Does the T'stat automatically switch to a lower fan speed in the heat mode or does it stay on high speed?
This is a Coleman Evcon MOD. DRHQ 0421CB

The furnace itself uses a lower speed for heat and higher for A/C if it is set up that way in the equipment

In theory, hot air is less dense than cold air, needing a lower fan speed. But that doesn't mean your furnace/air handler is setup to do that. Most tstats don't set the fan speed but rather call for heat or cool and it's the furnace/air handler that decides (or was preset for that mode).