Tools & Equipment ReviewHigh Efficiency HVAC Meets Green Ideals While Fulfilling Real-World Comfort Demands

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Size Right
Choosing energy-efficient heating and cooling units isn't enough; they must be sized correctly. "If you design the system to fit the house and its level of tightness, it will run more efficiently," says Brian Stamm, purchasing manager for Colorado green builder McStain Neighborhoods.

Improperly sized HVAC systems not only suck up more energy, they also can lead to comfort issues. An oversized air conditioning unit, in particular, can cause uncomfortable conditions, such as hot and cold spots or poor air quality, says Jeff Hurt, brand manager for Coleman Heating & Air Conditioning.

Too much capacity also can lead to short-cycling, which will prevent the system from drawing moisture out of the air, so the home is cool, but clammy. This effect leads homeowners to lower cooling set-points in an effort to make their homes more comfortable, which increases energy use, say Energy Star experts. Short-cycling can decrease energy efficiency by 30 percent or more, and also leads to more wear and tear on the air conditioning unit, decreasing the life of the system.

On the other end, "If it's undersized, it will run all the time because it can't keep up with the home's demands," points out Mickey Smith, product manager for York.

Estimating heating and cooling needs based solely on square footage will not yield the most accurate HVAC sizing. HVAC subs also will need "every scrap of information on the house: the type, size, and number of windows; whether they're insulated; how the house is insulated," says Reshetar Custom Homes president Shawn Reshetar.

Refer to the Air Conditioning Contractors of America's "Manual J8" to calculate heating and cooling loads and properly size home HVAC systems, or employ an HVAC engineering firm that has experience with optimizing more efficient systems.

Seal Tight
Finally, no discussion of high-performance HVAC would be complete without emphasizing the importance of duct sealing. According to Energy Star, sealing and insulating the ducts can improve the efficiency of a house's heating and cooling system by as much as 20 percent, potentially more. Leaving ducts unsealed and uninsulated allows conditioned air to leak into unconditioned spaces, which wastes energy and creates the potential for hot and cold spots.

"You could spend several thousand dollars to get the highest-efficiency HVAC system put into a home, but if your walls are full of spots where air can leak out, or if the ductwork is not installed properly, that system is not going to be very efficient," says Robert Smart, vice president of construction for Cary, N.C.-based Anderson Homes. "You can't look at one piece independently. You have to think of the whole house as a system." BP

Fresh Look
Despite the energy-saving advantages, tightly sealed houses mean less natural ventilation. Properly designed, mechanical ventilation can bring fresh air into the building and exhaust polluted or stale indoor air out to maintain air quality.

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers recommends an air exchange rate of one-third per hour for most houses, meaning that one-third of the air inside a house is replaced with fresh air from outside every hour. Mechanical ventilation controls the introduction of fresh air while diluting pollutants and removing excess moisture. Depending on the type of ventilation installed, the system also can condition incoming outdoor air before mixing it with indoor air, helping to maintain temperatures and efficiencies.

Installing one or more heat recovery ventilators (HRVs) in a cold climate helps save energy while exchanging fresh outdoor air for stale indoor air. The heat exchange provides about 70 percent efficiency, according to Energy Star, offsetting the energy used to condition the fresh air during the process. HRVs also retain indoor air's moisture, incorporating it into incoming air.

In warm, humid climates, installing energy recovery ventilators (ERVs) works in much the same way, but removes humidity from incoming air during the fresh-air exchange while conditioning it with cool air from inside the house.

Natural ventilation through windows uses one-third the energy of mechanical ventilation, according to Larry Brown, NAHB's program manager for energy efficiency. However, mechanical ventilation is automatic; windows require a homeowner to open and close them.

—S.L.M.

--This article originally appeared in Green Products and Technology magazine.