Equipment Features:

•    2-Speed, Zoned Heat Pump:  The building envelope design features described above greatly reduced the required air conditioner size. Manual-J HVAC equipment-sizing calculations showed the need for only 2½ tons of heating and cooling capacity.  In this case the owner opted for a two-speed compressor, which provides either 2½  or 5 tons of cooling or heating depending on the need.

Figure 40 (below) shows the air conditioner operation on a typical hot day summer day in Florida (August 24, 2001).  The Hoak home unit operated in the 2½-ton mode until the late afternoon when it switched to the 5-ton mode for a few brief periods.  In this home, energy use stays low because the low compressor speed operates the majority of the time.  But, when quick cool-down or excessive loads require more capacity, the high speed compressor can meet the need.

Table 11.  HVAC system cooling and heating efficiencies.

•          Variable-speed Air Handler:  Two benefits of using a variable-speed motor for air distribution are better moisture removal and energy efficiency. During the cooling season, slower airflow across a cold coil allows for more moisture removal.  Wintertime comfort also is enhanced with this operation, since the coil has more time to warm before the air is brought to full flow.

Indoor relative humidity tends to increase during the fall and winter months when air conditioning activity declines.  Without a dedicated dehumidifier, the air conditioner is the only means of reducing indoor relative humidity.  When there is a call for cooling - the low-speed compressor in a variable speed system operates more consistently than a larger system and keeps relative humidity from rising to unhealthy levels.

Figure 41.  Heat pump water heater produced all the hot water needs for a four-person household from April to September.

•     Heat Pump Water Heater:  Solar water heating would have been the first choice for this home, but poor orientation and too many shade trees forced a search for other options. (Please see Figure 41.)  Natural gas also was unavailable in the area. To avoid the inefficiency of electric resistance water heating, a 6,000 BTU/hour heat pump water

heater was connected to a standard 80-gallon electric water heater.  By locating the heat pump inside the home, homeowners gained a summertime benefit of additional cooling and year 'round dehumidification because the system removes moisture each time it operates.

This control scheme minimized the electric element use, reserving these units for backup heating when the heat pump failed to meet demand.  Data collected from April to September of 2001, showed that the unit produced all the hot water needs for a four-person household without requiring electric backup heating.  A problem that was later corrected, caused the heat pump to shutdown midway through September and offered a chance to compare heat pump and electric resistance performance. During the first 13 days of September the heat pump used 32.7 kWh, while 86.2 kWh was consumed during the following 13 days when hot water was supplied solely by electric resistance elements.  Note that hot water use was not monitored during this period.  Figure 42 illustrates the improved efficiency.

Figure 42.  Water heater energy use from September 1 to September 13, 2001.

•          Energy Recovery Ventilator:  The energy recovery ventilator acts as a conduit to flush out stale indoor air and replace it with outdoor air.  As the indoor air is expelled, a heat exchanger recovers up to 80% of the energy used to heat or cool the air and transfers it to the incoming air stream. This unit also transfers a portion of the moisture between the airstreams, which is useful during periods of high outdoor humidity.

•          Airtight Ducts:  Attic and duct heat gain contribute to about 22% of the cooling needs of a typical Central Florida home when are ducts located in a vented attic above the insulation.  While some home efficiency is lost by direct heat-gain through the duct insulation, a great deal more efficiency can be lost from unintended duct leakage from the ductwork into the vented attic.  Duct leakage test results showed only 50 CFM of air was lost at 25 Pa of pressure differential in the Hoak residence.  This leakage equates to 1.2% leakage per square foot of conditioned floor area - far below the leakage normally found in new Florida homes.