Air Seal HVAC Cabinet Seams

Scope

Seal seams and holes in the air handler or furnace cabinet of central forced air HVAC systems to prevent the loss of conditioned air.

  • Select an air handler with a manufacturer’s designation showing that air leakage is ≤2% of the design air flow rate.
  • Use mastic, mastic plus embedded fiberglass mesh fabric, or UL 181A or B tape to seal all cabinet seams and junctures between the air handler or furnace cabinet and the evaporator coil cabinet, the supply plenum and the return plenum.
  • Use putty around all conduit and wiring holes.
  • Use UL-approved gaskets to seal cabinet doors and access panels.
  • Seal all unused conduit knockouts with UL-listed tape or mastic.
  • Seal all fixed seams in the cabinets and all seams between the cabinet and the supply or return plenums with mastic or mastic and fiberglass mesh fabric.
  • Use a sealing putty to seal the inside of the high-voltage wire conduit termination point in the air handler after the wiring has been installed.
  • Check the insulation inside the air handler where the conduit enters. If the insulation has been compromised, repair it with approved spray glue and additional insulation.
  • Use a duct blower tester to test the airtightness of the air handler/furnace cabinet and ducts. Verify that the duct system meets code or program airtightness requirements.

See the Compliance Tab for related codes and standards, and criteria to meet national programs such as ENERGY STAR, DOE’s Zero Energy Ready Home program, and EPA’s Indoor airPLUS.

Description

Many homes are equipped with central forced air systems that rely on ducts to transport heated or cooled air from a furnace or heat pump to the rest of the home. If the ducts are leaky, they can be a source of energy loss through loss of heated or cooled air, poor HVAC performance through loss of air pressure in the ducts, and comfort problems. Air leakage problems can be worst at the HVAC furnace or air handler cabinet, where air pressures are highest. Cabinet seams, holes, and junctions should be sealed to prevent air leakage. The only place air should be able to leave the supply duct system and the furnace or air handling unit is at the supply registers. The only place air should be able to enter the return duct system and the furnace or air handling unit is at the return grilles (Building Science Corporation 2011).

In high-performance homes, all HVAC equipment, including the furnace or heat pump air handler and any ducts, should be located within the thermal envelope of the home. When the air handler is located within the conditioned space, it is tempting to think that sealing the cabinet is not that important because conditioned air will leak into the home rather than being lost to an attic or crawlspace. However, sealing cabinet air leaks is still very important for maximizing the performance of the HVAC equipment because it helps to ensure maximum air flow to the ducts. In a central forced air system, the highest air pressures are experienced at the air handler, with pressures increasing the closer one gets to the air handler fan. It is common for air pressures in the supply and return plenum at the air handler to equal or exceed 0.5-inches water column (125 Pascals). Therefore, it is critical to seal up the knockouts, seams, and slots in the air handler cabinet.

Gas- or oil-fired furnaces are often equipped with an add-on refrigerant coil (called the evaporator coil) to provide cooling during the summer months (Figure 1). The connection between the evaporator coil cabinet and the furnace cabinet is likely the highest point of pressure in the system and can be a large source of leakage if care is not taken to properly seal this juncture during installation. Refrigerant coil cabinets do not always fit directly on top of the furnace; many coil cabinets have a larger footprint than the furnace so the seam between the two boxes is uneven. The coil cabinet should be sealed to the furnace using mastic or an Underwriters Laboratories (UL) 181 approved foil tape. For larger gaps (greater than 3/8 inch) mastic and fiberglass mesh tape should be used. 

Air seal all holes and seams in the furnace cabinet with mastic, foil tape, or putty

Figure 1. Air seal all holes and seams in the furnace cabinet with mastic, foil tape, or putty. Pay special attention to sealing the junction between the furnace cabinet and the evaporator coil cabinet.

Air handlers (Figure 2), furnaces, and evaporator coil cabinets come from the factory with holes in the form of knockouts, penetrations, and slots for installing piping and wiring. These holes are there for ease of installation and service. However, when installation is completed, any unused holes should be sealed, along with gaps around wiring and piping. Holes where the condensate line and refrigerant lines penetrate the evaporator coil cabinet will be the next highest pressure point, and depending on the model may be a point of negative pressure. Seal around these lines with non-hardening putty. Use non-hardening putty to seal around pipes, tubing, and conduit penetrations in the air handler cabinet as well. This putty comes in strips, slugs, and cords (see Figure 3) and does not dry out, but remains pliable so it can be removed and reapplied. Seal unused electrical and piping knockouts with mastic.

The third point of high (negative) pressure is the area of the cabinet that houses the indoor blower fan. With respect to indoor air quality, this may be considered the most concerning area for air leakage, especially if the furnace is located in a garage or any other area where chemicals are stored or where there is exposure to carbon monoxide. Any seams or unused holes should be sealed with mastic.

The connections with the supply and return plenums are additional areas that experience high pressures. These seams and other cabinet seams should be sealed with mastic, mastic and fiberglass mesh tape, or UL-approved tape.

There are some penetrations in a furnace cabinet that are not considered leakage points. The furnace in Figure 1 is a condensing gas furnace. The penetrations for the condensate line for the condensing gas furnace, vent pipes, gas line, and high voltage wiring (not shown) are not connected to the conditioned air stream and therefore are not areas of concern for leakage.

Regarding cabinet panels that must be periodically removed for routine maintenance of the HVAC equipment, some HVAC technicians suggest using cloth-backed duct tape to seal the panel seams because it is easy to remove or cut through.

Air seal heat pump or air conditioner air handler

Figure 2. Air seal a heat pump or air conditioner air handler cabinet at all seams, holes, and junctions.

Removable putty can also be used to seal around wiring holes

Figure 3. Non-hardening removable putty can be used to seal around wiring holes in the HVAC cabinet.

How to Air Seal the HVAC Cabinet

  1. Install the furnace or air handler and all associated ducting within the conditioned space of the home. Select an air handler with a manufacturer’s designation showing that air leakage is no more than 2% of the design air flow rate when tested in accordance with ASHRAE 193 (per 2012 IECC R403.2.2.1).
  2. Use Underwriters Laboratory (UL) approved gaskets, mastic, mastic plus embedded fiberglass mesh fabric, or UL 181A or B tape to seal all cabinet seams and junctures between the air handler or furnace cabinet and the evaporator coil cabinet, the supply plenum and the return plenum.
  3. Use putty around all conduit and wiring holes.
  4. Seal all unused conduit knockouts with UL-listed tape or mastic. Seal all fixed seams in the cabinets and all seams between the cabinet and the supply or return plenums with mastic or mastic and fiberglass mesh fabric.
  5. Use a sealing putty to seal the inside of the high-voltage wire conduit termination point in the air handler after the wiring has been installed.
  6. Check the insulation inside the air handler where the conduit enters. If the insulation has been compromised, repair it with approved spray glue and additional insulation.
  7. Use a duct blower tester to test the airtightness of the air handler/furnace cabinet and ducts. Verify that the duct system meets code or program airtightness requirements.

Ensuring Success

Install the air handler within the conditioned space of the home.

Seal all seams and holes in the air handler cabinet.

Test the airtightness of the cabinets and ducts with a duct blower test. Verify that the duct system meets code or program airtightness requirements.

Climate

No climate specific information applies.

Training

Right and Wrong Images

None Available

Presentations

  1. Zero Energy Ready Home Training
    Presentation: Zero Energy Ready Home Training
    (19 MB)
    Author(s): Rashkin
    Organization(s): DOE

Videos

  1. Seal Seams in Air Handler Cabinets
    Publication Date: July, 2015
    Courtesy Of: Zero Energy Homes

    Video describing how to seal air handler cabinet seams.

CAD Images

None Available

Compliance

The Compliance tab contains both program and code information. Exact code language is copyrighted and may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

U.S. Department of Energy Zero Energy Ready Home

The U.S. Department of Energy’s DOE Zero Energy Ready Home National Program Requirements includes, as a mandatory requirement (Exhibit 1, Item 6) that all labeled homes be certified to the U.S. Environmental Protection Agency’s Indoor airPLUS criteria. Indoor airPLUS requires that homes meet ENERGY STAR for Homes criteria including the requirement that all duct systems are installed to be substantially airtight and properly balanced. Indoor airPLUS also advises that seams in the HVAC cabinet, plenum, and adjacent ductwork should be sealed with mastic systems, tape that meets the applicable requirements of UL 181a or UL 181b, or gasket systems.

Another mandatory requirement (Exhibit 1, Item3) of DOE’s Zero Energy Ready Home program is that duct systems be located within the home’s thermal and air barrier boundary.

DOE Zero Energy Ready Home Footnotes

(16) Exceptions and alternative compliance paths to locating 100% of forced-air ducts in home’s thermal and air barrier boundary are:

  1. Up to 10’ of total duct length is permitted to be outside of the home’s thermal and air barrier boundary.
  2. Ducts are located in an unvented attic, regardless of whether this space is conditioned with a supply register
  3. Ducts are located in a vented attic with all of the following characteristics:
    1. In Moist climates (Zones 1A, 2A, 3A, 4A, 5A, 6A and 7A per 2012 IECC Figure R301.1) and Marine climates (all “C” Zones per 2012 IECC Figure R301.1), minimum R-8 duct insulation with an additional minimum 1.5” of closed-cell spray foam insulation encapsulating the ducts; total duct leakage <= 3 CFM25 per 100 sq. ft. of conditioned floor area; and ductwork buried under at least 2” of blown-in insulation.
    2. In Dry climates (all “B” Zones per 2012 IECC Figure R301.1), minimum R-8 duct insulation; total duct leakage <= 3 CFM25 per 100 sq. ft. of conditioned floor area; and ductwork buried under at least 3.5” of blown-in insulation. Note that in either of these designs the HVAC equipment must still be located within the home’s thermal and air barrier boundary.
  4. Jump ducts which do not directly deliver conditioned air from the HVAC unit may be located in attics if all joints, including boot-to-drywall, are fully air sealed with mastic or foam, and the jump duct is fully buried under the attic insulation.
  5. Ducts are located within an unvented crawl space
  6. Ducts are located in a basement which is within the home’s thermal boundary
  7. Ductless HVAC system is used.

Please see the DOE Zero Energy Ready Home website for state-specific requirements.

ENERGY STAR Certified Homes

The ENERGY STAR Certified New Homes National Program Requirements lists the following criteria related to air handlers.

HVAC System Quality Installation Rater Checklist:
4.1 Total Rater-measured duct leakage meets one of the following two options: (16)
4.1.1 Rough-in: <= 4 CFM25 per 100 sq. ft. of CFA with air handler and all ductwork, building cavities used as ductwork, & duct boots installed. In addition, all duct boots sealed to finished surface, Rater-verified at final.
4.1.2 Final: <= 8 CFM25 per 100 sq. ft. of CFA with the air handler and all ductwork, building cavities used as ductwork, duct boots, & register grilles atop the finished surface (e.g., drywall, flooring) installed.
4.2 Rater-measured duct leakage to outdoors <= 4 CFM25 per 100 sq. ft. of conditioned floor area. (16,19)

ENERGY STAR Footnotes

(16) Duct leakage shall be determined and documented by a Rater using a RESNET-approved testing protocol. Leakage limits shall be assessed on a per-system, rather than per-home, basis…

(19) For homes that have <= 1,200 sq. ft. of conditioned floor area, measured duct leakage to outdoors shall be <= 5 CFM25 per 100 sq. ft. of conditioned floor area. Testing of duct leakage to the outside can be waived if all ducts and air handling equipment are located within the home’s air and thermal barriers AND envelope leakage has been tested to be less than or equal to half of the Prescriptive Path infiltration limit for the Climate Zone where the home is to be built. Alternatively, testing of duct leakage to the outside can be waived if total duct leakage is <= 4 CFM25 per 100 sq. ft. of conditioned floor area, or <= 5 CFM25 per 100 sq. ft. of conditioned floor area for homes that have <= 1,200 sq. ft. of conditioned floor area.

Many states have adopted state- or region-specific ENERGY STAR Certified Homes criteria - Please see the ENERGY STAR Certified Homes website for regional specifications.

2009 IECC

403.2.2 Ducts. Sealing (Mandatory).  All ducts, air handlers, filter boxes, and building cavities used as ducts should be sealed. Duct tightness should be verified by duct leakage testing (testing is not required if the air handler and all ducts are in conditioned space.) Testing can be done at rough-in or when construction is complete.

If testing is done at rough in, total leakage must be <= 6 cfm/100 sq. ft. of conditioned floor area when tested at a pressure difference of 25 Pascals (Pa) across the roughed in system, including the manufacturer’s air handler enclosure, with all register boots sealed. If the air handler is not installed at the time of the test, total air leakage must be <= 4 cfm/100 sq. ft. of conditioned floor area.

If testing is done post construction: Leakage to outdoors must be <= 8 cfm/100 sq. ft. of conditioned floor area or total leakage must be <=12 cfm/100 sq. ft. of conditioned floor area at 25 Pa across the system including the air handler enclosure.

2012 IECC

R403.2.2 Ducts. Sealing (Mandatory).  All ducts, air handlers, and filter boxes should be sealed. Duct tightness should be verified by duct leakage testing (testing is not required if the air handler and all ducts are in conditioned space). Testing can be done at rough-in or when construction is complete.

If testing is done at rough-in, total leakage must be <= 4 cfm/100 sq. ft. of conditioned floor area when tested at a pressure difference of 25 Pa across the roughed in system including the manufacturer’s air handler enclosure, with all register boots sealed. If the air handler is not installed at the time of the test, total air leakage must be <= 3 cfm/100 sq. ft. of conditioned floor area.

If testing is done post construction, leakage to outdoors must be <= 4 cfm/100 sq. ft. of conditioned floor area or total leakage must be <=12 cfm/100 sq. ft. of conditioned floor area at 25 Pa across the system including the air handler enclosure.

R403.2.2.1 Sealed air handler. The air handler should have a manufacturer’s designation showing that air leakage is no more than 2% of the design air flow rate when tested in accordance with ASHRAE 193.

2009 IRC

M1601.4.1 Duct Installation: Joints and Seams. The joints of duct systems shall be made airtight by means of tapes, mastics, liquid sealants, gasketing, or other approved closure systems.

2012 IRC

M1601.4.1 Duct Installation: Joints, Seams, and Connections. The joints of duct systems should be sealed with welds, gaskets, mastics, mastic plus embedded fabric, or tape.

2012 International Mechanical Code

603.9 Joints, seams and connections. Securely fasten all joints, seams, and connections with welds, gaskets, mastics, mastic plus embedded fabric, liquid sealants, or tapes that are listed and in accordance with Underwriters Laboratory: For connecting rigid duct use UL 181A products marked "181A-P” for pressure-sensitive tape, "181 A-M” for mastic, or "181 A-H” for heat-sensitive tape. For flexible ducts use UL 181B sealants marked "181B-FX” for pressure-sensitive tape or "181B-M” for mastic. The connections of ducts to the flanges of air distribution system equipment should be sealed and mechanically fastened.

More Info.

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

Case Studies

  1. Author(s): PNNL
    Organization(s): PNNL, FSEC, BA-PIRC
    Publication Date: January, 2013

    Case study about a DOE Building America 2012 Top Innovation on ventilation strategies in hot and humid climates developed and assessed by Florida Solar Energy Center.

  2. Author(s): PNNL
    Organization(s): PNNL
    Publication Date: August, 2012

    Case study about retrofitting a leaky air handler. 

References and Resources*

  1. Author(s): Air Conditioning Contractors of America
    Organization(s): Air Conditioning Contractors of America
    Publication Date: December, 2013
    Standard outlining industry procedure for sizing residential duct systems.
  2. Author(s): Air Conditioning Contractors of America
    Organization(s): Air Conditioning Contractors of America
    Publication Date: January, 2015
    Standard providing a universally accepted definition for quality installation for residential and commercial heating, ventilating, and air conditioning applications.
  3. Publication Date: January, 2011

    Document detailing the requirements, roles, and obligations for participants in an organized effort, ensuring that HVAC installations comply with the ANSI/ACCA 5 QI – 2010 (HVAC Quality Installation Specification) QI Standard.

  4. Author(s): DOE
    Organization(s): DOE
    Publication Date: August, 2015
    Standard requirements for DOE's Zero Energy Ready Home national program certification.
  5. Author(s): BSC
    Organization(s): BSC
    Publication Date: May, 2009
    Brochure with details on HVAC duct air sealing.
  6. Author(s): EPA
    Organization(s): EPA
    Publication Date: October, 2015
    Document outlining specifications that were developed by the U.S. Environmental Protection Agency (EPA) to recognize new homes equipped with a comprehensive set of indoor air quality (IAQ) features.
  7. Author(s): ICC
    Organization(s): ICC
    Publication Date: January, 2009
    Code containing 2009 ICC language for mechanical draft systems.
  8. Author(s): Beal, McIlvaine, Fonorow, Martin
    Organization(s): BA-PIRC, NREL
    Publication Date: November, 2011
    Document illustrating guidelines for efficient installation of interior duct systems in new housing.
  9. Author(s): Air Conditioning Contractors of America
    Organization(s): Air Conditioning Contractors of America
    Publication Date: January, 2010
    The Technician's Guide equips practitioners with the knowledge to properly implement all of the measurement procedures required in the HVAC QI Specification.
  10. Author(s): Fairey, Parker, Vieira, Martin
    Organization(s): FSEC
    Publication Date: September, 2014
    Preliminary results of two monitored lab homes constructed to represent the thermal characteristics of typical existing Florida homes are presented.

Contributors to this Guide

The following Building America Teams contributed to the content in this Guide.

Building Science-to-Sales Translator

Tight Duct Sealing =
Professionally-Sealed Comfort Delivery System

Technical Description: 

Leaky ducts can dump conditioned air into attics and crawlspaces, or pull in air from these same types of spaces. Both outcomes wastes energy and reduce the amount of heated or cooled air that reaches its destination, and can cause other problems with odors and contaminants. Professionally sealed comfort delivery systems are well sealed at all seams and connections with approved sealing tape or mastic, a type of paint-on adhesive, before they are insulated. Thus sealing helps air get where it is needed, at the correct temperature, without contaminants from crawlspaces and attics.

Alternate Terms

Optimum-Flow Comfort Delivery Duct Sealing
Contaminant Sealed Comfort Delivery System
Energy Saving Comfort Delivery Duct Sealing
Professionally-Sealed Comfort Delivery Ducts
Moisture Control Duct Sealing
Professionally-Sealed Comfort Delivery System
Sales Message
Professionally-sealed comfort delivery systems help ensure that heating and cooling can flow optimally to each room. This means less wasted energy along with enhanced room-by-room comfort. Wouldn’t you agree it’s important that your comfort delivery system is installed to deliver optimum performance?
Last Updated: 04/19/2016

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