CfmDuctCFM per room to duct diameter, with static pressure counted.

CfmDuct

CFM per room to duct diameter, with static pressure counted.

SectionsLatest
Duct SizingCFM to round and rectangular duct, by friction rate.68
Airflow per RoomHow much air a room needs, and how to check what it is getting.72
Static PressureThe measurement that explains most underperforming systems.37
Flex, Rigid & FittingsWhat each fitting costs you in equivalent length.43
Ductwork CostNew runs, replacement and the cost of a full redesign.1650
  1. What Size Duct for a Given CFM — And the Number That Decides ItDuct Sizing · 3 min read

    A duct size chart is the last step. The friction rate is the first, and it is the number most people skip.

  2. How Much Air Each Room NeedsAirflow per Room · 3 min read

    Dividing the system's total CFM by the number of rooms is the mistake, and it is the mistake most duct systems were built on.

  3. Static Pressure — The One Reading That Explains Most Duct ProblemsStatic Pressure · 3 min read

    If a house has one number worth measuring, it is this one, and almost nobody has it.

  4. Flex or Rigid — The Same Diameter Does Not Carry the Same AirDuct Sizing · 3 min read

    Flex duct is cheap, fast and forgiving, and it is only forgiving of the installer.

  5. Fittings Behave Like Extra Duct, and That Is Where Systems Are UndersizedDuct Sizing · 3 min read

    The tape measure gives one number and the design uses another, and the gap between them is why so many duct systems are too small.

  6. The Return Side Is Where Most Systems Are ShortDuct Sizing · 3 min read

    Air has to get back to the equipment, and in most houses that is the half of the system nobody designed.

  7. Balancing a Duct System — Dampers at the Branch, Not Vents at the RoomAirflow per Room · 3 min read

    Every duct system is out of balance when it is built, and most of them are never balanced.

  8. Sealing Ducts — Mastic, Not Duct TapeFlex, Rigid & Fittings · 3 min read

    The product named after the job is the one product that does not do it.

  9. Duct Insulation — R-Value, Where It Is Required and Why It SweatsFlex, Rigid & Fittings · 3 min read

    Insulating a duct is only half the job. Keeping humid air away from the cold surface inside is the other half.

  10. The Filter Is Often the Largest Single RestrictionStatic Pressure · 3 min read

    Better filtration is worth having. Getting it from a thicker medium in the same small frame is what causes the problem.

  11. Getting Air Out of a Closed BedroomAirflow per Room · 3 min read

    The door closes and the room stops being conditioned, which is the moment it most needs to be.

  12. Duct Leakage Testing — What CFM25 Means and Which Number MattersStatic Pressure · 3 min read

    Twenty per cent of the air a system moves is a plausible leakage figure for an existing house, and it is measurable in an hour.

  13. Registers and Grilles — Free Area, Throw and Why They WhistleAirflow per Room · 3 min read

    The duct is sized carefully and then the air leaves through whatever register matched the paint.

Ductwork replacement cost by city

1,494 calculations · Ductwork Cost · browse all

Ductwork replacement cost by state

153 calculations · Ductwork Cost · browse all

Flex against rigid duct

40 calculations · Flex, Rigid & Fittings · browse all