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.