Replacing old room thermostat (mains wired)

[scep]

The current room thermostat is a c. 40 year old Danfoss mechanical thing.
There are only three wires coming in an out - live and (switched), neutral and earth. The thermostat mechanism itself just directly switches this circuit on and off (which I presume is part of the pump/boiler circuit).

What are my options for fitting a prgrammable digital thermostat? I've looked around the forum but still not clear - can I simply wire this setup into a modern thermostat or do I need a particular one that can handle 230v (instead of a low voltage/no voltage control circuit?).

(N.b. Boiler is a Nettaheat Profile with a Potterton EP2001 controller)

Cheers

 

[D_Hailsham]

There are only three wires coming in an out - live and (switched), neutral and earth.

I make that four wires: Live; Switched live; Neutral; Earth.

What are my options for fitting a prgrammable digital thermostat? I've looked around the forum but still not clear - can I simply wire this setup into a modern thermostat or do I need a particular one that can handle 230v (instead of a low voltage/no voltage control circuit?).

As far as I know, all programmable thermostats can handle 230V as well as "volt-free" switching. It's just a case of how you wire it up.

The EP2001 will have to be retained to control hot water times.

To save you asking, the firm favourites are the Honeywell CM900 series.

 

[scep]

Thanks for the quick & useful reply. That's an errant comma! I was trying to say (in my garbled layman's way ) that it's a neutral wire but the unit switched it...

 

[D_Hailsham]

Thanks for the quick & useful reply. That's an errant comma! I was trying to say (in my garbled layman's way ) that it's a neutral wire but the unit switched it...

That should be OK then. Modern programmable stats are battery operated and double insulated so you don't need the earth. The existing live and switched live (aka neutral) are all you need to replace the existing thermostat with a programmable one in the same location.

 

[scep]

Nice one - off to buy a CM907 then

cheers!

 

[JohnD]

Thanks for the quick & useful reply. That's an errant comma! I was trying to say (in my garbled layman's way ) that it's a neutral wire but the unit switched it...

that doesn't sound right. Do you mean its a black wire?

if yes and it is switched, then it will be the switched live (and ought to have red sleeving pushed over the black at each end to show it)

if there is no neutral then you will have to use a battery-operated digital stat, as there is no mains supply of L & N (I was looking yesterday, and some do run off the mains).

BTW it is very bad practice to use the earth conductor for anything but an eath. Just sleeve and park it.

 

[scep]

My fault - explanation was bad. I'll start again

1) Three wires come into the stat i.e. if you pulled the stat off the wall there would just be a 'normal' 3-core cable like you would find in a light or plug socket (old colours, red & black + unsheathed earth).

2) When the stat is "off" (i.e. boiler/pump is off) there is 230v across the black terminal and the red terminal.

3) When the stat is "on" and has triggered the boiler/pump, there is no voltage across these terminals (the internal wiring is hidden so can't see what's going on).

I'm thinking that this is a switched live as per JohnD and so I'll need a battery powered stat? Here's a few pics to help clarify:

 

[londonboy]

You are correct, you will need a '2 core' or '2 wire' programmable thermostat. These are battery powered as you say.

A quick look on screwfix: Horstmann DRT2, Drayton Digistat +2, Siemens RDJ10-GB are all suitable.

 

[D_Hailsham]

Nice one - off to buy a CM907 then

The CM907 is also suitable.

 

[scep]

CM907 installed and working nicely - cheers