Where Are the Returns?

By Hal Alles

This is not about elections or unwanted gifts. It is about one (of many) reason residential HVAC systems don't work as well or as efficiently as they could. Forced air heating and cooling systems operate like a conveyor belt for heat. Airflow is the belt that carries heat between the HVAC equipment and the rooms. The blower (fan) in the air handler (usually called the furnace) drives this conveyor, pushing airflow to many supply vents around the house and pulling air back through one (or a few) centrally located returns. To load heat onto the conveyor, the temperature of the airflow is increased. To unload heat from the conveyor, the temperature of the airflow is decreased. The furnace loads heat onto the conveyor and it is removed near the supply vents (to heat); the A/C unloads heat that is loaded near the supply vents (to cool). The change in temperature between the return air and the supply air is called "delta TO," and is proportional to the heat transferred. HVAC equipment consumes about twice the energy to produce twice the delta TO for a fixed airflow.

For the conveyor to work, the air needs to get from the supply vents to the return. The halls and open areas of the house function as big ducts to get the air from the supply vents to the return. But what if the return path is blocked by a closed door? The airflow through the supply vent still forces air into the room, but without an easy path back, the airflow is reduced as the pressure in the room increases, so the room receives less conditioned air. The pressure increase forces air out of the room through windows, outlets, light fixtures, bathroom vents, etc. Room temperature air is forced out to condition the great outdoors.

Even worse, the escaped air is replaced by the same amount of air from outside. A slight vacuum is created, pulling outside air in through the easiest path, perhaps another bathroom vent, windows, exterior doors, etc. This replacement air requires extra conditioning to get it to room temperature, so energy is wasted. For example, to remove the added heat when it is 100° outside and 75° inside, the A/C needs to condition a volume of air about equal to the air from outside, wasting that portion of its capacity. With many doors closed and poor returns, this waste can be over 20%.

So how much return is enough? The cross section area of the return path should be at least 20% larger than the supply. The largest available return path is usually the gap under the door. A 6-inch duct can supply about 100 cfm and has a cross section of about 28 square inches, so the gap needs to be more than 1 inch - not likely to find that in a new home. Some new homes have up to five vents serving the master suite, so the gap needs to be more than 5 inches - clearly impractical. The best solutions are "pass through vents" or "jumper ducts." These allow the supply air to easily pass from a closed room into the open area with the return. It is a small addition to get a big improvement in comfort and efficiency. Faulty returns will still occur in elections, but they should not occur in homes.

June 2007 Builder Architect Edition Issue

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