Updraft

• Updraft gas furnaces draw air from the exterior of the property into the bottom of the unit, forced through the heat exchanger, and then allowed to disperse through the home’s vent system via the natural phenomenon of warm air rising. As such, they are usually installed in the basement.
  
  

Downdraft

• Downdraft gas furnaces draw air in at the top of the unit, force it down through the heat exchanger, then release it at the bottom of the unit, typically through floor or ceiling level vents, depending on whether the unit is installed on the ground floor or in the attic. Downdraft units may have a fan that forces the air out through the venting system.
  
  

Common Features

• Both types of gas furnaces, updraft and downdraft, are fitted with thermostats, which allow the householder to modify the flow of heated air into the house. The residents of the house set the thermostat to the desired temperature. The furnace, via an internal sensor, senses when the temperature has fallen below that required, triggering the furnace to release more heated air until the sensor registers the chosen temperature. Both types of units also contain filters to clean the air of pollutants and irritants, and condensation drains to release the byproduct of heating air, condensation.
  
  

Choosing

• The choice between an updraft and a downdraft gas furnace is usually dependent upon the available space in which it is to be installed. Premises and buildings that lack a basement or crawl space are typically fitted with a downdraft unit, as an updraft unit needs to be positioned below a house to enable the heat air to rise naturally through the vents. Because updraft units use this natural rising, they tend to be more energy-efficient than downdraft units, which use combustion to force the dispersal of the heated air.