Replacing an old room thermostat

[untrepid]

Hi,

I tried replacing an existing, and very dated, Honeywell room thermostat with a new one, the maker of which escapes me now, but seemed like a decent enough piece of kit. [Well, to be honest, my father, an electrician for 30 years had never heard of them...] We found though, that as soon as the new thermostat turned on from its timer, it immediately and reliably blew the fuse on the circuit. Putting the old thermostat back on and everything was working fine again. Trying to compare the two vague schematic diagrams we could see, there seemed to be possible differences in the resistive loads (or a total lack of, if that makes any sense?) between the two which seems like a reasonable cause for the fuse going pop every time.

Does it sound like there is a good reason that the new one didn't work? I'd say there was 20 years between the two bits of kit, so maybe some standard or convention as changed over that time? Or did I just buy junk? It was *fairly* cheap, about £10 to £15 for a 5/2 programmable features set...

Is there anything specific I might need to look for on another room thermostat if the new one is a no go?

 

[cross thread]

sounds like you have fitted switched live to the neutral connection

 

[ajrobb]

Old Honeywell thermostats were good pieces of kit and some included a heating resistor to narrow the hysteresis between turning on and off while keeping enough spring to make a clean break. These resistors needed a neutral connection. Digital thermostats should only need live and switched live connections.

 

[untrepid]

sounds like you have fitted switched live to the neutral connection

Well, I can't remember much about trying to fit it outside of the confusion and the swearing, but I'd be pretty confident it was wired in correctly. Maybe.

 

[untrepid]

Old Honeywell thermostats were good pieces of kit and some included a heating resistor to narrow the hysteresis between turning on and off while keeping enough spring to make a clean break. These resistors needed a neutral connection. Digital thermostats should only need live and switched live connections.

Does this lead to any real end? Does it suggest I really probably just stuck a wire were I shouldn't have?

 

[Gasop]

Old Honeywell thermostats were good pieces of kit and some included a heating resistor to narrow the hysteresis between turning on and off while keeping enough spring to make a clean break. These resistors needed a neutral connection. Digital thermostats should only need live and switched live connections.

Does this lead to any real end? Does it suggest I really probably just stuck a wire were I shouldn't have?

Yes!

Try leaving the blue wire isolated.

 

[Happyplumber]

I see the electricians are as usaul ........over complicating things!! Just terminate the neutral and use the other two cables

 

[untrepid]

Hi again,

My wife tried wiring the same thermostat again this afternoon with the same results (surprise surprise!) I did realise I gave pretty useless information last time I posted...

The thermostat is this one http://www.plumbworld.co.uk/glowarm-rt500-programmable-1932-17046 and I can't see any way to wire it up differently. The Live and Switched Live are connected, and there's nothing else to connect at all, other than Earth which doesn't go anywere at all, just isolated by itself on the the plastic holding bracket. On that page there's a link to the bracket wiring diagram - http://www.plumbworld.co.uk/link/12/pwsc0005wiring.gif

From where I stand literally the only other thing I could do would be to swap the L and SL wires, but that should surely be utterly pointless, assuming that it's just a normal relay connecting the two wires together when it turns on.

So as there's nothing else I can imagine doing, I'm not sure what I'm hoping for here, but any thoughts would still be appreciated.

 

[mikely]

What wires do you have at the thermostat and which was connected where on the old thermostat?

 

[untrepid]

Just Live, Neutral and Earth, nothing that seems "interesting" to me.

 

[ServiceNumpty]

You may find that the earth cable is actually the switched live. Sometimes the colours used were red, blue and solid yellow.

 

[untrepid]

Thank you all for your help.

We isolated the blue wire and used the "earth" as the switched live.

All working now.

Hurrah!