Ductless heat pumps, sometimes referred to as min-split heat pumps, consist of a single outside compressor/condenser unit connected to wall- or ceiling-mounted indoor air handler units. They use inverter technology with a variable-speed compressor that varies the refrigerant flow to continuously match the heating/cooling load. Without ducts and mounted inside conditioned spaces, a ductless heat pump can provide zoned heating with increased energy savings over standard heat pumps. Advanced units offer SEER ratings up to 26 and can perform at a much wider temperature range than standard heat pumps, eliminating the need for backup heat sources in many locations. Geothermal or ground-source heat pumps use the constant temperature of the earth as the exchange medium instead of the outside air temperature making them very efficient for heating and cooling, but initial installation costs can be prohibitive.
Manuals T and D are your guides for designing the air distribution system. Manual T provides best practice tips about where the supply registers, diffusers, and grills should be located, and how big they should be. Locating air handlers and return ducts in the garage is not recommended because of the potential for drawing carbon monoxide and hazardous fumes into the home. IRC says that ducts located in the garage may not have any openings in the garage and furnaces and air handlers that supply air to living spaces shall not supply air to or return air from a garage. All ducts, air handlers, and filter boxes should be air-sealed and joints and seams should comply with the 2009 UL 181-approved sealing methods. The Building America Best Practices Series recommends water-based mastic and caulk, foam sealant or mastic for duct-drywall connections.
Manual D details the duct system design, helping designers to evaluate duct lengths, fittings, and turns and select the best ducting material with the goal being to deliver the correct cfm to each room against the ideal friction created by the ducts and fittings with the static pressure available from the blower. Numerous studies have shown significant energy savings by placing ducts and air handlers inside the conditioned space. Ducts may be run through open-web floor trusses in a two-story home, through a dropped hallway ceiling in a one-story home, or through a conditioned attic. Duct runs should be as short as possible, and duct sizes and layouts should be shown on plans, per Building America’s recommendations. Alternatively, ducts can be located on the ceiling deck of an unconditioned attic and “buried” with blown-in insulation, however, condensation may occur at the duct exterior surface in humid climates; buried ducts may be permissible if they are directly insulated to R-8, apart from the piled-on insulation. But since the mixed-humid and hot-humid climates often exceed Building America’s maximum permissible dew points for this option, it specifically does not recommend buried ducts.
Building America states that all new homes should be equipped with whole-house mechanical ventilation and meet The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 62.2. A supply-only ventilation system that brings filtered outside air to the intake side of a home’s central air handler offers the advantage of positively pressurizing the home and reducing the intrusion of moisture and radon. Building America also recommends the addition of electronic controls, including a motorized damper and timer to open the damper in sync with operation of the HVAC fan for automatic ventilation of the home at regular intervals throughout the day. This approach would run the central fan on a timer basis, even when the compressor is not running. Exhaust only ventilation may draw too much air out of a home, creating negative pressure that pulls outside air in through leaky walls in older structures and potentially backdrafting combustion from fuel-burning appliances or fireplaces in a tightly sealed newer home.