Can Heat Pumps Also Cool?

February 7, 2024

Heat pumps are one of the most energy-efficient options for heating a home. They are typically two to three times more efficient than furnaces that burn fossil fuels. This efficiency is because heat pumps transfer heat rather than burning natural gas or heating oil, which means that installing one can significantly reduce heating costs. Another major advantage of a heat pump is that it can also function as an air conditioner to cool your home in summer.

Understanding How Heat Pumps Cool

Heat pumps are very similar to traditional air conditioners in many ways, as they use the same process to cool a building. All air conditioning systems cool using a special chemical refrigerant to facilitate heat transfer. This heat transfer process works naturally due to the laws of physics. Heat energy naturally flows from a heat source or higher temperature area into a heat sink. A heat sink is anything cooler than a heat source. In a heat pump or traditional AC system, the refrigerant acts as the heat source and heat sink at various points in the heat transfer process.

Before the refrigerant is pumped inside the building from the heat pump, it moves through an expansion valve. This causes its temperature to drop. At this point, the refrigerant is an extremely cold liquid that gets pumped inside and flows through the evaporator coil. When the system runs, the blower constantly pulls in hot air from inside the house through the return air vents and ductwork. As this hot air flows over the evaporator coil, much of the heat it contains naturally flows into and is absorbed by the refrigerant.

As the refrigerant removes heat from the air, it transforms from a cold liquid into a hot liquid as it flows through the evaporator coil. When it flows back outside to the heat pump, it then enters the heat pump compressor. When refrigerant is compressed and its pressure increases, it gets “superheated.” This essentially means that it instantly boils and turns into an extremely hot gas. At this point, the refrigerant is now somewhere around 120-140 degrees when it flows out of the compressor and into the condenser coil.

The compressor fan in the heat pump constantly runs when the system is cooling and blows air over the compressor coil. As long as the refrigerant is still hotter than the air blowing over the condenser coil, most of the heat that it absorbs and removes from the building gets released and naturally flows out into the air. The refrigerant then returns to the expansion valve, which cools it down to the point where it can again absorb heat from the warmer air inside the house.

Understanding How Heat Pumps Heat

A heat pump is different from a traditional air conditioner because it has a reversing valve. When the valve activates, it switches the direction the refrigerant moves through the system and enables the heat transfer process to work in reverse. That means the system absorbs heat from the air outside and transfers it to the air inside to heat the building. When a heat pump switches to heating mode, the coil inside the building and the coil in the heat pump switch roles.

The cold refrigerant now flows through the outdoor coil in the heat pump and absorbs heat from the warmer air blowing over it. The warmer refrigerant is then again compressed and superheated, which transforms it into a gas that is much hotter than the air temperature inside the building.

The blower still constantly pulls air in through the return ducts so that it flows over the coil inside. However, heat energy is naturally released since the air flowing over the coil is lower than the refrigerant’s temperature. The blower then directs the heated air into the return duct system and out of the vents in each room.

Many people find it hard to believe that a heat pump system can capture heat from cold winter air. However, no matter how cold air gets, it always contains at least some heat energy. That means the heat transfer process always works naturally as long as the refrigerant is colder than the air. In heat pumps, the refrigerant traveling through the outdoor coil when in heating mode is usually 10 to 20 degrees lower than the outdoor temperature so that it can act as a heat sink.

Many cold-climate heat pumps remain effective until outdoor temperatures drop below 5 degrees. They will still heat your home but become less energy efficient and may not be able to keep your house as comfortable as you prefer.

Understanding Heat Pump Defrost Cycles

One issue when a heat pump heats in colder weather is that frost and ice form on the outdoor coil since the refrigerant flowing through it is so cold. When the coil gets covered with ice, the heat transfer process can’t work. This is because the ice prevents the air from directly contacting the coil, which in turn prevents the refrigerant from absorbing heat.

This issue is why heat pumps have a defrost cycle or mode that they will occasionally switch to in order to thaw the coil and melt the ice. When the defrost cycle starts, the reversing valve activates so that the refrigerant changes direction and the system switches to cooling mode. This means that the refrigerant again starts capturing heat from inside the building, and hot refrigerant flows through the outdoor coil to melt all of the ice on it.

The fact that a heat pump pulls heat from out of the home when defrosting is why heat pump systems must have an auxiliary heat source to work effectively in colder temperatures. In auxiliary heat mode, the unit generates warmth with electric resistance heating coils, similar to baseboard heating. If there were not an auxiliary heat source that ran during the defrost cycle, the home would quickly become much colder since the system is actively pulling heat out of it.

The auxiliary heat source is also necessary in case the temperature is too cold for the heat pump to work, and it also serves as emergency heating in case the heat pump stops working. If the home is more than a few degrees colder than whatever temperature the thermostat is set to, the auxiliary heating will run alongside the heat pump. This helps the system to heat the home and raise the temperature more quickly. Once the desired temperature is reached, the auxiliary heating will shut off, and the heat pump will continue running as needed to maintain the temperature.

Contact the Pros

At Brennan Heating & Air Conditioning, we have been serving the residents of Seattle, WA, and the surrounding areas since 1987. We install, maintain, and repair heating and cooling systems, including heat pumps. In addition, we provide plumbing, electrical, and indoor air quality services. To learn more about the benefits of installing a heat pump system in your home or to schedule a consultation or service call, contact us today.

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