What is an air source heat pump and how does it work? These are good questions, and depending on where you live and what utilities are available to you, the answers could help lower your monthly heating costs.
Types of Heat Pumps
There are many types of heat pumps, such as Ground Source Heat Pumps, Mini-Split Air Source Heat Pumps, Air to Water Heat Pumps, Packaged Unit Air Source Heat Pumps, and Split System Air Source Heat Pumps. This article will focus on the Split System Air Source Heat Pumps, which has an outdoor unit just like an air conditioner, and an indoor unit with a coil. Regardless of the type of heat pump, they will provide heat to a building as shown below in How A Heat Pump Works.
What is a Heat Pump
Simply put a heat pump has been called an air conditioner in reverse. Air conditioners take heat from inside the house and shed the heat outside. Heat pumps recover heat from outside of the house and transfer it to the inside of the house. The heat pump is different than an air conditioner because it has a reversing valve that allows the system to bring heat into the house.
Picture yourself driving a car that can only go forward or can stop. Going forward you can speed up, slow down, make turns and even drive in a circle. An air conditioner is like that type of car. Now add a reverse gear, and the car can now go forward, stop, or go in reverse. Going in reverse is like a heat pump. Adding a gear to go backwards changes the direction of car. Adding a reversing valve to an air conditioner does the same thing, it changes the direction of the air conditioner to that of a heat pump.
Therefore a 3 ton 16 SEER Carrier heat pump has the same components as a 3 ton 16 SEER Carrier air conditioner except for the following basic elements.
- The heat pump has a reversing valve, the air conditioner does not.
- A little extra tubing is required inside the heat pump to properly operate the reversing valve.
- A defrost board is required in a heat pump melt any frost or ice that accumulates on the outdoor coil during the cooler months.
- The low voltage wiring at the heat pump is different.
What is an Air Source Heat Pump
An Air Source Heat Pump is a system of mechanical components that use the refrigeration cycle to transfer heat from a place that the heat is not wanted, to a place that the heat is wanted. In the residential application, the Air Source Heat Pump is used to transfer heat from outside of your house to inside your house. How is this transfer of heat possible when the outside temperature is much less than the temperature inside your home?
Heat is generated by molecular activity. With increased molecular activity there is an increase of heat, and a decrease in molecular activity means there is less heat. The Second Law of Thermodynamics states that heat seeks cold, so increased molecular activity and thus increased heat demands the transfer of heat to an object that is lower in temperature. At absolute zero or −459.67 °F, the molecular activity is so low and there is no transfer of heat.
Consider your refrigerator and freezer. Based on the Refrigeration Cycle heat is removed from the refrigerator/freezer, and in the process a fan passes air over the condenser coil and heat is blown into the kitchen area. With average refrigerator temperatures around 35*F and freezer temperatures around 0*F, where is the heat coming from, and how does it leave the refrigerator and freezer? The residential temperatures just cited are far above absolute zero, or -459.67*F, so the molecular activity in ice cream, ice cubes, milk, or butter, is still fairly high.
The transfer of heat is governed by the temperature that a liquid boils, or the boiling point. At a young age we are taught that water boils at 212*F at sea level. Did you know that on the top of Mount Everest water boils at 160*F? The differences in pressure (in this case altitude) and temperature show a pressure temperature relationship. The temperature pressure relationships are key components to the refrigeration cycle and our discussion of Air Source Heat Pumps.
In the refrigeration cycle for residential air conditioners and heat pumps, the liquid used for heat transfer is called a refrigerant. Freon or R-22 refrigerant has been used for decades in air conditioners and air source heat pumps, and it has a boiling point of -41.4*F. Puron or R-410a has replaced R-22 for environmental reasons, and it boils at -61*F. Because the box temperatures in a refrigerator or freezer are much higher than the boiling points of refrigerants, they will cause the refrigerants to boil.
Let’s use R-410a as an example of removing heat from a refrigerator. There is a 96*F difference between the -61.0*F boiling point of R-410a and the +35*F temperature in the box. When the refrigerant is circulated in the box it will boil and remove heat. Going back to water that boils at 212*F, isn’t it true that it will also boil when the water is heated 96*F more, or to 308*F. The excess temperature difference causes both water and refrigerant to boil. Neither R22 nor R-410a are used in refrigerators and are mentioned here for temperature comparisons.
This article is published in January 2017 and at this time + 5*F is considered by many experts as the lowest outdoor temperature that an Air Source Heat Pump can provide heat without an auxiliary source. The following excerpt published in 2013, followed a 2009 study done by the U.S. Department of Energy, in which a residential Air Source Heat Pump was effective to -30*F. “ A 4-ton low temperature heat pump (LTHP) manufactured by Hallowell International…claimed to operate down to –30°F, and to supply essentially all needed heat to a home via its compressors with very minimal use of a supplemental heat source…Standard air source heat pumps more rapidly lose heating capacity as the outdoor temperature drops below about +30°F and rely increasingly heavily on supplemental heat…As this report will demonstrate, however, its technology lived up to its promise…”
In 2013 the outside operating temperature range difference between a low temperature heat pump and a standard heat pump was 60*F. When considering an Air Source Heat Pump, the consumer should do thorough research on the makes and models being considered. For example, Carrier’s Model 38GRQ air source mini-split delivers 80% of its potential heating capacity at -22*F, using an outdoor unit with Inverter Compressor Technology!
Selecting an Air Source Heat Pump
In addition to price, there are a number of factors the consumer should analyze before making a decision. Here are my Top Four considerations when considering an Air Source Heat Pump over a conventional furnace and air conditioning system.
Choosing a Contractor is the most important decision made when purchasing new heating and cooling equipment the subject of another article, but the principles are simple. The contractor must be registered in your state to do business, and have a Master’s license for HVAC issued by the prevailing authority. A full insurance package including workers compensation, positive feedback with your local Better Business Bureau, and a list of current references are mandatory in selecting a contractor. What does your gut tell you about the contractor?
The Manufacturer is very important. Hallowell International manufactured the Low Temperature Heat Pump mentioned in the study published by the U.S. Department of Energy, and is no longer in business. Many top name manufacturers have higher price points on their equipment for a reason, the quality. Does the manufacturer allow just anyone to install their equipment, or must the contractor qualify before representing a brand.
Each Air Source Heat Pump manufacturer publishes data for their equipment. The efficiency of an air conditioner is rated by its SEER. The efficiency of an Air Source Heat Pump is mainly rated by its HSPF. Since Air Source Heat Pumps provide air conditioning and heat, the equipment will have SEER and HSPF ratings. What are SEER and HSPF ratings, how do they differ, and why are they significant in the equipment selection process? These ratings will be included in a future article.