I would like to replace my existing EWC thermostat with a Honeywell RTH7600D. Here's what I have:
- hydro-air system, 1 oil furnace with two air handlers that also do A/C. - control panel is an EWC-ST-2D
- two zones:
Zone 1 - two thermostats for basement/first-floor zone, one is a simple dial thermostat in the basement and the second is the EWC programmable that I would like to replace because it seem to fail to auto-start the fan every few weeks.
Zone 2 has just one thermostat, same EWC model but it has been more reliable.

I'm getting the impression that the new Honeywell RTH7600D isn't compatible. The existing EWC has seven wires connected, W/RC/RH/G/B/Y/O. The Honeywell installation instructions don't deal with the O except for heat pump installs, which I don't have.

Any thoughts on how to wire this?

I think I mis-stated the above. I guess it's really two zones for the basement/first-floor air handler and then 1 zone for the second floor (attic air handler). Not sure if that helps or doesn't matter but there it is.

Before trouble starts please visit

EWC Controls (http://www.ewccontrols.com/)

This is where everyone ends up when using EWC's control panels. There is a list of thermostats that are approved for use with there panel somewhere on there site or better yet call them and tell them what you intend to do. If you search this board you will find a few other posts of people trying to do exactly what you want to do.
Other wise you are spinning your wheels. Go to the source. EWC.

Yup, I've arrived there ;-) Looks like I need to find one of the few compatible stats. Thanks