Combi boiler room thermostat connection

[franks.flite.]

I have a 3 year old Protherm 100 EC. I have purchased a Danfoss TP5, no voltage stat and Danfoss have told me (by email) which 2 terminals to connect to the boiler (2 +3) but they will not tell me what gauge wire I can use. I would like to use "bell wire" and as there in no volt I can't see why not. A qualified electrician could not give me a straight answer. Also the boiler wiring diagram shows 4 wires in a connector block, 2 with a bridge which I know I have to remove, and 1 marked blue -. and one marked red +. It is not clear which 2 to use and whether the Danfoss will be polarity sensitive.

I would appreciate any knowledge on this as I cannot trace Prothem on the web to ask the questions.

 

[chrishutt]

The stat will probably switch an extra low voltage (24 volts I think) from the boiler (but check this via manual or wiring diagram or multimeter). If the wire is too thin there will be too high a voltage drop, so I wouldn't assume that bell wire would do. You could ask on the electrical forum, or just try the bell wire first and upgrade to 1mm cable if it doesn't work. In most cases it doesn't matter which of the two boiler terminals (the ones with the bridge) goes to which on the stat.

 

[croydoncorgi]

Not clear from my copy of manual whether thermostat carries mains voltage or not. (Boiler DEFINITELY requires volt-free type thermostat but that does NOT necessarily imply low voltage on the wire.)

To be safe (unless you can confirm low voltage control) use 1mm twin-&-earth cable. Bell wire is NOT rated for mains voltage.

For a 2-wire thermostat connection, polarity is not an issue.

 

[franks.flite.]

Thank you, Chrishutt and Croydoncorgi. You have highlighted something I overlooked, although the Danfoss TP5 is battery powered it could still switch a voltage without using that current to power the thermostat. This perhaps is also the reason why the "Switch rating of the relay contact" is specified as 6(2)A, 10-264Vac, I assume anything from 10 volt to mains, with a "Rated impulse voltage of 2.5Kv. Also the "Switching action of output relay" is specified as 1 x SPDT.Type18. I asked Danfoss what it meant but they failed to tell me.

On the other side of the debate, the Protherm manual states "on no account must any electrical voltage be applied to either of the terminals of the external controls connection". The diagram for the connection is not clear, it only says "remove the link (bridge).

I shall ask the qualified electrician again but they seem to over specify and I don't fancy a large cable nailed down the lounge wall!

 

[bster]

I always wire terminal 1 (supply) and 3 (switched live) and assumed that terminal 2 was for a 3 wire configuration (incorparating a neutral to smooth the on\offs) but the TP5 instructions say 'do ont connect existing earth or neutrals', so when is term 2 used?