Replacing a room thermostat

[RichieHammer]

Hi

I am in the process of replacing an old Honeywell room thermostat (no model number but probably about 25 years old) with a digital programmable thermostat from Horstmann (Centaurstat 7).

There are three wires for the existing thermostat (red on 1, blue on 2 and yellow on 3). Reading various posts on this subject, I have assumed that red is live, blue is neutral and yellow is the switched live. The new thermostat has four connections as follows: 1 L (Line), 2 Load (Heat), 3 Load (Cool) and 4 N (Parking).

I have connected the red wire to 1, the yellow wire to 2 and the blue wire to 4 (to park the neutral as this is not needed).

When I switched on the programmer, the heating came on immediately, regardless of the settings on the thermostat. My first thought was to connect the yellow to 3 but the documentation clearly states that this is for a cooling system switch.

If there is anybody with experience with these thermostats then I would be grateful for your comments.

 

[breezer]

try it and see.

oh and put the neutral in a choc bloc just in case

 

[chrishutt]

Are you aware that the Centaurstat does not switch off completely, but has a "night set back" temperature which might be preset at 12C or so? So depending on the programming, the Centaurstat will either be switching on the heating for the set day time temperature or the night set back temperature.

 

[RichieHammer]

Thanks for the messages.

I also contacted Horstmann and they sent me a conversion table for a number of different thermostats. On closer examination, my Honeywell thermostat is a T6160B model. Their table states that the T6160B should have four wires connected, with 1 being Common, 2 being Neutral, 3 being Call/On and 4 being Sat/Off.

However, my connection to the T6160B only had three wires connected, with no wire connected to terminal 4 (Sat/Off). Is it possible that this is why the new thermostat is not switching the boiler off?

I have emailed Horstmann again with this information and I await their reply. As my Honeywell thermostat has worked fine for 25 years with three wires, I am puzzled by this problem.

 

[simond]

Hello there

Checked the Horstmann data sheet.

This unit has volt free contacts and is purely a changeover switch controlled by a battery operated relay.

The original Honeywell unit when wired correctly uses a live in, live out and a neutral for a small heater inside the case. NB:It will also work without the neutral, but not as accurately.

The Horstmann unit 'park' terminal is just for you to marshal the neutral wire safely - it doesn't connect to anything inside and will now be redundant.

You just need to connect the remaining two wires to terms 1 and 2. Doesn't matter which way round.

There are three likely reasons why the unit is on all the time;
1. You haven't programmed it correctly (as per Chris H)
2. The unit was faulty out of the box
3. You have, at some point, inadvertently connected the neutral to one of the switch terminals and welded on the relay terminals (breezer was alluding to this)

You would be surprised how often we see (3).
You can't rely on the neutral being blue - heating engineers have been known to take liberties with their colour coding or even use earth wires, we've all seen it.

My advice is based upon the unit being wired correctly originally. Had a few shocks myself, can't say i enjoyed any of them.

 

[RichieHammer]

Thanks Simon for this advice. I have now changed this device for a different model and now everything is fine.