AHRI certificate
Documentation that a specific condenser-and-coil pairing was tested together and actually earns its rated SEER2/HSPF2. Ask for it in writing.
Air handler
The indoor half of a ducted system: blower, coil, filter rack. Replaces the furnace cabinet.
Auxiliary / backup heat
A secondary heat source, electric strip heat or a paired furnace, that takes over below the balance point.
Balance point
The outdoor temperature at which the heat pump's output exactly matches the house's heat loss. Below it, backup heat runs.
BTU (British Thermal Unit)
The unit heating and cooling capacity is measured in. One ton of AC capacity equals 12,000 BTU/h.
Defrost cycle
A brief reversal that melts frost off the outdoor coil. The steam it produces is normal and lasts a few minutes.
Cold-climate heat pump
A model specifically rated to hold most of its heating capacity at very low outdoor temperatures, often down to −5°F or colder.
Dual fuel / hybrid
A system pairing a heat pump with an existing gas furnace, switching between the two based on outdoor temperature.
Inverter / variable speed
A compressor that modulates instead of cycling on and off. The main reason modern units are quieter and more efficient.
Manual J
The industry-standard room-by-room heat loss and gain calculation. The only correct basis for sizing.
Reversing valve
The component that swaps which coil is hot, turning a heat pump into an air conditioner and back.
Strip heat
Electric resistance elements used as backup. Effective and expensive; poorly configured controls run it far too often.
Short cycling
Turning on and off too frequently, usually from oversizing. Causes poor humidity control and premature wear.
Line set
The insulated copper pair carrying refrigerant between outdoor and indoor units. Often reused, sometimes shouldn't be.
R-454B / A2L refrigerants
The lower-global-warming refrigerants replacing R-410A. Mildly flammable, so they require updated tooling and training.
Static pressure
Resistance the blower fights against. High readings mean undersized ducts, and no equipment upgrade fixes that.
Switchover temperature
On a dual-fuel system, the setpoint where control hands off from the heat pump to the furnace.
Zoning
Dividing a home into areas with independent thermostats and dampers or separate indoor units, so unused rooms aren't conditioned unnecessarily.