HVAC Reference Library · Technical Reference

Heat Pumps: How They Work and What to Stock

A heat pump is an air conditioner that can run in reverse to heat. This sheet explains the reversing valve, defrost and auxiliary heat, and the parts that are unique to heat pumps on a counter.

Ref HV-0005AHRI 210/24010 CFR 430

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

An air-source heat pump moves heat from outdoor air into the building in winter and from the building to outdoors in summer. A reversing valve switches the direction of refrigerant flow, so the outdoor coil acts as the condenser in cooling and as the evaporator in heating. Because the outdoor coil runs cold in heating mode, frost forms on it, and a defrost control periodically reverses the system to clear it. Auxiliary heat, usually electric heat strips in the air handler, adds heat when the heat pump alone cannot keep up and during defrost. Ratings are set under AHRI 210/240 and the DOE test procedures in 10 CFR 430.

How to identify

A heat pump outdoor unit has a reversing valve (a four-way valve with a solenoid coil) near the compressor and usually a defrost control board. The thermostat has heat pump settings and an "emergency heat" mode. The data plate says "heat pump" and lists both cooling and heating data.

Heat pump parts and what they do

Part	Function	Stocking notes
Reversing valve and solenoid coil	Switches heating and cooling	Coils are sold by voltage; valves by tonnage and connection size
Defrost control board	Starts and ends defrost	Often brand specific; match the OEM part number
Defrost sensor or thermostat	Senses outdoor coil temperature	Clip-on style; match the board
Bi-flow filter drier	Filters in both flow directions	Standard driers are one direction only
Bi-flow TXV or dual metering	Meters refrigerant in both modes	Per OEM design
Heat strips (electric heat kits)	Auxiliary and emergency heat	Sold by kW and air handler model
Sequencers or relays	Stage heat strips	Match the heat kit
Outdoor thermostat (on some systems)	Locks out strips above a set point	Optional accessory
Heat pump thermostat	Controls O or B valve terminal, aux and emergency heat	Confirm O or B before selling

Key points

Thermostat terminal O energizes the reversing valve in cooling. Terminal B energizes it in heating. Brands differ. Wiring the wrong one makes the system heat when it should cool.
Auxiliary heat (W2) comes on automatically as a second stage. Emergency heat (E) runs the strips and shuts off the heat pump.
Steam or fog from the outdoor unit during defrost is normal.
Cold-climate heat pumps keep more capacity at low outdoor temperatures. Compare published capacity data at the temperatures that matter.

Common mistakes

Selling a standard one-way filter drier for a heat pump line.
Setting the thermostat for B on a system that uses O.
Treating the emergency heat light as a fault code.
Replacing a defrost board with a "universal" board without matching the sensor and timing settings.

Governing standards: AHRI 210/240; 10 CFR 430 (DOE energy conservation program for consumer products); manufacturer installation instructions.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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