Is is possible to replace old 3 wire 24v DC? Mercury thermostat, for automag 24v DC system, with a new electronic Honeywell 3 wire 24v AC thermostat? Model #ct87k thermostat. Honeywell support wasn't much help other than telling me new thermostat was 24v AC. I do have a CNV relay for a two wire thermostat coming from the boiler mate. Do I need a relay for a 3 wire? Is there such a beast?

The one time I've had to work on an Automag system, I called Automag. They were most helpful. Here's a link to their website: AUTOMAG - Technical Information

Figured it out myself. Used the 24v AC relay and powered the new 24v AC thermostat. Used only red and white from thermostat and not third yellow wire. Automag wiring diagram from AUTOMAG helped a lot.

Good deal. Glad you got things up & running.