I have two zones on which there is a Honeywell Chronotherm IV Plus Thermostat. Right now I have a/c running on both units with no problem - fan is on "auto". However, my plumber needed me to turn the fan on both units from "auto" to "on". Seemed easy enough to change the setting, but it only worked on one of the thermostats. The fan connected to the second unit never turned on. Any thoughts on problem and correcting it? Thanks.

Take the thermostat off the wall and jumper the red wire to the green wire, if the fan comes on the thermostat is the culprit, if it does not come on then the problem is in the furnace or air handler, possibly that some one did not have enough wires to stat and connected green to yellow at furnace and no green wire at thermostat. Best , Mike