Duct leakage testing

Duct leakage testing

A duct leakage tester is a diagnostic tool designed to measure the airtightness of forced air heating, ventilating and air-conditioning (HVAC) ductwork. A duct leakage tester consists of a calibrated fan for measuring an air flow rate and a pressure sensing device to measure the pressure created by the fan flow. The combination of pressure and fan flow measurements are used to determine the ductwork airtightness. The airtightness of ductwork is useful knowledge when trying to increase energy conservation.


Uses of Duct Leakage Testing

Duct leakage testers are used in residential single family, residential multi-family and commercial buildings that have a forced air delivery systems for heating and cooling.

How Duct Leakage Testers Work

A basic duct leakage testing system includes three components - a calibrated fan, a register sealing system and a device to measure fan flow and building pressure. Supply registers or return air grills are sealed using adhesive tapes, cardboard, or non-adhesive reusable seals (Vent Cap Systems™). One register or return is left unsealed. The calibrated fan is then connected to that unsealed register. Pressure is monitored in one of the branches of the ductwork while the calibrated fan delivers air into the system. As air is delivered into the ductwork, pressure builds and forces air out of all of the holes in the various ductwork connections or through the seams and joints of the furnace or air-conditioner. The tighter the ductwork system (e.g. fewer holes), the less air you need from the fan to create a change in the ductwork pressure.

A duct leakage test can be performed by either pressurizing or depressurizing the ductwork. Ductwork that is outside the building envelope, meaning in an unconditioned attic or crawlspace, should be pressurized so as to not bring in unwanted contaminates.

Duct tester airtightness measurements are presented in a number of different formats including but not limited to:

Air Flow (CFM)

CFM25 is defined as the air flow (in cubic feet per minute) needed to create a 25 Pascal pressure change in the ductwork. CFM25 is one of the most basic measurements of ductwork airtightness. A pressure of 25 Pa is equal to 0.1 inches (0.25 cm) of water column.

Leakage Area

Leakage area estimates are a useful way to visualize the cumulative size of all leaks or holes in the ductwork. There are a variety of standard calculation methods used to calculate leakage areas.

Commercial and industrial ductwork is often tested to standards developed by the Sheet Metal Air Conditioning Contractors' National Association, SMACNA. Ductwork is subjected to higher pressures and then given a rating or classification rather than a leakage estimate.

Additional Test Methods

Duct Leakage to Outside

The above test procedure describes how to determine total duct leakage, how much leakage there is for all of the ductwork connected to the HVAC system. Another test is duct leakage to outside. Depending on the location of the house in the United States, some hvac systems are completely inside the thermal envelope, some completely outside the thermal envelope and some are a combination of the two. Energy conservation is really only achieved by sealing ductwork that is outside the thermal envelope or connected to the outside. With mixed systems it is possible to determine amount of leakage to the outside by simultaneously pressurizing the house while pressurizing the ductwork and measuring the amount of flow that is required to equalize the pressure.

Blower Door Subtraction

Another form of total duct leakage test is to use a blower door to measure the total amount of leakage of the house. Then seal off all registers and returns and measure the total amount of leakage again. The difference in the two readings is an estimate of duct leakage in CFM.

Pressure Pan Test

A third test method to determine if ductwork is leaking to the outside is to use a pressure pan, a cover over the register with a pressure tap for a hose connection. With the house pressurized (or depressurized) to 50 Pa (-50 Pa) using a blower door, a pressure gauge, by means of a hose, is attached to the pressure pan. If the pressure value is near 0 then it is an indication that the ductwork associated with that particular register is not connected to the outside. If the pressure is 5 Pa or above then it is an indication that the duct work is connected to, or leaking to, the outside. The closer the value to the house pressure indicates greater leakage. This method does not quantify duct leakage but serves to identify locations of ductwork that are leaking to the outside. It is more of a qualitative measure.


There are several manufacturers of duct leakage testers, and are listed alphabetically:

TSI-Alnor leading mfg of Test & Measurement Equipment

See also

  • Sheet Metal Air Conditioning Contractors' National Association, SMACNA
  • American Society of Heating Air-conditioning Refrigerating Engineers, ASHRAE
  • Air Conditioning Contractors of America, ACCA


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