Digi-Key is your friend.  Build yourself a daughter card with a bunch of relay driver IC&#x27;s and 24VAC relays.  Bare minimum is three, but there are several different arragements for home HVAC, and more doesn&#x27;t hurt.  Some high-SEER rating HVAC units also have multiple speeds.  Say, 6-8 relays for some flexibility.  Check behind your existing thermostat and see how many wires it gets to be sure.<p>Most units just use bang-bang control around some known limits.  Something I&#x27;ve always wanted to try is to have a &quot;periodic recirculation&quot; mode for the times that I&#x27;m relying on passive solar heating, just to turn over the stratified air.