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Balancing Act

By Steve Vacha

We all know that air has volume.  If air is removed from a room it creates a vacuum.  This vacuum will naturally equalize itself by pulling air in from another space.   And if air is being pushed into a room in the form of a warm air vent, the old air must have a place to escape to make room for the new conditioned air to enter. This is where cold air returns come into play.

This balancing act of pressure is very important to a home’s comfort and safety. For instance, an older 1 1/2 story house that has had the 2nd floor finished years after the main floor often only has warm air registers and no air return vents.  This makes cooling the 2nd floor very difficult. Not because there aren’t enough registers, but because there are no air return vents to take out the warm air that has gathered at this highest point in the house. There is literally no room for the cool air coming from the register to enter the room.  By adding proper return air vents that pull air out of the area, the cool air could much more efficiently cool that 2nd floor.

Safety concerns enter the picture with combustible gas fired appliances. If there is negative pressure at the utility room this can cause the water heater or furnace to back-draft.   The carbon monoxide fumes are literally pulled back from the chimney to fill a vacuum present in the room. Periodically we will find flues that are not well sealed at a masonry chimney which would make it even easier for carbon monoxide to be pulled back into the utility room. It is important to make sure the heating and air system has been balanced for optimum air flow.

Some basements have high humidity in the warm season of the year.  Many species of mold can begin to grow from humidity alone if the humidity stays higher than 55% for long enough. To prevent this musty and moldy condition, adding cold air returns will reduce this high humidity.

At a recent ASHI training seminar, we heard a story of a home owner who was having trouble with air movement in her house. She had been getting sick and she was convinced it was something in the air.  She preceeded to spend thousands of dollars in air purification systems and electronic filters for the furnace. An HVAC expert was hired to come and try to diagnose the problem. Upon inspection of the house he noticed there was room with a chinchilla and only a return vent in it. This room had negative pressure and all the odors, dust, and fumes from the chinchilla cage were being pulled from the room by the furnace through the return vent and distributed throughout the house. The chinchilla was moved to the garage and the home owners health concern was solved!

Indications of improper presuration in homes.

  • Uneven temperatures noted from room to room when the system is operating
  • Air flow varies when all the registers are open


  • Add return vents as needed  /   Manually adjust heat register flow at the register
  • Add mechanical or automatic zone damper controls