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 stale air to return to the return side of a central air handler.
- ENERGY STAR Certified Homes requires that the dedicated return ducts, transfer grilles, jump ducts, and/or door undercuts provide 1 square inch of free area opening per 1 CFM of supply air, as reported on the contractor-provided balancing report. Or, 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.
- 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.
- If transfer grilles or jump ducts are used, refer to the balancing report provided by the HVAC contractor for the bedroom air flows to size the grilles or ducts. Ensure that both openings have the required free area. Seal all seams, gaps, and holes in the ducts and connections.
- If door undercuts are used, framers should cut the bottom of the doors to approximately ¾ inch above the finished floor.
- Test the pressure differential with the bedroom doors closed.
See the Compliance Tab for related codes and standards, and criteria to meet national programs such as ENERGY STAR Certified Homes, DOE’s Zero Energy Ready Home program, and EPA’s Indoor airPLUS.
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
- 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.
- 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.
- 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.
- Consider size when locating central returns. Central return grilles are much larger than most supply grilles.
- Install return ducts as you would supply ducts.
- Seal all seams, gaps, and holes of the return duct system with mastic.
- Seal the return box to the floor, wall, or ceiling with mastic, caulk, and/or foam.
- Do not use building cavities as return air pathways.
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.
No climate specific information applies.
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
(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 2002. AABC National Standards for Total System Balance 2002. The manual details the minimum standards for total system balance.
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.
This topic is not specifically addressed in the 2009 IECC.
This topic is not specifically addressed in the 2009 IRC.
This topic is not specifically addressed in the 2012 IECC.
This topic is not specifically addressed in the 2012 IRC.
Ducted Returns = Comfort System Return Ducts