Building Systems

Once the building design is established, the next major area to address is the major building systems. Passive cooling and heating, daylighting, and good envelope design reduces heating and cooling demands placed on building systems, but in most climate zones, some mechanical heating and cooling systems are still needed. Careful selection and sizing of equipment is critical, as heating and cooling account for about 56 percent of the annual energy bill for U.S. residences.3

Heating, ventilating, and air-conditioning (HVAC) systems need to be designed with thermal comfort, energy efficiency, life-cycle cost, and the need for healthy indoor air in mind. A well-ventilated living space features the frequent exhaust of indoor air to remove pollutants and reduce moisture and the regular introduction of fresh air.

Historically, many affordable housing units were designed with individual heating and cooling systems. A forced-air furnace or electric-resistance heater, an evaporative swamp cooler or window-mounted air conditioner, and individual 30- to 40-gallon tank hot-water heaters are common. As energy codes have improved and fuel costs have increased, a growing number of projects are turning to central systems for domestic hot-water and space heating. With this approach, hot water is pumped through the building for use in showers and sinks and, when needed, in floor or wall radiators. A central hot-water-based system permits elimination of gas lines to individual units, as appliances and air-conditioning (if provided) can be electric. Another approach is to use a high-efficiency heat pump for both heating and cooling. Either strategy improves energy efficiency and can lower utility costs by 15 percent or more.

Energy modeling is a valuable and effective tool in the decision-making process. Several alternative options for major systems can be defined during the design charrette and then modeled to identify what option is projected to deliver the greatest savings over the life of the systems for the least installed cost.

When considering alternatives for the major building systems, also consider the impacts on utility metering and utility allowances (see Chapter 5). Installing separate electric and gas meters gives residents more incentive to conserve resources. However, master metering provides a way to average out utility costs among residents, which is valuable if the owner is responsible for the bills, as is typical for senior, single-room occupancy (SRO), and special needs developments. Providing multiple meters also requires space and additional piping and wiring. In some locations, submetering is permitted, a choice that offers the energy efficiency of central HVAC systems and the accountability of individual meters.

Recommended strategies include the following:

• Install ceiling fans in the living room and bedrooms to augment air movement in buildings designed with passive heating and cooling principles. In mild climates, fans can help eliminate the need for air-conditioning. In hotter climates, they can reduce the need to use air-conditioning.

• Use RemRate or other Residential Energy Services Network (ResNet) accredited software to simulate building energy demand and estimate equipment needs for low-rise buildings, and EnergyPlus or other DOE-2-compatible software for buildings over three stories.4

• Provide at least 15 cubic feet per minute (cfm) of fresh air per occupant, consis tent with the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standard 62.2, to dilute pollutants and prevent moisture buildup.

• Determine the heating and cooling loads of the building using Air Conditioning Contractors of America (ACCA) Manual J.

• Size ductwork using ACCA Manual D and locate ductwork in conditioned spaces. If ductwork must run outside conditioned areas, it should be well sealed and insulated.

• Size the HVAC equipment using ACCA Manual S to prevent the purchase of oversized equipment that may short-cycle (constantly turn on and off), which leads to substandard comfort, buildup of moisture in the equipment, and shorter equipment life.

• Seal ductwork with fiber-reinforced mastic. Do not use cloth duct tape, foil tape, or silicon.

• Balance the air supply being distributed to each room on the basis of the calculated room loads. Make sure return grills and ducts are adequate to maintain the desired airflow, provide balanced flow to each room, and ensure that air has a path to the return grill, even if doors are closed.

• Specify ENERGY STAR® -rated heating and cooling equipment.

• Specify hot-water heaters with an energy factor (EF) of 0.8 or better, and install a drainwater heat recovery unit.

• Test HVAC systems after installation to ensure that the refrigerant charge is correct and the system is balanced.

• Provide thermostats that are easy to use and accessible.

• Consider hydronic radiant heat systems, as an alternate to electric resistance or gas-fired-furnace forced-air systems.

• Consider using whole-house fans for single-family homes in climates with significant swings between afternoon and evening temperatures.

• In the dry Southwest and mountain regions, consider evaporative coolers as an energy-saving alternative to conventional air conditioners.

• In climates with significant heating needs, consider a heat recovery ventilation unit, which transfers heat from the air being exhausted from the dwelling to the air entering the heating system.

• In climates with both a high number of heating and cooling days, consider geo-thermal heat pumps, which use the constant temperature of the earth to assist in heating and cooling. In the summer, heat is carried away from the building and released into the earth; in the winter, the process is reversed, and heat is pulled from the ground and released into the building.

• Consider using tankless hot-water heaters, alone or, for multiunit buildings, in a series.

• Consider using a solar hot-water system to preheat the water before it enters either the central boiler or individual water heaters.

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