An air-source or air-to-air heat pump can provide both heating and cooling.
- In the winter, a heat pump extracts heat from outside air and delivers it indoors.
- On hot summer days, it works in reverse, extracting heat from room air and pumping it outdoors to cool the house.
Nearly all air-source and air-to-air heat pumps are powered by electricity. They have an outdoor compressor/ condenser unit that is connected with refrigerant-filled tubing to an indoor air handler. As the refrigerant moves through the tubing of the system, it completes a basic refrigeration cycle, warming or cooling the coils inside the air handler. The blower pulls in room air, circulates it across the coils, and pushes the air through ductwork back into rooms.
When extra heat is needed on particularly cold days, supplemental electric-resistance elements kick on inside the air handler to add warmth to the air that is passing through.
Instructions: Place your cursor over the numbers below to find out how the heating cycle of an air-source heat pump works:
Click here to open a text description of how an air-source heat pump works.
How an Air-Source Heat Pump Works
An air-source heat pump consists of an indoor air handler, a blower, an outdoor unit, a compressor, and and expansion valve. Here's how it works.
A cold liquid refrigerant (located behind the expansion valve in the tubing outside the house) absorbs heat from the air outside (absorption occurs because the refrigerant is much cooler than the air outside).
As the liquid refrigerant absorbs the heat, it evaporates; hence the outside coils are referred to as "evaporator coils". The temperature of the refrigerant at this point is almost equal to the outside temperature.
The evaporated refrigerant gas is compressed in the compressor to higher temperatures (so heat can be transferred to inside) and pressure. The compressor is often thought of as the "heart" of the heat pump, since it does most of the work of forcing heat "uphill."
The high-temperature and high-pressure refrigerant gas passes through the indoor coil, where the refrigerant gives up its heat to the indoor air.
The fan blower pulls in room air, circulates it across the coils, and pushes the air through ductwork to distribute heat to the house. When extra heat is needed on particularly cold days, supplemental electric-resistance elements kick on inside the air handler to add warmth to the air that is passing through.
The fan also cools the refrigerant to the point where much of it condenses, forming a liquid. Therefore, in the heating season, the indoor coil is called the "condenser coil." This change of state results in a large transfer of heat energy.
The warm refrigerant liquid now goes outside via the tubing to the expansion valve. This device reduces the pressure, causing the refrigerant to become cold again--cold enough so that it is once again ready to absorb heat from the cool outdoor air and repeat the cycle.
In the winter, a heat pump extracts heat from outside air and delivers it indoors. In the summer, the heat pump extracts heat from room air and pumps it outdoors to cool the house.
Instructions: Observe the heating and cooling cycles of a heat pump. First click on the button of the cycle, and then press "play".
The Balance Point
As we have learned, air-source and air-to-air heat pumps work by extracting heat from the outside air. These heat pumps require a backup system to supplement their heating ability when the outdoor temperature gets below a certain temperature.
As the outdoor temperature drops, the heating requirement of the house increases and the output of the heat pump decreases. At some point, the temperature of the home’s heating requirement and the heat pump output match. This temperature is called the balance point and usually falls between 30-45 degrees Fahrenheit. For any temperatures below the balance point, supplemental heat will be required.
To locate the balance point, the heating requirement (BTUs/h) of the house and the heat pump output (BTUs/h) are plotted against the changes in outside temperature. The place where the home heating requirement and heat pump output lines cross is the balance point.
Take a look at the graph of the Balance Point.
Efficiency of a Heat Pump
Efficiency of a heat pump is measured using a term Coefficient of Performance (COP), and it is the ratio of the useful heat that is pumped to a higher temperature, to a unit amount of work that is put in. We will look at COP in terms of air-source heat pumps.
A general expression for the efficiency of a heat engine can be written as:
Using the same logic that was used for heat engines, this expression becomes:
Where, Q Hot = Heat input at high temperature and Q cold= Heat rejected at low temperature. The expression can be rewritten as:
Note: Thot and Tcold must be expressed in the Kelvin Scale.