Heat Pumps: How They Heat and Cool With One Refrigerant Loop

What the work looks like

A heat pump is an air conditioner that can run in reverse. In cooling mode it pulls heat out of the house and dumps it outside. In heating mode a reversing valve swaps the indoor and outdoor coils' roles: the outdoor coil becomes the evaporator (absorbing heat from outdoor air), and the indoor coil becomes the condenser (dumping heat into the house). Same compressor, same refrigerant, one extra valve.

You will install and service air-source heat pumps (the most common), mini-splits (ductless, inverter-driven, very efficient), and the newer cold-climate heat pumps that work below 0 F (Mitsubishi Hyper-Heat, Fujitsu Halcyon). Dual-fuel systems pair a heat pump with a gas furnace and switch to gas below the balance point (usually 25 to 35 F). The federal tax credits under the IRA (25C, 25D) have made heat pumps the fastest-growing residential HVAC segment in 2024-2026.

Job titles: HVAC Installer, HVAC Service Tech, Heat Pump Specialist. Pay $22 to $50 per hour; cold-climate and mini-split specialists earn a premium.

Safety and tools

Heat pumps use the same refrigerants as AC systems (R-410A on older units, R-454B or R-32 on 2025+ new installs due to A2L refrigerant transition). A2L refrigerants are mildly flammable, which changes the tools and practices:

• A2L-rated recovery machines and manifolds.
• No open flame near the system during servicing.
• Leak detection using electronic sniffers rated for A2L.
• Brazing requires an inert-gas purge (nitrogen) inside the line, same as any refrigerant work.

EPA 608 certification is required to touch any heat pump that holds more than 5 pounds of refrigerant. For residential mini-splits with factory-charged line sets, you still need the cert for any recovery or evacuation.

Tools: Fieldpiece SMAN 480 or Testo 557s manifold (A2L-rated), vacuum pump (Fieldpiece VP85 or similar) pulling down to 300 microns, micron gauge (Testo 552), torque wrench for flare fittings (Asada or Imperial; flare torque is critical on mini-splits), scale, recovery machine.

Your first exercise

Find a heat pump (residential or commercial). Look at the outdoor unit. The reversing valve is the cylinder-shaped valve with four refrigerant lines going into it (usually near the compressor). In winter, the reversing valve is energized on most manufacturers (some do cooling-energized instead). Feel the lines: in heating mode, the large-diameter suction line is hot; in cooling mode, it is cold. That tells you what mode the system thinks it is in.

Check the outdoor coil in winter on a cold day. Frost is normal; the system periodically runs a defrost cycle (reverses to cooling mode, heats the outdoor coil with indoor heat, melts the frost, reverses back). If the coil is a block of ice, the defrost sensor or board has failed.

Where to go next

Build on Heat Pumps with HVAC Fundamentals (Introduction to HVAC), Refrigeration Systems (Introduction to Refrigeration), Refrigeration Troubleshooting (Introduction to Refrigeration Troubleshooting), HVAC Controls and Thermostats (Introduction to HVAC Controls), EPA 608 (Introduction to EPA 608 Certification), and Gas Furnace Service (Introduction to Gas Furnace Service). Safety: Hazardous Materials Handling, Fire Safety & Prevention.