HVAC Business is Plagued by Return Air Problems

The Air Conditioning Business is plagued by return air problems. Inadequite return air is the most common problem in HVAC system installations.  Forget all the different rules of thumb and size your return air components based on the facts and sound engineering.

An important fact to understand is that “fans can’t suck in air” like pumps can’t suck in water.  The air and water are pushed into these similar machines.  Air and water have weight.  When you stack up air the thickness of the earth’s atmosphere, the air weighs about 14.696 pounds per square inch (PSI) at the earth’s surface.  When you turn on a fan or pump the suction or return of the machine eliminates the atmospheric pressure in the pipe or duct and the atmospheric pressure outside the pipe or duct pushes in air to replace it.  If the return/suction of the duct is restricted, friction loss reduces the amount of air that can enter the fan.  Fan return/suction design requires very little restriction or pressure drop.  A bigger fan or motor won’t help restricted return air.

Low pressures in duct systems are expressed in inches or feet of water pressure.  At standard conditions one inch water column is .036126 pounds per square inch (PSI).  This pressure is measured by an instrument called a manometer.

The blower/fans in residential central air conditioning systems are designed to overcome .50 inches of total static pressure.  That means if the friction loss in the supply duct and the restrictions in the return duct are more that .50 inch W.C. or .02 psi, the fan will not produce the 400 CFM per ton needed for efficient operation.

Designers allocate 25% of the system static pressure to the return air.  This is .125 inches W.C. for the duct, filter and return air grill.  To test the pressure drop, the system should be in cooling with a wet evaporator coil.  Read the pressure drop behind the filter using a manometer with a right angle static pressure tip.  If the drop is under .125 inches water column, you’re good.

If pressure drop across the return components is too high a filter with a larger surface area (pleated) or increasing the opening surface area will solve the problem.  Generally less than 200 square inches of filter grill per ton should be suspect.  CFM can be measured at the return grill with a CFM Flow Hood.

Return and filter grill air noise is another issue.  The noise is generated by air velocity entering the grill.  Consulting manufacturer’s data can give you the noise rating.  Generally 400 FPM feet per minute are the upper limit for acceptable noise.   Velocity can be measure with several instruments.