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Static pressure

External static pressure is the resistance to airflow that a ducted heat pump's blower works against, measured in inches of water column.

Editorial staffEdited by heatpumpwiki

History 1

External static pressure is the resistance the blower pushes against from the ducts, filter, registers and coil outside the unit. It is measured in inches of water column (in. w.c.).

The federal test for ducted systems now assumes a minimum of 0.50 in. w.c. for conventional systems, up from 0.10 to 0.20 under the old test.Source 1Source 2 That change is part of why SEER2 and HSPF2 numbers are lower than SEER and HSPF.

High static pressure in a real house reduces airflow. DOE says heat pumps need "about 400 cubic feet per minute (cfm)... per ton," and that performance can "deteriorate if airflow is much less than 350 cfm per ton."Source 4 NRCan notes heat pumps move more air than furnaces, and if that exceeds what the ducts can carry, "you may have noise issues or increased fan energy use."Source 3 See Ducted vs ductless heat pumps.

Sources

  1. 1.10 CFR Appendix M1 to Subpart B of Part 430, Uniform Test Method for Measuring the Energy Consumption of Central Air Conditioners and Heat Pumps. Electronic Code of Federal Regulations (U.S. DOE rule). Accessed Oct 25, 2026
  2. 2.Energy Conservation Program: Test Procedures for Central Air Conditioners and Heat Pumps (Final rule, 82 FR 1426, January 5, 2017). U.S. Department of Energy (Federal Register). Accessed Oct 25, 2026
  3. 3.Heating and cooling with a heat pump. Natural Resources Canada. Accessed Oct 25, 2026
  4. 4.Air-Source Heat Pumps (archived copy of energy.gov/energysaver/air-source-heat-pumps, captured June 20, 2026). U.S. Department of Energy. Accessed Oct 25, 2026

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  • DefrostDefrost is the cycle in which a heat pump briefly reverses to melt frost off its outdoor coil.
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