing stat, which I assume are live, neutral and switched live.

The new stat receiver requires a live, neutral, switched live in and switched live out.

As I've only got three wires in the old stat, can I take a live feed from the brown via a small length of wire and use it to power the switched live in? I don't really want to have to run a new cable through the wall and under the floor boards.

Thanks in advance....

 

[jackthom]

I think you have the right idea but can you say which model you are fitting just to be certain.

 

[sseager]

It's a Programmable RF thermostat type RFWRT, made by grasslin. The user manual is online at www.tfc-group.co.uk if that helps?

 

[jackthom]

Ok thanks, so for each grasslin terminal the connections will be

1) Neutral (Usually Blue)
2) Live

3) A short wire link from Live (common )

4) Switched Live (usually yellow)

You can't always trust the original installer to have wired to any expected colour code but you should be able to identify each cable using the old thermostat wiring.

 

[sseager]

Works fine, thanks for that.

But...

The live feed to the thermostat is switched by a timer in the boiler cupboard. Does the receiver need a permanent, as in NEVER off live, or will it be ok being switched on and off twice a day?

The boiler also heats the water tank, so I don't really want to go down the route of leaving the timer switched on permanently and relying on the programmer to regulate the demand, as it'll only do so for the heating - I don't need my hot water topping up at 3am.

Cheers

 

[jackthom]

The receiver does need a permanent live and the Grasslin should be taking sole control of all the programmed periods for heating.

Is the old timer a single channel unit so both heating and hot water are tied together?

With a 2 channel timer you could set the heating independently to be on 24 hrs and that would effectively provide the permanent live without any wiring change.

With a single channel you will have to find the other end of the thermostat wiring, which could be in a wiring centre box or at the old timer itself, then reposition the thermostat brown wire to be connected to Live.

If you need help with that some pictures of the relevant wiring would be useful.

 

[sseager]

The old timer is indeed a single channel timer so everything is off or on together, with separate thermostats controlling the flow to the central heating system and hot water tank via a motorised valve.

I've had a poke around in the centre box and have located the wire which runs to the thermostat for central heating. The box has no permanent live connection anywhere within it - if I switch the timer off, the entire contents go dead.

I would guess that my only option is to steal a permanent live from elsewhere and run it into the box to feed the thermostat?

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[jackthom]

Can you post a picture of the single channel timer wiring and what is the model number

 

[sseager]

Yeah, one like you've pictured.

 

[jackthom]

Do you have the model number of the old timer and can you post a picture of the wiring?

To get the 3-port motorised valve to work properly it will also need to have power available permanently so this overall control of mains power that the timer has will need to be looked at.

 

[sseager]

Pics as requested...

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[sseager]

Having slept on it, and looked again at the pictures, would there be anything wrong with the following set up...

Find a permanent live to run into the centre box. I'm looking at stealing the one from a redundant immersion heater we never use - obviously changing the size of the fuse in the spur...

Use the vacant block to wire the new permanent live to the CH thermostat live, neutrals and earths in with the others at the far right.

Timer then controls the boiler on off, motorised valve and tank thermostat. CH stat permanently on, but obviously only able to call for heat at time boiler set on.

Change the timer to be on morning and afternoon/evening. Tank stat controls hot water demand, programmable ch stat controls heating demand during periods of boiler activity.

Messy I know, and doesn't solve the problem you mention of no permanent live to motorised valve, but in theory at least, I think that should get me what I'm after, bearing in mind the existing set up works fine with an old style mechanical room stat.

 

[jackthom]

There's a basic problem with the existing Danfoss programmer because it almost certainly can't provide the signals you need to correctly operate the 3-port valve at those times when you want heating but don't want your hot water cylinder to be heated (ie Danfoss set to OFF).

To get properly independent control of both heating and DHW, a new single channel programmer which can supply HW_ON and HW_OFF signals would be required. It would need a small amount of rewiring at both the programmer end and also in the wiring centre.

eg// This one can do three on/off periods per day...
http://www.honeywelluk.com/products/Time-Controls/Wired/ST9100a/


Although you could connect a mains Live feed to the thermostat wiring to keep the receiver powered permanently, that most likely still won't give you heating when the Danfoss programmer has turned OFF. (Stealing a Live from another circuit could be dangerous if someone else comes to service the thermostat/wiring centre, turns off the boiler supply etc and is surprised to find it's still live in there)

So I think it's a case of either keeping the Danfoss switched ON whenever you might need heating or considering a new programmer and a bit of rewiring.