Replacing Horstmann HRT2 Thermostat

[Sensor]

Hello,

I'm looking to replace my Horstmann HRT2 with a programmable such as a Horstmann HRt4-B or Danfoss RET-M but I'm a little confused with the current wiring.

The HRT2 datasheet shows:
1 - L (Line)
2 Do Not Use
3 Load Heat
4 N

The current wiring :
1 Brown (multimeter shows 230 Vac)
2 Blue
3 Green/Yellow earth sleeving
4 No cable fitted

Rather concerned by N to terminal 2 where datasheet says not to use.

Boiler is an old Glowworm Space Saver 50, pumped system.

http://www.horstmann.co.uk/download.../ThermoStats/HRT2UserandInstallationGuide.PDF

Anybody able to shed a little light for me?
Many thanks for your time.

 

[jackthom]

Maybe just a wiring error which presumably hasn't totally prevented the thermostat from working.

Use the multimeter, with one probe kept on the blue neutral cable, to check that the Live remains 230V regardless of the thermostat setting and that the wire on terminal 3 only goes Live when the thermostat is calling for heat.

Then you have identified the 3 wires that the new thermostat will need.

(Just don't ever forget that the wire colour which would normally be used for an Earth connection is actually switched Live here.)

 

[D_Hailsham]

The HRT2 datasheet shows:
1 - L (Line)
2 Do Not Use
3 Load Heat
4 N

The current wiring :
1 Brown (multimeter shows 230 Vac)
2 Blue
3 Green/Yellow earth sleeving
4 No cable fitted

I think the installer thought he was wiring up a Honeywell stat, where 1 = L, 2 = N and 3 = Load.

Are you saying that the earth wire is sleeved with green/yellow sleeving? If so, it would be better to change it to red or brown sleeving to show that it is carrying voltage.

You said you wanted to install a programmable stat: the two you mentioned are not programmable.

 

[Sensor]

Yes, the cable that has been used is standard 3 core cable (not T+E), so using the multimeter as jackthom suggested shows the 'earth' wire is carrying the switched live. Will mark it/sleeve it.

On the subject of the 'stats I meant to say digital rather than programmable. Recommendations for digital (non-RF) room 'stats welcome...

Many thanks for taking the time to reply, the advice is appreciated.

 

[D_Hailsham]

Yes, the cable that has been used is standard 3 core cable (not T+E), so using the multimeter as jackthom suggested shows the 'earth' wire is carrying the switched live. Will mark it/sleeve it.

On the subject of the 'stats I meant to say digital rather than programmable. Recommendations for digital (non-RF) room 'stats welcome...

Recommendations first

Honeywell DT90E.

As for wiring,

HRT2 ---- DT90E
1 -------------- A
3 -------------- B

If I was doing it, I would use the blue wire, currently in 4, as the switched live and make the green/yellow safe. But it does mean finding the other end of the cable, moving the blue to the terminal holding the green/yellow and moving the green/yellow to the earth terminal. The switched live should still be sleeved in red or brown at both ends.