Start with the Rest of the House—
Improve the Building Envelope
One reason HVAC contractors oversize systems is to make up for other thermal deficiencies. These deficiencies include uncontrolled air leakage, improperly installed or missing insulation, and minimum efficiency windows, all of which contribute to comfort problems in houses. In many cases even oversizing the HVAC system may not solve the problem.
Building envelope airtightness and proper installation of insulation is important not only to reduce energy consumption, but also to alleviate customer comfort complaints resulting from drafts or uneven temperatures. The most complicated areas are typically overhang areas such as fireplaces, bay windows, and rooms beside or over unconditioned spaces like attics or garages. Making sure the house has a properly installed continuous insulation system and all air leakage pathways are sealed using solid sheathing materials, spray foam, or caulk helps to ensure that the HVAC system is not the sole means for creating a comfortable environment for your customers.
Properly installed double glazed, low-emissivity windows will also improve the performance of the thermal envelope by reducing heating and cooling loads, and can minimize drafts and comfort complaints. Use windows with a low U value (0.35 or less) and a Solar Heat Gain Coefficient appropriate for the climate zone.
The Distribution System—Engineered Ductwork and Duct Sealing
Would you buy a sports car that had skinny little tires? Would you expect a motor boat with a souped up engine and a little tiny propeller to get you anywhere? The same issue is at play with a heating and cooling system. A state-of-the-art furnace or air conditioner is only effective if the air it heats and cools gets to the spaces where the people are. Performing engineering calculations allows the HVAC contractor to determine proper duct sizes according to the appropriate airflows for each room, taking into consideration the duct lengths and number of transitions in the various branches. Properly sizing registers helps to ensure good mixing of the air in the room, and installing balancing dampers where necessary will help ensure proper flow of conditioned air to and from all rooms in the home. Additionally, duct tightness and location are also important factors in designing a high quality air distribution system.
When ductwork is located outside the insulation layer of a house or in exterior walls, it is exposed to outdoor temperatures. Moreover, leaks in ducts located outside the house cause conditioned air to be dumped directly outside or into the attic or crawlspace rather than delivered to the house. Even leaky ducts inside the house can cause distribution and balancing problems, resulting in lower comfort and a higher probability of homeowner complaints. These problems can be minimized by locating the ductwork inside of the thermally insulated, air-sealed envelope of the home and sealing the entire duct system with a UL-181 approved closure system.
Properly Sizing HVAC Equipment
Programs like Energy Star Homes® and Building America show that builders are interested in improving the performance of their homes. Because changes to one system in a home may affect others, it’s important to determine the size of an HVAC system in relation to other improvements, like airtightness and better windows.
Unfortunately, many contractors still follow 20-year-old rules of thumb. The only way to properly size heating and cooling equipment is to perform an engineered analysis according to industry standards, like the Air Conditioning Contractors of America’s Manual J. By properly sizing the equipment, the annual operating efficiency is improved even though the equipment is actually operating for longer periods of time. This is similar to the way a car gets better gas mileage when running at a steady 55 miles per hour rather than in stop-and-go traffic.
In addition, it’s important to locate the HVAC equipment within the conditioned building envelope. A furnace or air conditioner in the attic, for example, will be fighting the winter cold and summer heat at the same time it is trying to heat or cool the house. Putting HVAC equipment in unconditioned spaces typically means having to buy a larger system, to make up for these losses.
The size of heating and cooling equipment for a home can be reduced through a better building envelope and an HVAC optimization strategy. Installing high performance windows, increasing building airtightness, and improving insulation contributes to a better envelope, and helps to reduce the heating and cooling loads. An HVAC optimization strategy includes doing a detailed HVAC design that takes into account the actual (lower) loads, HVAC equipment specifications and placement, optimized ductwork pathways, and improved duct system airtightness. The combination of an improved envelope and an HVAC optimization strategy can result in one air handling unit with a zone control system being used instead of two separate systems, thereby saving construction costs while not sacrificing comfort.
Many HVAC contractors initially resist decreasing the number of units typically used in a home because they’re worried about comfort complaints. By showing them you are taking control of many comfort related issues by improving the thermal envelope, changes to the HVAC system strategies are more readily accepted, including best practice advances like designing an engineered duct system, sealing the ducts, and proper sizing of the HVAC equipment.
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