Basic physics for those considering a heat pump
Most people are familiar with heat pumps, and know that they can provide both heating in the winter and cooling in the summer. They also know that heat pumps are considered to be a very efficient way of heating a structure, since they draw heat from the outside air. But many people also wonder how a heat pump works to draw out that heat, and how low the outside temperatures can be and still have the heat pump work.
First, you need to know a couple of basic principles of physics. One is that heat will always move from a warm surface to a colder one. Another is that liquids absorb heat as they boil, and give heat off as they condense. A third is that when a liquid or a vapor is compressed, its temperature rises, and when that pressure is released, its temperature falls. Heat pumps, like refrigerators and air conditioners, utilize all of these basic principles to move heat from one location to another.
A heat pump has two primary components: an outdoor unit, which contains a compressor, an expansion valve, a reversing valve, a fan and a series of coils called an evaporator; and an indoor unit, which contains a fan and a series of coils called a condenser. Connecting the two units are a pair of tubes which contain a refrigerant that circulates inside the tubes in a closed loop. A refrigerant is a type of fluid that has the special property of boiling at temperatures well below 0 degrees F.
The refrigerant enters the outdoor unit as a liquid, which is colder than the outside air. Latent heat in the outside air is drawn to the cold refrigerant, and this heat causes the refrigerant to boil and turn to a vapor. This process occurs inside the evaporator.
The vapor moves next into the compressor, which compresses the vapor and causes its temperature to rise to about 100 degrees. The hot refrigerant vapor moves into the indoor unit and through a series of tubes, where a fan blowing across the tubes causes the heat in vapor to be given off to the air inside your house.
As the heat is given off, the cooling vapor condenses back into a liquid, a process which occurs in the condenser. This liquid then moves back to the outdoor unit and enters an expansion valve. Inside the expansion valve, the pressure on the liquid is released, and the liquid’s temperature drops back down to below 0 degrees. This cold liquid moves back into the coils of the outdoor unit, absorbs more heat from the outside air, and the cycle begins again.
An important part of the outdoor unit is the reversing valve, which allows the refrigerant to move in the opposite direction: the evaporator in the outside unit and the condenser in the inside unit exchange functions and the heat pump now acts as an air conditioner, absorbing heat from the inside air and releasing it to the outside air.
Air at any temperature down to absolute zero (approximately -460 degrees F) contains some amount of latent heat that can be given off to the heat pump. In a practical sense, however, heat pumps operate most efficiently down to an outside temperature of about 25 to 35 degrees, known as the balance point. Below the balance point, the amount of extracted heat is insufficient for heating a house by itself, and electric strip heaters inside the indoor unit begin to come on. These heaters, called supplemental heaters, come on one at a time -- there are typically three or four strips -- to make up the amount of heat necessary to keep the indoor temperature at the desired level.
When the supplemental heating strips are not on, the heat pump is at its most efficient, since it is using electricity only to move heat, not to create it. In areas where the winter outdoor temperatures rarely reach below 25 degrees, a heat pump is an excellent way to keep utility costs down. In much colder areas where the temperatures are often below the balance point, some or all of the supplemental heat strips are almost always on, causing the heat pump to lose energy efficiency.
Another thing to be aware of when considering a heat pump is that the temperature of the air being delivered to the house is typically between 80 and 100 degrees, as contrasted with a standard gas or electric furnace which delivers heated air at 130 to 140 degrees. This air may feel relatively cool to people used to conventional forced air furnaces, and is a common complaint when switching to a heat pump.
Because of this lower air temperature, a greater volume of air needs to be provided to the house in order to maintain the desired indoor temperature. This increased air volume requires larger ducts for delivery. In new construction, installing ducts of the proper size is no problem, but when converting from a standard forced air furnace to a heat pump, you may find that your existing duct system is undersized.
Heat pumps present an excellent value in some areas, while their higher cost may not be justified in others. When considering a heat pump versus a standard forced air furnace, consult with a qualified heating contractor to discuss your options and decide which is best for your particular location.
