Ducted Returns

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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. Most HVAC distribution systems are designed with at least one supply register in each room, but typically only one larger return register is installed on each floor of the home—usually in a central hallway floor or ceiling. 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). 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 has lower airflow requirements to meet the load (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). Utilizing a central return strategy is a simple and effective way to get air back to the air handler. When locating a central return, it should be placed in a central hallway where it is adjacent to the main living space of the house.

The use of building cavities (the space between wall studs or “panned” floor joists) as return air pathways is not appropriate in an energy-efficient house. 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.

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 at 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 recommends that returns provide 1 square inch of free area opening per 1 CFM of supply air, or achieve a rater-measured pressure differential <= 3 Pa with respect to the main body of the house when bedroom doors are closed and the air handler is operating. 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 one of these in combination with transfer grills, jump ducts, and/or undercut doors. Consider filter placement and air handler unit 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.
    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.

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

Figure 1 - Return ducts are air sealed with mastic, just like supply ducts Reference

A complete HVAC system includes ducted returns

Figure 2 - A complete HVAC system includes ducted returns 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.

Scope

Bedrooms pressure-balanced and provide 1 sq. in. of free area opening per 1 CFM of supply air or achieve a Rater-measured pressure differential ≤ 3 Pa

Duct Quality Installation

 

Option A.

Bedrooms pressure-balanced using any combination of transfer grilles, jump ducts, dedicated return ducts, and/or undercut doors to provide 1 square inch of free area opening per 1 CFM of supply air, as reported on the contractor-provided balancing report:

  1. Refer to the balancing report provided by the HVAC contractor for the bedroom air flows to size the transfer grilles and/or jumper ducts.
  2. Install and seal properly sized transfer grilles during framing. Both openings of the transfer grille must have the required free area.
  3. If transfer grilles are not used, install and seal jumper ducts during framing. Both openings and ducts must have the required free area.
  4. ENERGY STAR recommends that doors are undercut to approximately 3/4 inch above the finished floor.

Option B. 

Bedrooms pressure-balanced using any combination of transfer grilles, jump ducts, dedicated return ducts, and/or undercut doors to achieve a Rater-measured pressure differential <= 3 Pa (0.012 in. w.c.) with respect to the main body of the house when bedroom doors are closed and the air handler is operating

  1. ENERGY STAR recommends that transfer grilles, jumper ducts or dedicated returns be installed and doors undercut to approximately ¾ inch above the finished floor.
  2. Test the pressures of each bedroom.
Rooms supply air flow Free area opening required Height required for 10 in. wide transfer grille* Height required for 12 in. wide transfer grille* Height required for 14 in. wide transfer grille* Jumper duct diameter
CFM  in 2 in in in in
50 50 6.7 5.6 4.8 8
75 75 10 8.3 7.1 10
100 100 13.3 11.1 9.5 12
125 125 - 13.8 11.9 14
150 150 - - 14.3 14
175 175 - - - 16
200 200 - - - 16

* Assumes the net free area of the transfer grille as .75 in.

Option C.

Bedrooms pressure-balanced using any combination of transfer grilles, jump ducts, dedicated return ducts, and/or undercut doors to achieve a Rater-measured pressure differential <= 3 Pa (0.012 in. w.c.) with respect to the main body of the house when bedroom doors are closed and the air handler is operating

  1. ENERGY STAR recommends that transfer grilles, jumper ducts or dedicated returns be installed and doors undercut to approximately ¾ inch above the finished floor.
  2. Test the pressures of each bedroom.
Rooms Pressure (PA) with Respect to Main Body
Bedroom 1  
Bedroom 2  
Bedroom 3  
Bedroom 4  
Bedroom 5  
Bedroom 6  

 

ENERGY STAR Notes:

This checklist item does not apply to ventilation ducts. For HVAC system with multi-speed fans, the highest design fan speed shall be used when verifying this requirement.

Pressure Relief Tips

  • ENERGY STAR recommends that HVAC contractors install transfer grilles, jumper ducts, or dedicated returns.
  • ENERGY STAR recommends that framers undercut doors to approximately ¾ inch above the finished floor.
  • If transfer grilles are used, contractors must install and seal properly sized transfer grills according to the load calculation.
  • If jumper ducts are installed, contractors must seal all seams, gaps, and holes of the ducts and connections.
  • If return ducts are installed, 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.

Pressure Testing Tips

  • Prior to testing pressures:
    • Verify all supply and return terminations are unrestricted.
    • Turn the HVAC system on to cooling mode. If there is no cooling mode, set it to heating mode.
    • Verify air is blowing out of the supply terminations.
  • Verify the reference pressure is measuring the outdoor pressure.
  • Test all pressures by placing the pressure measuring device in each bedroom with the door shut.

Additional Information

For additional information and specific duct testing protocols please refer to RESNET Chapter 8 (Standard for Performance Testing and Work Scope: Enclosure and Air Distribution Leakage Testing).

Training

Right and Wrong Images

Presentations

None Available

Videos

None Available

CAD Images

None Available

Compliance

ENERGY STAR Version 3, (Rev. 07)

HVAC System Quality Installation Rater Checklist, Duct Quality Installation.  This checklist item does not apply to ventilation ducts. For HVAC system with multi-speed fans, the highest design fan speed shall be used when verifying this requirement.

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.

More Info.

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): Burdick
    Organization(s): IBACOS, NREL
    Publication Date: December 2011

    Document providing guideance and considerations for duct design in an energy efficient house.

  3. 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.

  4. Author(s): EPA
    Organization(s): EPA
    Publication Date: June 2013

    Standard document containing the rater checklists and national program requirements for ENERGY STAR Certified Homes, Version 3 (Rev. 7).

  5. 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.

  6. 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.

  7. 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.

Last Updated: 08/15/2013

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