Thermostat change

[emphasis303]

Hi guys

A friend of mine has a honeywell thermostat.

Now he has 3 wires going in, a blue brown and earth wire.

He wants to change it to this thermostat but it says its only a two wire installation. Im guessing it doesnt take an earth so am I correct in saying we dont need one?

http://www.screwfix.com/prods/40819...rols/Honeywell-DT200-Digital-Room-Thermostat#

Thank you very much

 

[Taylortwocities]

Many thermostats need three wires plus the earth. The 3 wires are live, switched live and neutral.
Your friends old stat doesnt have a neutral so the stat he is buying (two wires) is the right choice.
The earth wire should be connected to the metal back box that the stat screws on to.

 

[briwire]

The three wire system was really needed on the old style boilers. This was because the thermostat would energise a coil on a gas valve directly. So that as the stat turned on and off you would get a spark of back emf from the valve coil that would appear as a flash across a TV screen or a crackle on a radio. To overcome this, a interference surpressor was included in the stat, but this needed a neutral, hence the three wires.

The new type boilers use the stat to provide a signal to the control PCB. No load, no interference, so the neutral is not need.

 

[Taylortwocities]

Not quite Briwaire. The neutral is needed to power a wee heater in the stat that is called the accelerator. Mechanical thermostats include an accelerator (or anticipator) which has the effect of smoothing out the temperature cycle, so that on and off periods are not too long and makes the stat more accurate.

 

[emphasis303]

thanks for all ur replies

he does have an old boiler, so I am going to connect the blue and brown cables and then the earth to the metal back box?

thank you

 

[Taylortwocities]

Yes

 

[bernardgreen]

Before you do verify the earth wire was an earth wire connected to earth at the boiler end.

There are hundreds if not thousands of thermostats where the naked copper wire intended for earth has been used for neutral. I have also seen it used as switched live and on one install it was the permanent live.

Twin and earth to an old mechanical thermostat means bodge and potential danger.