Single stage furnaces are the traditional equipment that is either on or off, sometimes leading to short on-off-on-off cycles.

Two stage furnaces default to a 70% burn rate and only go to 100% when needed. This leads to longer run times and better temperature regulation without increasing your monthly energy bill.

Variable speed blowers allow the furnace to slowly ramp up the air flow, meaning no more noisey *poof* when your system turns on. Instead you'll simply notice a slow and steady increase in air movement, leading to fewer hot and cold spots within your home

If you live in California, be sure to look for "Low NOx" units due to regulations, unless you live in one of the following counties, in which case regulations restrict you to "Ultra-Low NOx" units. (Fresno, Kern, Kings, Los Angeles, Madera, Merced, Orange, Riverside, San Bernadino, San Joaquin, Stanislaus, or Tulare)