In article ,
Tim Smith wrote:
I was just doing some testing on my existing thermostat as I dont
believe it is working quite right anyway. It is an old(ish) Honeywell
T40 electro-mechanical type.

It has 3 connections.

Terminal 1. Red Terminal 2. Blue Terminal 3. Yellow

The Red appears to be live (approximately 196V measured against terminal
3 on alternating voltage scale on the voltmeter).

Yup. That will be the line in.

The Blue appears to be either 1.5V or 3V (also measured against Termianl
2) depending whether the thermostat is turned right down or right up

That's likely to be a neutral - needed for the accelerator coil in a
mechanical stat.

The yellow will be the switch return to the pump. It should vary from
near 0 to mains depending on whether the thermostat is satisfied or not.

Does this sound right ? I was expecting that the switched live would
either be 0V or live. I have had problems with the heating being on but
never really reaching the desired level (although I assume that these
old mechanical types are reasonably inaccurate).

They're not terribly accurate in terms of operating at the temp on their
scale, but should be reasonably consistent. And of course any thermostat
can only measure the local temperature. Where you sit in the room may well
be colder.

Modern programmable electronic types are battery operated so don't need
the neutral. The reason for this is they require continuous volts for
their clock and electronics, and of course the live feed to a thermostat
gets switched off if you switch off the system. Suppose they could have
used a re-chargeable, but in practice the battery is cheap and has a long
life.

--
*Dancing is a perpendicular expression of a horizontal desire *

Dave Plowman London SW
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