Iris CT101 Fan Only

My thermostat is an Iris CT101 and it's for heat only. The issue that I have is I want to use only the fan occasionally when I'm running my pellet stove. I have a four conductor thermostat wire and I've got the white on W of thermostat and furnace,red on RH of both,green on G of both and blue on C of both. When I turn furnace on off at thermostat and press Auto to fan on the fan doesn't respond. Any ideas? Thanks