Heating, Ventilating, and Air Conditioning

HVAC (heating, ventilating, and air conditioning) systems have a significant effect on the health and comfort of a home. Issues such as improper ventilation and poor indoor air quality are often linked to HVAC system design and operation. A well-designed house should have an HVAC system properly sized to its demands. Proper equipment sizing ensures a comfortable environment and provides opportunities to recapture some of the expense of an efficient building envelope.

Rules of thumb for equipment sizing do not work in modern homes and should not be used. Unfortunately, rules of thumb are still prevalent. A Florida survey points out some of the practices of HVAC contractors (Viera, Parker, Klonbergo, Sonn, and Cummings, 1996). Although only a small percentage of Florida's HVAC contractors responded, the survey found that about one-third of respondents sized air conditioning and duct capacities based on square footage or other rules of thumb.

Compounding the problem, the rules were not consistently applied. Some respondents provided twice as much capacity as others for a given square footage of floor area. More than one-third of respondents indicated intentional oversizing of HVAC equipment on some jobs, in order to avoid complaints, accommodate future expansions, enable quicker cooling of homes, and allow for lower cooling set points by homeowners.

For the best results in comfort, efficiency, and durability, integrate HVAC system design for both equipment and ducts in the overall architectural design. Work closely with a HERS rater, HVAC engineer, or HVAC contractor to properly design, size, and select HVAC equipment. If done properly, this single step goes a long way toward improved energy efficiency and comfort and substantial cost savings.

Sizing Air Conditioners

Along with "right sizing" the unit for optimum comfort, an efficient building envelope often allows for downsizing of air conditioning units, further reducing initial cost. In larger homes, downsizing may allow one unit to replace two, for additional savings in cost and maintenance. These dollar savings can help compensate for the extra cost of window and insulation improvements.

Three Sources for HVAC Design

The Air Conditioning Contractors of America (ACCA) has published simple but effective methods for determining loads and sizing ductwork and heating and cooling equipment.

Manual J tells how to calculate loads.
Manual D tells how to size ducts.
Manual S is a selection guide of appropriate heating and cooling equipment to meet identified loads.

For more information or to purchase these documents on the Web, go to the ACCA Web site.

Efficiency Measures for Air Conditioners, Heat Pumps, and Furnaces

Selecting an energy-efficient air conditioner or heat pump is made easier with the Seasonal Energy Efficiency Ratio (SEER) rating system. The SEER rating measures the cooling efficiency of air conditioner products. It represents the total cooling of a central air conditioner or heat pump (in Btu) during the normal cooling season as compared to the total electric energy input (in watt-hours) consumed during the same period. The higher the SEER rating, the more energy efficient the unit is. Minimum ENERGY STAR^® standards call for a SEER rating of 12, but a wide selection of air conditioning units ranging up to a SEER rating of 20 are available.

The Heating Season Performance Factor (HSPF) is a measure of a heat pump's energy efficiency over one heating season. It represents the total heating output of a heat pump (including supplementary electric heat) during the normal heating season (in Btu) as compared to the total electricity consumed (in Watt-hours) during the same period.

The Annual Fuel Utilization Efficiency (AFUE) measures the amount of fuel converted to heat at the furnace outlet in proportion to the amount of fuel entering the furnace. This is commonly expressed as a percentage. A furnace with an AFUE of 90 could be said to be 90-percent efficient.

Air Conditioners

Central air conditioners should be rated at a minimum of 13 SEER for air cooling and heat pumps should be rated at a minimum of 7.6 Heating Season Performance Factor (HSPF) for heating. In September 2006 DOE began enforcing a 13 SEER standard for all residential central air conditioners.

Until recently, SEER-10 air conditioning equipment has been standard across the country. But SEER-11 and SEER-12 equipment is becoming more widely used. SEER-12 equipment is nearly always cost-effective. Consider using SEER-14 air conditioning equipment to achieve performance levels greater than 30 percent savings. Equipment with SEER ratings up to 20 are now available. Currently, ENERGY STAR-labeled central air conditioners have a minimum rating of SEER 12.

Heat Pumps

Heat pumps are preferable to electric resistance heating in all but areas where there are fewer than 500 annual heating degree days. A unit with a HSPF of 7.7 or more will reduce the electric consumption during heating by more than 50 percent relative to electric resistance heating. The new standard will require that central heat pumps have a minimum rating of 7.7 HSPF. Lists of all ENERGY STAR-rated appliances can be found at the ENERGY STAR Web site.

In colder areas where temperatures often fall below 30 degrees F, typical air source heat pumps require an electric resistance system to properly heat a home. This can be an extremely costly method of supplemental heating; a backup gas or propane furnace may be a cost-effective alternative. A newly designed cold climate heat pump is under development but is not readily available.

Central Gas-Fired Furnace

Sealed-combustion gas furnaces should be specified for central gas-fired heating systems. Sealed combustion means than an appliance acquires all air for combustion through a dedicated sealed passage from the outside to a sealed combustion chamber, and all combustion products are vented to the outside through a separate, dedicated, sealed vent. ENERGY STAR labels furnaces that meet a minimum Annual Fuel Utilization Efficiency (AFUE) of 90. Lists of all ENERGY STAR-rated appliances can be found at the ENERGY STAR Web site.

Mechanical Ventilation

Building America recommends integrating mechanical ventilation into the HVAC system. Whole-house ventilation is a requirement in some state building codes. Central fan-integrated supply ventilation can be an easy and inexpensive way to provide outside air to the HVAC system. This system provides fresh, filtered, outside air in a controlled amount using the existing HVAC delivery system for even distribution and mixing.

Most of the Building America teams have designed and field tested these ventilation systems. The systems involve exterior air intakes, ductwork running to the return air side of the HVAC system, dampers to allow control of the air intake, and electronic controls to ensure that the HVAC fans operate frequently enough to draw in adequate fresh air.

Outside Vents

Diagram of attic air system featuring heating and cooling coils in a conditioned attic with 6" insulated flex-duct to the outside and a wall-cap where the air duct meets the gable end wall. There is a motorized damper to control open time and a manual damper to adjust flow rate on the outside air duct leading to the conditioning unit. The conditioning unit is connected to the living space below via an air duct with a central return box and a return grille running into the conditioner and another duct running out the vent into the living space. The conditioning unit also has a vent to receive 20CFM from the attic and to output 20CFM as well. Finally, there is a stand-alone dehumidifier in the attic as well, with a remote dehumidistat on the wall inside the house and vents running from the living space to the unit and back to the living space.

Compact Air Distribution System

A properly designed air distribution system is essential for an energy-efficient home. The following best practices describe important elements of an efficient air distribution system.

Make duct runs as short as possible. An efficient building envelope and efficient HVAC equipment allow for a compact air distribution system. Conditioned air may be discharged from inside walls or from ceiling diffusers up to 12 feet from the window wall in most cases without compromising comfort. Such "inside throw" layouts cut ductwork runs, saving money and reducing the amount of ductwork that may run in unconditioned space.

This diagram shows an air handling unit in the attic with transfer grilles in the walls of the living space below leading to a central hallway and the air return running from the hallway to the unit above. The conditioned air is distributed to the living areas, and flows back into the hallway through the transfer grilles and into the air handler above to be redistributed.

This diagram shows an air handling unit in a small attic space above a central hallway. The air handling unit receives air from the hallway and distributes it into the living areas, where it diffuses back into the hallway through transfer grilles between the different rooms and hallway. There is also a thermal boundary between the living areas and roof to serve as insulation.

This diagram shows an air handling unit within a closet in a hallway area; the air is distributed from the air handler to the living areas via ducts in a separate conditioned space above the unit but below the attic; it is then diffused into the hallway through transfer grilles and comes back to the air handler via a return at the ceiling in the central hallway.

Seal All Ducts and Air Handlers

Seal all ductwork seams and connections to air handlers with UL181-approved water-based mastic and seal drywall connections with caulk or foam sealant. Sealing ductwork is very important. Leaky ductwork in an unconditioned attic or crawlspace can draw unhealthy and humid air into the air distribution system. Sealing ducts with mastic is desirable even for ducts located in conditioned spaces. Properly sealed ducts make sure air gets to the spaces intended, rather than leaking into a plenum space. It also minimizes the chances of creating pressure differentials from space to space that would induce airflow through the envelope. Sealing each joint reduces the chances of unconnected ductwork, a surprisingly common mistake.

Mastic provides the most reliable duct sealing method for new construction. All ductwork, including the air handler compartment (which typically has many leaky joints), should be mastic sealed.

DOE research has found that some tapes perform adequately for sealing ducts, particularly fiberglass duct board. However, good performing tapes may be difficult to identify and traditional duct tape (cloth-backed rubber adhesive tapes) should never be used to seal ducts, even if it meets UL ratings. Sealing tapes should not be used for structural purposes. Tapes have low tensile strength and cannot mechanically support ducts. Technical Reports (PDF 2.2 MB) (Sherman and Walker 2004) and (PDF 690 KB) (Walker and Sherman 2005) on duct sealants can be found on the web. Download Adobe Reader.

Ducts and Air Handlers in Conditioned Space

Place ducts and air handlers in conditioned spaces or buried in insulation to the extent possible. High temperatures can be found in unconditioned spaces and create an unfavorable environment for ducts and air handlers. Ducts and air handlers perform best when placed within conditioned space. California code recognizes crawlspace placement of ducts as preferable to putting ducts in attics. Keeping ducts inside conditioned space may require one of several strategies, such as:

Placing ducts in a chase designed to run through a central corridor below the attic or on top of the ceiling through the attic. If the chase runs through the attic, it must fit within the roof truss design and should be covered with insulation.
Insulating and sealing the underside of the roof sheathing to create a conditioned attic. This strategy requires tightly sealing the roof structure, especially where it connects with the walls, to avoid the entry of outside air. This technique essentially requires building a nonvented roof assembly and may require a variance from local code officials. More information is available from BuildingScience.com.
Insulating and sealing the exterior walls of the crawlspace so that it becomes a conditioned space, such as a mini-basement. This strategy requires treating the crawlspace much like a living space with conditioned air supply, moisture control, and air returns to the HVAC system.

Duct Insulation

Ducts in unconditioned spaces must be insulated. To the extent possible, ducts should be placed inside conditioned space. In conditioned spaces, they require minimal insulation. If the ducts are placed in unconditioned spaces, due to the extreme summer temperatures in these spaces, 10 to 30 percent of the energy used to cool the air can be lost to conduction through the duct surfaces. Therefore, they must be insulated. Codes typically require R-8 insulation levels for ducts in unconditioned attics and crawlspaces. More information is available at the ENERGY STAR Web site.

Transfer Grilles and Jump Ducts

Use jump ducts and transfer grills and other return pathways to maintain balanced pressure in each room of the house. This practice is especially useful in rooms that are often isolated from the rest of the house by a closed door, such as a bedroom.

To maintain balanced pressure, air must be returned from each room to the central HVAC equipment. One way to do this is to add a ducted return from each room, but this approach is expensive and consumes a lot of space. A more reliable and cost-effective approach is to provide a central return and make sure that there are transfer grilles or transfer ducts, of adequate size, that allow air to pass from individual rooms to the central return even when doors are closed. Figure 18 illustrates different approaches to creating paths to equalize air pressure and allow air to return to HVAC equipment. Registers and transfer grills should be placed high on the wall in areas where furniture may block air movement.

Jump Ducts

Source: Building Science Corporation

Other Considerations

Draw Duct Layouts on Plans

Clearly identify on plans and drawings the locations, sizes, and types for all duct work and registers, including the heating and cooling supply ducts, passive return air ducts or transfers, the mechanical ventilation air inlets (at least eight feet away from any exhausts or condensers), and all exhaust outlets. Indicate any chases or other spaces that are dedicated to duct runs. This level of detail can be referenced in contract documents, which provide guidance in the field for proper installation.

Energy Performance and Commissioning

After installation, evaluate air conditioners and heat pumps with a duct pressure test and, if needed, a smoke test to identify the location of leaks.

Printable Version