Room Thermostat with 330k internal resistor

[Ezhik]

I have just replaced a 'mechanical' type thermostat with an electronic programmable Honeywell CM51. The old one had 3 wires connected, red wire live input to the switch, then two outputs, yellow wire direct from the switched contacts and blue wire which connected to the switch contacts via a 330k (half watt or so) internal resistor.

The CM51 has only two contacts, so I put a 330k in series with the blue wire and connected it the contacts with the yellow wire to make the circuit just the same as in the old one.

So there is a yellow wire straight from the switched contact, a blue wire from the switched contact via a 330k, and a red wire live input. It works as expected, but I'm curious about the blue wire. The wiring diagrams that come with the CM51 don't show any connections that have a need for a resistor.

Anyone know where the blue and yellow wires would be going?

[Christian McArdle]

They aren't required. Remove the resistor and disconnect (and insulate) the blue wire to it. The red goes to 'C' and the yellow to 'NO' (usually, or NC, sometimes). You've already got these right, if the thing is working as expected.

Christian.

[Set Square]

The resistor in the old stat was a heater which generated a bit of heat when the stat switched the heating on, and improved the response of the temperature sensing mechanism. The third wire was a neutral return for it. Modern stats don't need this - requiring just a live and switched live. The neutral wire - and your unscheduled resistor! - should be disconnected and made safe.

[Andrew Gabriel]

The resistor is compensating for the hysteresis in the mechanical thermostat by providing a little heat when the stat is off. It's sometimes called an accelerator resistor.

You don't want it with an electronic stat -- actually it will lead to inaccuracy if you've got it anywhere near the temperature sensor. Leave the blue (neutral) disconnected if the new stat doesn't need a netral connection.

[MikeS]

Andrew, Am I correct in thinking the old type thermostats where ether bimetal strip or wax filled mechanisms, whereas modern are electronic solid state thermostats.

MikeS

[Ezhik]

Thanks for your reply. I've removed and insulated the blue wire and still works correctly. Appreciated...cheers!

[Ezhik]

Thanks very much for all the replies. I now understand what the resistor was there for, and why its not needed with the new electronic stat.

I did say it was working as it should in my first post, but having now reomoved the resistor I can say it was having an effect. The heating was switching off (ie reaching temperature) too soon, now that I've taken the resistor out I can turn down the stat by at least 1 degree!

So thanks again to everyone for your replies, job now done!

[Set Square]

Surely, it provides a little heat when the stat is *on* - thus compensating for a lag in the stat responding to a rise in room temperature.

The other way round: a) would delay the response to room cooling b) wouldn't need a third wire - 'cos you could connect the resistor across the contacts, using the the load for the return.

[Andrew Gabriel]

Yes, I got it wrong way round...