Ducted Returns

Scope

A ducted central return brings air from central return registers back to the air handler through insulated, air-sealed ducts
A ducted central return brings air from central return registers back to the air handler through insulated, air-sealed ducts

Provide for pressure balancing between bedrooms and the rest of the house.

  • Install ducted returns or a combination of ducted returns, transfer grilles, jump ducts, and/or door undercuts in bedrooms to allow pressure balancing between bedrooms and the rest of the house in homes with ducted heating and cooling systems by providing a path for room air to return to the central air handler, thereby increasing the volume of conditioned air circulating in the room.
  • ENERGY STAR Certified Homes requires that the dedicated return ducts, transfer grilles, jump ducts, and/or door undercuts together achieve a rater-measured pressure differential of ≤3 Pascals (0.012 inch water column) with respect to the main body of the house when bedroom doors are closed and the air handler is operating on the highest design fan speed. A rater-measured pressure differential of ≤5 Pascals (0.020 inch water column) is acceptable for rooms with a design airflow ≥150 cfm.
  • If dedicated return ducts are installed in each bedroom, contractors must seal all seams, gaps, and holes of the return duct system with mastic and seal the return box to the floor, wall, or ceiling with mastic, caulk, and/or foam.
  • Refer to the balancing report provided by the HVAC contractor for the bedroom air flows to size the return ducts. If a balancing report was not provided, the flow of the supply register when the air handler is on high speed may need to be measured using a flow hood, anemometer, or other flow measurement tool.
  • Test the pressure differential with the bedroom doors closed.

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

Description

For central “forced air” furnace and air conditioning systems to operate properly, the HVAC distribution system should be designed with adequate supply and return registers to provide conditioned air to all parts of the house and return stale air to the furnace for reconditioning. Inadequate return air pathways can cause pressure imbalances from room to room, which can cause drafts and temperature differences between rooms or floors, leading to comfort complaints. Pressure imbalances can also cause the furnace and air conditioning equipment to work harder than necessary. A well-designed return air strategy is critical for the performance of the HVAC system in an energy-efficient house, which may have lower airflow requirements to meet the lower heating and cooling loads (Burdick 2011). The return air must have a clear path back to the air handler from every room that has a supply outlet, with the exception of bathrooms or kitchens due to the potential for spreading odors through the house (Burdick 2011).

Each room can be individually ducted to the return side of the air handler; however, installing that much ducting is costly and there may be space constraints that limit the feasibility of this approach. Utilizing a central return strategy is a simple and effective way to return stale air to the air handler (Figure 1). When utilizing a central return strategy, one or more return registers should be placed in central hallways or stairwells adjacent to the main living spaces of the house, with at least one return per floor. These central returns should be ducted to the return side of the HVAC air handler with air-sealed ducts that are insulated if located in unconditioned space (Figure 2). Building cavities (the space between wall studs or “panned” floor joists) should not be used as return air pathways; if unducted, these spaces are very difficult to air seal. Return air pathways that leak will draw air from unintended places in the house and can lead to undesirable pressure differences. A fully ducted return system will be easier to air seal and will have better airflow characteristics than building cavities used as return air pathways.

To ensure that “stale” air is able to return to these central returns from rooms that have closeable doors such as bedrooms or offices, builders will often rely on door undercuts. Typical door undercuts (1/2 to 3/4 inch) alone do not allow adequate return volume, especially when carpet is installed, and are not appropriate for an energy-efficient house. Door undercuts are not approved in ACCA Manual D (Rutkowski 2009). Other methods for providing an air pathway from closed rooms to central return registers are jump ducts and transfer grilles.

Return ducts are installed by the HVAC contractor. Return duct locations should be indicated on the HVAC design plans. Tasks associated with this installation should be included in the contract for the appropriate trade, depending on the workflow at a specific job site.

How to Install Return Ducts

  1. Calculate the amount of return air needed. A target value for return capacity is two times the volume of the total supply air with an airflow velocity within the return of less than 500 feet per minute and the net free area of the grille sized 1.5 times the cross-sectional area of the return duct (Burdick 2011). ENERGY STAR requires that returns achieve a rater-measured pressure differential 3 Pascals (0.012 inch water column) with respect to the main body of the house when bedroom doors are closed and the air handler is operating on the highest design fan speed. A Rater-measured pressure differential of ≤5 Pascals (0.020 inch water column) is acceptable for rooms with a design airflow ≥150 cfm. The bedrooms can be pressure-balanced using any combination of transfer grilles, jump ducts, dedicated return ducts, and/or undercut doors.
  2. Determine whether you will use individual return ducts, one or more central ducts, or central ducts in combination with transfer grills, jump ducts, and/or undercut doors. Consider filter placement when making this decision. With individually ducted returns, the filter will need to be located at the equipment return air inlet. With a centrally located return, the filter can be located at the return grille. This configuration may make it easier for the homeowner to change or clean the furnace filter, if plans called for locating the furnace in a hard to reach location, such as an attic or crawlspace.
    1. Consider noise when determining placement of returns. A return duct that has a direct connection to the blower motor could transfer that blower noise to the living room.
    2. Consider size when locating central returns. Central return grilles are much larger than most supply grilles.
  3. Install return ducts as you would supply ducts.
    1. Seal all seams, gaps, and holes of the return duct system with mastic (Figure 3).
    2. Seal the return box to the floor, wall, or ceiling with mastic, caulk, and/or foam.
    3. Do not use building cavities as return air pathways.

A complete HVAC system includes ducted returns

Figure 1. A complete HVAC system includes ducted returns Reference

Figure 2. A ducted central return brings air from central return registers back to the air handler through insulated, air-sealed ducts. (Image courtesy of Steven Winter Associates).

Return ducts are air sealed with mastic, just like supply ducts

Figure 3. Return ducts are air sealed with mastic, just like supply ducts Reference

 

Ensuring Success

Duct blaster testing equipment can be used to test pressure differences between each bedroom and the main body of the house when bedroom doors are closed and the air handler is operating. Builders should achieve a rater-measured pressure differential <= 3 Pa with a combination of ducted returns and door undercuts and/or transfer grills or jump ducts. Alternatively, the combined return pathways should provide 1 square inch of free area opening per 1 CFM of supply air, as reported on the contractor-provided balancing report.

Climate

No climate specific information applies.

Training

Right and Wrong Images

None Available

Presentations

None Available

Videos

  1. Ducted Returns
    Publication Date: July, 2015
    Courtesy Of: Risinger Homes

    Video describing jumper ducts and how to pressure balance rooms. 

  2. Ducts Buried in Attic Insulation and Encapsulated
    Publication Date: September, 2015
    Courtesy Of: BMI

    Video describing how to properly bury ducts in attic insulation.

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.

ENERGY STAR Certified Homes

ENERGY STAR Certified Homes (Version 3/3.1, Revision 08), Rater Field Checklist

6. Duct Quality Installation 

6.2 Bedrooms pressure-balanced using any combination of transfer grills, jump ducts, dedicated return ducts, and / or undercut doors to achieve a Rater-measured pressure differential ≤ 3 Pa with respect to the main body of the house when all bedroom doors are closed and all air handlers are operating. See Footnote 34 for alternative.34

Footnotes:

(34) Item 6.2 does not apply to ventilation or exhaust ducts. For an HVAC system with a multi-speed fan, the highest design fan speed shall be used when verifying this requirement. As an alternative to the 3 Pa limit, a Rater-measured pressure differential ≤ 5 Pa is permitted to be used for bedrooms with a design airflow ≥ 150 CFM. The Rater-measured pressure shall be rounded to the nearest whole number to assess compliance.

ENERGY STAR Revision 08 requirements are required for homes permitted starting 07/01/2016.

Associated Air Barrier Council

Associated Air Barrier Council 2002. AABC National Standards for Total System Balance 2002. The manual details the minimum standards for total system balance.

National Environmental Balancing Bureau

National Environmental Balancing Bureau (NEBB) Section 15990 – Testing, Adjusting, and Balancing. NEBB is a certification association whose members perform testing, adjusting and balancing (TAB) of heating, ventilating, and air-conditioning systems and commission and retro-commission building systems. This document is the TAB procedural standards.

2009 IECC

This topic is not specifically addressed in the 2009 IECC.

2009 IRC

This topic is not specifically addressed in the 2009 IRC.

2012 IECC

This topic is not specifically addressed in the 2012 IECC.

2012 IRC

This topic is not specifically addressed in the 2012 IRC.

2015 IECC

2015 IRC

In existing homes, the steps to install ducted returns are the same as with new construction methods. Access to the attic or basement is required. While it is not desirable to locate ducts in unconditioned spaces (e.g., a vented attic), if that is where they are already located, it may be beyond the budget of the retrofit project to move them into conditioned space. In this case, insulate the ducted returns to or above new code requirements to minimize the negative impacts (heat loss/gain to unconditioned space). It may be possible to bury the ductwork under the attic insulation and/or to encapsulate the ductwork with spray foam to minimize the impact of locating the ducts in unconditioned spaces.

When doing work in an existing home, make sure to refer to the following documents for safety guidelines:

  • Site Assessment of Ceiling, Attics, and Roofs Guide
  • Site Evaluation of Foundations and Floors
  • Assess, Remove, and Remediate Hazardous Materials

If cutting through painted surfaces, check for lead paint and mitigate as required. If cutting through plaster and wood lath ceilings, additional care will be required to minimize crumbling of the plaster. High-speed cutting tools, such as powered multi-tools or an angle grinder with a diamond blade, can assist in making clean cuts through the plaster and lath (but will create a considerable amount of dust, so you may want to have a vacuum operating next to the tool while cutting. Another trick is to apply a sealing compound to the attic side of the lath to harden it up (minimize flex) prior to cutting.

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
    Publication Date: April, 2012

    Case study about design and testing 10 high-performance homes in Farmington, Connecticut.

References and Resources*

  1. Author(s): Associated Air Barrier Council
    Organization(s): Associated Air Barrier Council
    Publication Date: January, 2002
    Standards book discussing changes, additions and enhancements over the 5th edition of the AABC National Standards for Total System Balance.
  2. 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.
  3. Author(s): Burdick
    Organization(s): IBACOS, NREL
    Publication Date: December, 2011
    Document providing guideance and considerations for duct design in an energy efficient house.
  4. Author(s): Baechler, Gilbride, Hefty, Cole, Williamson, Love
    Organization(s): PNNL, ORNL
    Publication Date: September, 2010
    Report providing builders in marine climates with guidance for building homes that have whole-house energy savings of 40% over the Building America benchmark with no added overall costs for consumers.
  5. Author(s): DOE
    Organization(s): DOE
    Publication Date: August, 2015
    Standard requirements for DOE's Zero Energy Ready Home national program certification.
  6. Author(s): EPA
    Organization(s): EPA
    Publication Date: September, 2015
    Document outlining the program requirements for ENERGY STAR Certified Homes, Version 3 (Rev. 08).
  7. Author(s): Raymer, Moyer
    Organization(s): Tamarack Technologies, Florida Solar Energy Center
    Publication Date: July, 2006

    Correcting pressure imbalances in your HVAC system can result in a healthier, more efficient home.

  8. Author(s): National Environmental Balancing Bureau
    Organization(s): National Environmental Balancing Bureau
    Publication Date: January, 2005
    Standard that includes testing, adjusting and balancing (TAB) to produce design flows for air and hydronic systems.
  9. Author(s): Lstiburek, Brennan
    Organization(s): BSC
    Publication Date: December, 2006
    Document with important building science considerations, designed for members of the residential construction and remodeling industries, as well as owners and managers who work in affordable housing.

Contributors to this Guide

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

Last Updated: 03/14/2016

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