Room thermostat

[christopher]

I want to change a 25 year old Honeywell 42003878 on off 240v 2 wire plus earth thermostat for a programmable one , using the same wiring.

Can anyone recommend something or tell me what to avoid

Chris

[Dave Plowman (News)]

If the original is 2 wire, a programmable one is a direct replacement with no gotchas.

[Graham.]

Actually it probably should have been wired with 3 wires + earth so the anti-hysteresis resistor had a neutral return which would have made the thing more responsive. A modern electronic replacement won't necessarily need a neutral

Ebay item number: 110627488174

I like the type that has a knob that mimics an old electromechanical one rather than fiddly "up/down" buttons.

[Dave Liquorice]

same

Big "if" though an old thermostat like that should be three wire plus earth. Live from programmer, switched live to boiler and neutral for the compensation heater.

How ever even if it does turn out to be three wire[*] most programmable stats are battery powered and have "voltage free" contacts. Just connect the live from programmer to the switched live to boiler via the terminals that make on demand for heat.

[*] It's not unknown for ordinary T&E to be used for a stat with the CPC being hijacked for one of the other required uses.

[Graham.]

[**] It's not unknown in the pre-ELCB/RCD era for the earth to be connected normally and the neutral terminal strapped to it. I couldn't possibly tell you how I know this ;-)

[<me9]

A thing to watch out for is a new one with changeover contacts. If connected in place of one that needed a neutral there may be a big bang!

[Dave Plowman (News)]

I don't know of any which does - they're all battery operated if designed for the replacement market. The clock needs to be kept running.