Air Flow and Fan Watt Verfication


Anchors Aweigh Energy LLC




Fan Watt Draw / System Air Flow


When a completely new system is installed where all the mechanical components are replaced and more than 40 feet of duct work are replaced, a compliance test called fan watt draw/airflow verification is required. This sounds like two tests but it is really one compliance form which uses both measurements as a metric.


The industry standard for air flow through an air conditioning system is 400 CFM per ton of cooling. This is what most engineers in the factories design their equipment to operate most effectively at. The minimum amount of air flow for this test is 350 CFM per ton. The maximum amount of power in watts the fan in your mechanical system can use is 0.58 Watts per CFM of the measured air flow for this test.


If your systems air flow is too low then heat will not be removed from the air properly which leads to short cycling of your system. Short cycling is when the system energizes for a few minutes then de- energizes again because the low air flow has tricked the system into thinking the load has been met or the low air flow has caused a fault in the system which shut it down. Low air flow is usually caused by undersized duct work. The fan in the conditioning equipment can move over 350 CFM per ton but the duct work connecting your home to the equipment is so undersized that it is impossible to move that much air. When the air hits the resistance of the smaller ducts the fan motor has to work harder to push the air through the increased resistance resulting in increased energy usage and decreased air flow.


There are no other options for meeting compliance on this HERS measure. When fan watt draw / system air flow is called for as a HERS verification measure, the system must not use more than 0.58 watts per CFM of measured air flow. Where AAE sees this issue most often is when an existing unit was replaced with a larger unit but the existing ducts remain. The existing ducts may only be sized for a three ton system and if a five ton system is installed, then the ducts for the three ton system are going to be two tons too small for the five ton system.