How Energy Efficient Are Heat Pumps?

Being located in the middle or southern part of the United States, you are probably more than familiar with the term heat pump. Heat pumps are heating and cooling systems that use Freon in conjunction with electrical heat strips to heat and cool a designated area. Given that these systems do not use natural gas or propane, they are supposed to be more efficient, cleaner, and all-around safer. The only problem is the systems rely heavily on the changing of states of the Freon. Changing from a gas to a liquid and back. Meaning, outdoor temperatures play a huge role.

Why Heat Pumps Are Energy Efficient

In theory, a heat pump takes the heat inside the home and forces it outside. If you go outside on a warm or hot day and place your hand above the operating outdoor unit, you will feel it blowing out hot to warm out. This is the air inside the home, and it is being transferred outside via the refrigerant and copper lines. A lower pressure, a refrigerant can absorb energy from the outdoor air and use it to provide heat. This works because when a refrigerant is under pressure, it emits heat. The compressor, usually located outside and considered the heart of the HVAC system, reduces these pressures and allows the refrigerant to absorb more energy. If you are like most people, you are probably having a hard time following this. That is understandable, and all you need to know is that a heat pump is more efficient because it transfers, absorbs, and moves heat instead of generating it. When you compare that to a unit that constantly burns fuel to create heat, it should be easy to see how heat pumps are much more effective. Another neat thing about a heat pump is that it can reverse the flow of refrigerant and provide cooling as well, using a similar theory. During normal cooling operation, the refrigerant or Freon will flow from the compressor to the condenser where it is changed from a liquid to a vapor. It is later changed back to vapor when it hits the evaporator. You can reverse this flow, which is what the reversing valve does, and you’ll get heat. Most heat pumps also use electrical resistance heating because Freon is extremely susceptible to outdoor temps. When the outdoor temp drops, it becomes increasingly harder for the changing of states. This along with other factors can determine the overall efficiency of a heat pump system.

Energy-Efficiency Ratings

You’ll notice that today’s heat pumps come rated in a variety of energy-efficiency ratings. The higher, the more effective the unit is considered. For instance, a 21 SEER (Seasonal Energy Efficiency Rating) is more efficient than an 18 SEER. As you can imagine, a heat pump loses efficiency as the weather gets colder outside. Whether you are training to become a tech or simply interested in purchasing a unit, you’ll hear a lot of terminologies and industry lingo being tossed around. Coefficient of heating performance along with seasonal energy efficiency rating are two terms you’ll hear constantly. The coefficient of heating performance is the ratio of a unit’s input energy compared to its output energy. A SEER is the measure of the unit’s cooling annual performance. SPF (Seasonal performance factor) is another term you’ll commonly see or hear. This is a measurement of the overall efficiency of the heat pump at generating thermal energy. This is the number that will certify the efficiency of the heating side, whereas the SEER is a measure of the cooling side. Electrical heat resistance comes into lay when there are huge temperature variations between the input and output ratings. In certain climates, it is impossible for refrigerants to effectively change from liquid to gas or gas to liquid. Therefore, electrical heat must be supplemented. The electrical heaters are also sometimes used to defrost the outdoor unit. As outdoor temperatures drop, Freon becomes less and less efficient. Like water, it will tend to lose efficiency and change state at 32 degrees F.

Temperature Ranges Of A Heat Pump

Since heat pumps operate by transferring energy, the outside air will play a crucial role. Most heat pumps units are designed to operate efficiently at 25 to 40 degrees F. However, as was said above, when outdoor temperatures hit that 32 degrees F mark, you’ll notice a huge drop in the overall effectiveness of the unit. It’ll take the system much longer to satisfy the thermostat if it can even satisfy it at all. This is where electrical resistance heat helps. All that in mind, there are some areas that can efficiently operate year around without the assistance of backup heat, AUX heat, or heat strips. Take the San Francisco area for instance. Given that temperatures rarely drop below 25 degrees F, there simply isn’t a need for the additional backup heat. Because the introduction of electrical heat makes the system that much more ineffective, HVAC manufacturers are always looking for newer methods to increase efficiency. Geothermal has become increasingly popular in some areas. Geothermal heating and cooling units are systems that draw energy from underground water sources to aid in the transfer of energy. Although these units can be extremely effective and tied in with plumbing and hot water systems, they require many more installation restrictions. One must not only live near a river, lake, or pond but the soil and water must offer specific properties

Two-Stage Compressors

The advent of two-stage compressors is another solution for increased efficiency. Most of the earliest heating and air systems were designed to only regulate the heat at a single stage. Two-stage systems have proven to be much more effective over the years, as they provide a stage of high capacity and a stage of low capacity. The theory is, why run a motor on high when it can accomplish the same tasks in low. You get the same results with nearly half the energy consumption. A higher capacity compressor is specifically designed to handle more extreme temperature fluctuations. That said, two-stage systems are designed to operate on the lower capacity side 80% of the time, making them more effective. The more powerful, higher capacity second stage doesn’t come into play until it is absolutely necessary. Because two-stage compressors run longer, the removal of humidity is also much more effective during the cooling cycles.

Variable Speed Heat Pumps

Variable speed motors have come into play over the years as well. Think of a two-stage system as a system that offers an extra setting when more power is needed. It’s almost the same theory as a truck with 4-wheel drive. A variable-speed motor is something entirely different. It starts at a lower speed and slowly works its way to a higher speed, incrementally increasing as it goes. The motor might start at 20%, then go to 40%, then 60%, and work its way all the way up to 100%. The theory is that the temperature in the home will be satisfied before the motor reaches 100%. Not only have these systems proved to be more efficient and add years of life to units, but they are much quieter. There is also the additional control of dehumidification and more efficient filtration, however, that’s an entirely different topic.

Unit Sizing

Heat pumps and heating and air systems, in general, are extremely technical. Especially to an outsider. There are a lot of outside factors that can determine just how effectively the unit operates. You can clearly see the role that ambient temperatures play. Unfortunately, that is not the only consideration. A heat pump system can be doomed to fail right from the start if it isn’t properly sized. Not only must the system be big enough to produce energy to effectively satisfy temperatures in the area, but it must not be oversized. Putting a 5-ton unit in the place where a 3-ton unit should go is just as bad as sticking in a 1-ton unit. When the units are oversized, it causes them to satisfy the temperatures in the areas too quickly. HVAC systems must operate for desired times to effectively dehumidify and remove moisture. Other factors to consider are the building materials. Some doors, some wall materials, and even some windows offer more insulation as compared to others. All these things must come into play when properly sizing a heat pump unit for the desired area. Therefore, it is usually best to work with a highly trained and experienced engineer to properly size your unit. These individuals will not only be able to properly size the unit, but they’ll be able to properly size the ductwork as well. The sizing of the ductwork is just as crucial for the effective delivery of air throughout the desired area.

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