How to force furnace to fan to turn on

sousvide23June 7, 2011

Hi all

I switched to a Honeywell RTH8500 programmable thermostat from one of those mercury based dial thermometers. There were two wires coming out of the wall into the old thermostat.

I hooked it up without issue - heat blows out when it's supposed to. Now that summer is nearing, I wanted to use the furnace to circulate some air through the returns by switching it to fan mode (no heat). I've followed the instructions on how to set the thermostat to do this but I CANNOT get the fan to turn on - it will only engage when I set the desired temperature OVER what the current room temp is...and I don't want that in the summer.

Did I wire this thing up wrong? I checked my furnace...the manual does say it's capable of fan only operation.

cheers.

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