wiring for new roomstat help please

[thundercats]

hi

i have halstead finest gold and have just removed the old room stat that looks like it was installed about 20 yrs ago!
to my surprise the wiring to the stat is just a live and neutral using 2.5mm

i am replacing the stat with a heatmiser lcd stat see

http://www.heatmiser.co.uk/dt_brochure.pdf

put in the live and neutral and it doesnt come on - i suspect i need more than two wires, anybody ca help so i can get the stst working what kind of cable do i need? and what wires go to which at the boiler and stat ends

regards

dave

 

[kevplumb]

try the electrical forum

 

[MANDATE]

Hi thundercats. I don't know for certain, but you might be missing an important point. A two wire room stat is nothing more than a switch. It has no neutral because there is no 'load'. It has a 'live' input and output is called a 'switched live'. A 'load' in the form of a motor, heater, lamp etc does need a 'neutral wire' and there are some room stats that have three wires because besides being a switch there is an additional device. I'm thinking if your new stat incorporates a small 'load' then there will a neutral terminal as well as the 'live' and 'switched live' and what you may have done is used this neutral terminal to connect what you think is the neutral wire. If your new stat requires a neutral then you will have run an additonal wire from the stat to a convienient neutral terminal at the boiler or junction box.
hope this makes sense!

 

[CH4]

thundercats,

The existing two wires are NOT live and neutral. They just carry a signal between two terminals on your boiler. The old thermostat, as MANDATE pointed out, functioned as a switch - to interrupt the boiler signal whenever the set room temperature was reached.

Your new thermostat needs its own mains supply to power its internal electronics.

So, you actually need four wires: the existing pair connected to the thermostat's NC (normally closed) and COM terminals and a permanent Live and neutral connected to the thermostat's L and N terminals, respectively, to power the electronics.

NB It does not matter which way round you connect the existing two wires, because you are using the thermostat as a volt-free switch.

 

[breezer]

thundercats has asked in the uk electrics forum, see here

 

[moderator]

i have locked this thread, pending deletion, as thundercats has asked in uk electrics