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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)]

In article ,
wrote:
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

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

--
*Marathon runners with bad shoes suffer the agony of de feet*

Dave Plowman London SW
To e-mail, change noise into sound.

[Graham.]

wrote in message ...
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
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.

--
Graham.

%Profound_observation%

[Dave Liquorice[_2_]]

On Fri, 31 Dec 2010 10:48:14 +0000 (GMT), Dave Plowman (News) wrote:

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.

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

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.

--
Cheers
Dave.

[Graham.]

"Dave Liquorice" wrote in message ll.co.uk...
On Fri, 31 Dec 2010 10:48:14 +0000 (GMT), Dave Plowman (News) wrote:

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.

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

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.

[**] 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 ;-)

--
Graham.

%Profound_observation%

On 31 Dec,
"Graham." wrote:

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

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!

--
B Thumbs
Change lycos to yahoo to reply

[Dave Plowman (News)]

In article ,
Graham. wrote:
A modern electronic replacement won't necessarily need a neutral

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.

--
*You never really learn to swear until you learn to drive *

Dave Plowman London SW
To e-mail, change noise into sound.