Two wire/Three wire thermostats?

[niavasha]

What advantage does the three wire thermostat offer over a two wire thermostat?

I've got a Potterton Kingfisher 2 CF125 linked to a Horstmann CentaurPlus C27 with three MZVs off it, two for CH and one for HW.

Could I use a three wire thermostat with the above and would it offer any advantages or is it not suited?

 

[EliteHeat]

If by three wire you mean having a neutral connection, then these usually contain a component called an antcipator.

These are supposed to make the temperature sensing more accurate and also allow for an LED to be illuminated when there is a call for heat - but that is probably not a major advantage!

I have no idea how the anticipator works but am certain that someone much cleverer will be along soon to tell you.

 

[samparker]

the anticipator in some (mainly older ones but can still get them now) works by: the neutral wire is heated when the stat is calling, and as it heats up, it allows the temperature sensor in the room stat (bimetallic switch usually) to heat up a bit quicker (usually 5-6 degrees or so). This means that the room stat reaches its set temperatue quicker, and breaks contacts (is satisfied) thus stopping the boiler heated water from going to the rads (until the temperature drops again and it makes, and thus puts power back on to the pump and boiler)!

the reason for an anticipator is: if the room temperature reached it's setting, ie 21 degrees c, the heat sensor (bi-metallic strip inside it) would then close (switch off boiler), but in doing so (over minutes) as it is slower to react, would then have reached over 21 degrees, thus meaning that the room was quite a bit over 21 and not very comfortable!! it basically allows the room temp setting to be shut off more precisely!!

a lot of room stats now, especially the digital types, use a thermistor inside instead, which is even more accurate and doesn't require a neutral wire either. some are battery powered or some just use the normal 2 wires (COMMON and CALL terminals)!-not sure how they get their power from though without the battery!...

 

[bengasman]

alternatively, you can ignore the reading on the stat, which is not calibrated anyway, and just set the stat as low as you feel comfortable with.

 

[D_Hailsham]

the neutral wire is heated when the stat is calling

To be accurate, there is a small resistance between the switched live and the neutral which heats up when the stat is calling. It is important that the live and switched live are connected correctly, otherwise the resistor will be heating up all the time.

a lot of room stats now, especially the digital types, use a thermistor inside instead, which is even more accurate and doesn't require a neutral wire either.

I don't know why they still make stats which rely on bimetallic sensors, when thermistors are so cheap, more accurate and more sensitive to temperature change.

some just use the normal 2 wires (COMMON and CALL terminals)!-not sure how they get their power from though without the battery!...

I think you will find they do have a neutral, to power the electronics in the stat.