Duct Tightness Testing

This post originally appeared on the Green Building Advisor website.

Testing air leakage in ductwork used for forced air heating and cooling systems has been required since the 2012 IRC code.  The 2012 through the 2018 code allowed a testing exemption for all ductwork located inside the building envelope.  In other words, if you kept all ductwork inside the thermal and air boundary of the building, no testing was required.  That has changed for the 2021 energy code.

There are two standards allowed for duct tightness testing, ANSI/RESNET/ICC 380 (which also includes the blower door testing standards) and the ASTM E1554.  Both standards require testing to be performed at a pressure of 25 Pascals or 0.1 inches of water column.  Testing can be performed during rough-in, either with or without the air handler encloser installed.  Testing can also be performed postconstruction, testing at this point requires the air handler to be installed.  The 2021 energy code limits both the rough-in and postconstruction tests to an air leakage rate of 4.0 cubic feet per minute per 100 square feet of conditioned floor area when the air handler is present.  If the air handler is not present during the rough-in test, air leakage is limited to 3.0 cubic feet per minute per 100 square feet of conditioned floor area.  Both the rough-in and postconstruction testing requirements are when any part of the duct system is located outside the building thermal/air boundary.

Here is the major change to the duct tightness testing requirements in the 2021 IRC, if all the ducts remain inside the thermal/air boundary, ductwork will be tested with leakage rates limited to 8.0 cubic feet per minute per 100 square feet of conditioned floor area.  The 8 cfm/100 ft² of conditioned floor area is fairly easy to pass, but testing is required to verify the limit.  My guess is this 8-cfm limit will be tightened in future code additions.

Section R403.3.5 and R403.3.6 in the 2021 IRC is where the requirements for testing and leakage limits for heating and cooling ductwork are discussed.  Testing is required for all ducts serving heating and cooling systems with one exception.  That exception states:

“A duct air-leakage test shall not be required for ducts serving heating, cooling or ventilation systems that are not integrated with ducts serving heating and cooling systems.”

Clear as mud code language.  As an example of this exception, an isolated duct system used only for balanced mechanical ventilation (ERV/HRV) would not require testing.  My interpretation of this exception is that testing will be required if the duct system that served the ventilation system was integrated into the forced air heating and cooling duct system.  In other words, if you were supplying fresh air into or removing stale air out of the building by using the ductwork for the normal heating and cooling system, testing of all the ductwork would now be required.  Basically, if you don’t want to test all the ductwork, then keep the systems separated.

Why the change?

Some may argue that even if air ducts leak within the building envelope, the air remains inside the building.  This is true, but there still can be issues caused by the leaks.  Duct systems should be designed with a specific air flow rate to each room, leakage may reduce this designed air flow.  Another issue, ductwork is often installed between floor systems or in wall cavities.  Even though the ducts are within the thermal and air boundary of the home, air leaks in ducts at these locations may pressurize or depressurize these spaces, forcing conditioned air to move through building cavities or bring outside air into these cavities.  This can be detrimental if the air comes in contact with a condensing surface.

Heating and cooling forced air system efficiencies include not only the air handler and it’s associated equipment, but also the ductwork and how it is designed and installed.  The most efficient system will deliver the designed air flow exactly where it’s needed, not losing some capacity to areas that do not require conditioning.

ANSI/RESNET/ICC 380-2019 Standard

As I mentioned earlier, there are standards for performing duct leakage testing.  This article will focus on the total duct leakage and leakage to outside tests.  The 380 standard requires all components of the HVAC system design to be installed before performing any testing.  All supply and return ducts along with any associated devices connected to the system such as dampers and/or ventilation openings or ducts need to be installed.  The air handler unit also needs to be installed according to the standard, but there is an exception, if the authority having jurisdiction allows missing equipment, testing can continue but missing components need to be included with the final report.  This is also where the IRC code language and the 380 standards differ.  The code allows testing without the air handler present; the standard would require the air handler to be installed unless allowed by the AHJ.  It might be best to have a discussion with the inspector on what they are looking for before testing.

There can be different dampers tied to the HVAC duct system.  How they are addressed depends on the type and location of this connected system.  Non-motorized dampers connected to the system that remain inside the conditioned space, this includes balancing dampers, need to be left as found.  All zone and bypass dampers need to be fully open.  Motorized dampers in ducts that connect conditioned spaces to outside or unconditioned spaces should be closed without further sealing.  Non-dampered, continuously operating ventilation systems that connect conditioned space or space-conditioning duct systems to the exterior need to be sealed on the exterior when possible.  (Exhaust, supply and balanced mechanical ventilation systems.)  What is and is not allowed to be sealed during duct tightness testing can be confusing.  Refer to the 380 standard or ask your local code official for clarification.

Installing the testing equipment and running a total duct leakage test.

The two most common manufacturers of duct tightness testing equipment are The Energy Conservatory with their Duct Blaster and Retrotec’s DucTester System.  Both systems work similar, with a fan and duct that connects to the air handling system, a manometer/flow gauge and the tubing required to measure pressures within the system.

Testing requires the heating and/or cooling system be disabled so that it cannot start during testing.  Other air handlers and fans around the home also need to be disabled so that their operation does not affect test pressures.  Any other combustion appliance that may start during testing should also be turned off.  The filter in the duct system should be removed.  All supply and return vents need to be sealed.  This is typically achieved by using a tape to cover either the installed registers, or if the registers are not yet installed, seal at the face of the duct boot.  (There are other methods to sealing registers.)  A reference tube is placed in the supply register closest to the air handler and connected to the manometer/flow gauge.  This tube will register the pressure inside the ductwork during the test.

All interior doors must be opened.  If any ductwork is located outside the building envelope, a door or panel to those spaces must also be opened.  A door or window to the outside must also be opened to prevent pressure changes inside the structure while the test is being conducted.

The duct testing fan and associated supply duct can be connected to the air handler or to a large central return grill.  You have the option to conduct either a positive or negative pressure test.  With both manufacturer’s systems, you simply turn the fan around to change the air flow direction.  There are advantages to both, with a negative pressure test, the material used to seal the vent registers will be pulled tight against the register or register boot, creating a better seal, positive pressure testing wants to push the product used to seal off the register.  With a positive pressure test, smoke can be added to the ductwork during the test to assist in locating the leaks.

Duct tightness testing without the air handler present. In this case, I used the supply air duct for the spot to attach the duct testing equipment. The return duct next to the supply duct was sealed before testing began.

Once the duct system is prepped and the testing equipment is installed, a test can be conducted.  You have the option of running an automated test by using software or an app provided by the manufacturer of the test equipment, or performing a manual test which requires using the fan speed control and the digital manometer/flow gauge.  The information provided by the equipment will show a total duct leakage at a specific cubic feet per minute volume of air flow.  The total CFM of leakage divided by the conditioned square footage of the home times 100 ((CFM of duct leakage/total conditioned square footage)) x 100) will provide the pass or fail test result required by code.

The duct leakage to the outside test.

The total duct leakage test outlined above indicates the total duct leakage across all the ducts, whether the ducts are located inside or outside the building envelope.  A second type of test can be performed to determine how much of the total duct leakage is occurring with ducts that are outside the building envelope, such as ducts located in unconditioned attic or crawl spaces, or in attached garages.

The duct leakage to outside test requires a blower door to operate at the same time and at the same pressure of the duct testing fan, 25 Pascals (or to as close to 25 Pascals as possible when excessive duct leakage prevents that pressure from being met).  Basically, you are conducting a blower door test at 25 Pascals at the same time as running a duct tightness test.  The purpose is to have the house and all ductwork located inside the building envelope at the same pressure, eliminating duct leakage that is occurring inside the building envelope.  The duct leakage that is left is only the leakage that is occurring outside the building envelope.  Conducting the duct leakage to outside test is more involved than what I intend on covering in this post.  Both the Energy Conservatory and Retrotec have great videos showing how these tests are performed on their YouTube pages.

When would the information supplied by the two tests be used?

The 380 standard lists when the two tests are to be used, it is called; Procedure to Apply Results of Duct System Leakage Test.  This section of the 380 standard states:

“If the results of the duct system leakage test are to be used for assessing compliance with a limit on total duct system leakage, (defined by code or by an energy efficiency program) then the total duct leakage […] shall be used.”

“If the results of the duct system leakage test are to be used for assessing compliance with a limit on duct system leakage to the outside, (defined by code, by an energy efficiency program or for a home energy rating), then the duct system leakage to outside […] shall be used.  Alternatively, the total duct leakage […] is permitted to be used as if it were the leakage to outside (the total leakage value is permitted to be used in software as if it were leakage to the outside).”

“If the results of the duct system leakage test are to be used for conducting and energy audit or predicting savings from retrofits, then the duct system leakage to outside […] shall be used.”

Forced air heating and cooling equipment have listed efficiencies for the equipment, but the total efficiency of the system is also affected by the design and installation of the duct system.  Building codes are recognizing issues that are created when unintended air leaks are present in these duct systems.  Testing assures performance meets the intended design.  It’s nice to see we are moving in the right direction.

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