Can you install a 90 AFUE condensing furnace in the attic?

With the new Department of Energy gas furnace efficiency standards changing in the near future to 90% AFUE or greater, it presents the question of whether you can, or more importantly should you, install a condensing furnace in the attic of your home.

Here at Santa Fe Air we have always advised against putting a condensing furnace in an unconditioned space such as an attic or crawl space. Condensing furnaces produce water every time they operate, and it’s difficult to insure that this water will not freeze. Some HVAC companies feel that with the proper precautions they can eliminate any problems that the unconditioned space may bring. It’s true that we can install heat tape to the condensate drain trap of the furnace, as well as add additional heat tape around the drain, but where do you terminate this drain?

When the home was designed to accommodate a furnace in the attic, the only condensation that they had to worry about was that of the air conditioner. Of course it’s warm in the summer so freezing isn’t a concern. They could run the drain straight out to daylight, or frequently provided a drain that ran from the attic down to ground level on an exterior wall of the house. The water in this type of termination will freeze in the winter, leaving you with no heat and the possibility of property damage.

A manufacturer’s installation manual offers this advice.

  1. Furnace shall be installed in an area where ventilation facilities provide for safe limits of ambient temperature under normal operating conditions. Ambient temperatures must not fall below 32 degree F unless the condensate system is protected from freezing.
  2. Do not allow return air temperature to be below 55 degrees F for extended periods. To do so may cause condensation to occur in the main heat exchanger, leading to premature heat exchanger failure.
  3. If this furnace is installed in an unconditioned space and an extended power failure occurs, there will be potential damage to the internal components. Following a power failure situation, do no operate the unit until inspection and repairs are performed.

There are too many ways for something to go wrong with an installation of a condensing furnace in an attic or crawl space. No matter how well you insulate the drain trap and hose, it will not protect you when the power fails. So unless you have a whole home generator that automatically provides power when the utility company can’t, Santa Fe Air will not recommend a condensing furnace for your attic.

If your home currently has a non-condensing furnace in the attic and it’s over 12 years old, you might want to consider replacing it now while the non-condensing furnaces are still legal to install. Still have questions about attic installs? Give us a call and we will be happy to help answer them.