Hi,

I have an old 2-zone oil fired hot water heating system. I'm trying
to replace one of the existing mercury thermostats with a Honeywell
programmable one (CT3600).

I just bought it from Home Depot and I'm reading the compatibility
note on the package - it says it's compatible with hot water system
of 2-wire-zone-valve but not with 3-wire-zone-valve. However, there
are not zone valves on my heating system. There are 2 Taco cartridge
circulators and the thermostats connect to a relay boxes that control
the circulators. By the way, there are 2 wires on the thermostats,
R and W.

Can someone enlighten me in terms of whether this kind of system
is compatible with the main stream programmable thermostats or it
requires something special.

Thanks.

- Glacier