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What's the Difference between a Heat Pump and an Air Conditioner?

A heat pump is simply an air conditioner that is reversible.

Imagine for a moment that you have a window unit air conditioner mounted in a window of your house. When the unit is running it blows cool air on the inside of the house while exhausting warm air on the outside. If you turned the unit around in the window so that the warm air blows inside the house, you could heat with the unit.

A heat pump has a reversing valve that allows the flow of refrigerant to change direction when the thermostat shifts from "Heat" to "Air Conditioning" and vice versa.

In the summer, the system takes heat from inside the house and releases it outside…leaving the house cooler.

How a Heat Pump "Pumps" Heat…


heat pump in Summer diagram

Outdoor Section:

  • The outdoor section consists of a fan, outdoor coil, compressor, and reversing valve.
  • Refrigerant circulates between the outdoor and indoor sections. It exists in one of three forms…liquid, gas, or super high-pressure, high-temperature gas.
  • As the refrigerant in the high-presssure gas state condenses into liquid, heat is released. Managing where this condensation takes place allows the system to "pump" heat from one location to another.
  • The function of the compressor is to compress gaseous refrigerant into the super high-pressure state.
  • In summer, the compressor sends the high pressure gas to the outdoor coil, where it condenses into liquid form, thus releasing its heat to the outdoor air.
  • The liquid refrigerant is sent to the indoor coil where it is allowed to expand (boil) as it passes through the coil. As it boils into gas, it absorbs heat from inside the home.
  • The refrigerant in gaseous state is then sent back to the compressor to be compressed into high-pressure gas and circulated through the outdoor coil…begining the cycle again.
  • In winter, the reversing valve shifts and the cycle is reversed. The compressor sends its high-pressure gas to the indoor coil where it condenses into liquid, releasing heat inside the home.
  • The liquid refrigerant is then sent to the outdoor coil, where it is allowed to expand (boil) back into gas and absorb heat from the outdoors.
  • The cycle begins again as the compressor compresses the gaseous refrigerant into it's high-pressure form and sends it back to the indoor coil.

Indoor Section:

  • The indoor unit is comprised of several different components.
  • There is an indoor fan (also called a "blower") that moves the heated or cooled air across the coil and into the house.
  • The indoor coil allows the exchange of heat from the refrigerant lines.
  • The drain pan catches any water condensation on the outside of the coil.
  • The auxiliary heating element provides supplemental heat when the weather is very cold and backup heat if the outdoor unit becomes non-operational.
  • The filter helps keep the indoor coil clean.
  • A humidifier further enhances the comfort of air circulating inside the home.

Additional Parts of a Heat Pump System

A heat pump system has several other components not found in air conditioning systems. It has electric heating coils inside to provide supplemental heat during the coldest weather when the heat pump alone is not able to supply enough heat for the house.

A heat pump also has a defrost thermostat on the outdoor unit to control the defrost cycle of the unit when the system is operating in the heating mode. During the winter, the system reverses itself every 90 minutes to release warmth to the outside and melt any frost or ice that may have formed on the unit.

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  • 9375-J Gerwig Lane, Columbia MD 21046
  • Lic. HVACR MD 01-2939