How Does a Heat Pump Work?

The basic idea: moving heat, not making it

A heat pump is essentially an air conditioner that can run in reverse. Heat naturally flows from warm to cool. Refrigerant, which boils and condenses at very low temperatures, lets a heat pump push heat the other way. Even cold outdoor air contains heat energy, and a heat pump can extract some of it and bring it inside.

The main components

• Compressor: pressurizes the refrigerant and drives it around the system.
• Outdoor coil: releases heat in cooling mode and absorbs heat in heating mode.
• Indoor coil: absorbs heat in cooling mode and releases heat in heating mode.
• Expansion device: lowers refrigerant pressure and temperature before it enters the coil that must absorb heat.
• Reversing valve: changes the direction of refrigerant flow, which switches the system between heating and cooling.
• Blower and thermostat: move air through the ducts and tell the system what to do.

Cooling mode versus heating mode

In cooling mode

The indoor coil gets cold and absorbs heat and humidity from the air in your home. Refrigerant carries that heat to the outdoor coil, where it is released into the outside air. This is the same cycle as a standard air conditioner.

In heating mode

The reversing valve redirects the refrigerant so the outdoor coil becomes the cold one, absorbing heat from outside air. The refrigerant carries that heat to the indoor coil, where it warms the air passing through, and the blower distributes it through the ducts.

Why heat pumps can be efficient

Because they move heat instead of burning a fuel or heating an element, heat pumps can deliver more heat energy than the electrical energy they use. This ratio is called the coefficient of performance, or COP. A COP of 3 means roughly three units of heat delivered for each unit of electricity consumed. COP varies with outdoor temperature and the equipment, and it falls as outdoor temperatures drop. Efficiency ratings such as SEER2 for cooling and HSPF2 for heating describe seasonal performance.

The defrost cycle and emergency heat

In cool, damp weather, frost can form on the outdoor coil when it is cold. To clear it, the system briefly switches into cooling mode, which warms the outdoor coil. During this defrost cycle you may see steam rising from the outdoor unit and the fan may stop for a few minutes. That is normal. Some indoor air may feel cooler during defrost, and many systems use electric auxiliary heat strips during this time.

Most heat pump thermostats also have an Emergency Heat setting, which runs only the electric backup heat. It is intended for situations where the heat pump itself is not working. Electric resistance heat is far more expensive to run than the heat pump, so do not use it as a normal setting.

Are heat pumps a good fit for Houston?

Heat pumps work well in mild climates, and the Gulf Coast has short winters with relatively few days at deeply low temperatures. That means the system can handle heating efficiently most of the time, with a single piece of equipment providing both cooling and heat. Houston does see occasional hard freezes, and a properly sized system with backup heat, or a gas furnace paired with an AC in a dual-fuel setup, can cover those. Whether a heat pump is a good choice for you depends on your home, your ducts, your utility rates and your priorities, and a contractor can help you compare.

What you can safely do as a homeowner

1. 1Replace the filter on schedule.
2. 2Keep the outdoor unit clear of leaves, debris and snow or ice, and leave about two feet of space around it.
3. 3Do not pour hot water on the outdoor unit or chip ice off it. If heavy ice builds and does not clear, turn the system to Off and call for service.
4. 4Learn what your thermostat's Emergency Heat setting does, and use it only if the heat pump is not working.
5. 5Schedule maintenance twice a year, since heat pumps run in both seasons.

What a technician checks on a heat pump

In addition to the standard cooling checks, a heat pump visit typically includes testing the reversing valve operation, defrost controls and sensors, auxiliary heat elements and sequencers, and checking temperature rise in heating mode. Refrigerant charge and airflow are verified because both strongly affect performance in each mode.