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HVAC School - For Techs, By Techs

May 4, 2023

Steve Rogers from The Energy Conservatory (TEC) returns to the podcast to discuss the impacts of duct leakage on occupant comfort and HVAC system performance.

Duct leakage has more significant negative effects in heat pump systems than in furnace systems, especially in climates with high heating, cooling, or latent loads, due to pressure imbalances and moisture problems. You can measure duct leakage by masking off all supply and return registers, attaching a calibrated fan, and running the duct blaster to pressurize the duct work to 25 Pascals. 

Exhaust-only ventilation presents many of the same problems as duct leakage, particularly in the humid South. The duct leakage allowable by code (in Florida) is almost equivalent to a 50-CFM bathroom fan.

Leakage often happens on the supply side, and it is important to determine whether the leakage is happening on the supply or return side; you may lose significant capacity on the supply side, and you may lose a little less capacity if the leakage is primarily on the return side. That capacity, however, is often heavily latent, leading to potential moisture problems (though less so in cold climates). Duct leakage may go outside or merely into an unconditioned space within the home; you can test the duct leakage outside with a duct blaster and a blower door simultaneously.

Steve and Bryan also discuss:

  • Duct leakage problem differences in the North and South
  • Regulations and their effects on changeout practices
  • Using powered flow hoods and a TrueFlow grid to measure duct leakage
  • Duct leakage allowable by code
  • Home construction types and duct configuration
  • Envelope leakage
  • Leakage testing and pricing
  • Low-Income Heating Assistance Program (LIHEAP)


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