Heat pumps function both as a heater and air conditioner using the same equipment that operates in reverse modes. When in heating mode, rather than primarily burning a fuel or heating an electrical element to generate heat, a heat pump concentrates, extracts, and transfers warmth from the outside to the inside of your home or building or vice versa. The primary benefit: in the right circumstances, a heat pump can use more “renewable” energy (natural temperature differentials from the air, ground, or water) for its primary indoor heating and cooling source. Exception: when in “auxiliary heat” mode, heat pumps generally use electrical resistance “strip heat”.
Like other HVAC technologies, heat pumps are constantly improving.
Visit our website dedicated to heat pumps
The first decision point: what power or fuel sources are available? Equipment costs more and You are running a compressor to generate heat.
In some parts of the country or even neighborhoods within a city, natural gas (or its cousins propane or butane) are not available as a heat source to a furnace. This leaves electricity as the main commercial source of heat. Since heat pumps are usually all-electric, many people whose only heat source is electricity buy them. Why? The other alternative, an electric heat strip central heater, produces all its heat from resistance heating (no heat transfer from external sources like the heat pump has).
For those accustomed to central heating using a gas furnace, heat pumps have a few inherent characteristics that take some getting used to. One example: in normal heating (not auxiliary heat) mode, the air coming out of indoor vents from a heat pump is cooler than the warm air from a gas furnace. The air from the heat pump in normal mode may be cooler than your body or skin temperature. Since blowing air that is cooler than body temperature can feel chilly to some people, this might determine where you sit or where furniture is placed in relation to the room vents. Some people describe more sensitivity to this issue than others. If you have questions about this, ask your HVAC contractor about it. If you want to make a specific comparison between a heat pump and some other type of heating unit (gas or strip electric), you can ask what the measured temperature will be at the vent into your rooms in your heating choices.
Another factor: due to the way they “move” heat from inside to outside or vice versa, heat pumps work best in certain types of climates. Although the subject of debates, as a practical matter, air source heat pumps appear to us to be designed to work best in moderate climates with significant swings in daily warming. For ground or water source designs, they could be considered in places with affordable access to ground or water temperature differentials with less concern for air temperature differentials. Why? A cool or cold climate with lots of cloudy days would not have as much differential between inside and outside heat in the air as the same climate with more sunshine warming would. Taken further, areas with many sub-freezing days would require more auxiliary heat. Since auxiliary heat in a heat pump usually comes from electricity (called “heat strips”), this would cause high electricity bills.
The majority of this page describes split heat pumps. This simply means that the major components (the evaporator coils and condenser coils and compressor) are divided into two cabinets, one located inside and another outside. The other alternative is for the major components to be inside one cabinet, or “package”. Further down on this page, we also included an introduction to the ductless heat pump, which has been designed for situations where ductwork is not needed.
How A Heat Pump Works
Heat pumps “move” heat from the outside to inside (in heating mode) and inside to outside (in air conditioning mode). With the help of electricity to power the compressor, blower, and provide auxiliary heat, heat pumps use various external sources for the heat source. Air source heat pumps are currently the most common, especially for moderate climates, but there are also ground (geothermal) source and water source designs.
It is a bit counterintuitive, but even in cooler weather there are still useful heat differentials in the outdoors. The differentials provide a heat source which can be “moved” from the outside, then, in alternation, compressed and expanded to warm indoor space. For air conditioning mode, this process is reversed. There is a valve type switch that tells the heat pump which operating mode to use.
Although heat pumps derive their heat differentials aided by electricity, they have auxiliary electric (or less commonly gas) heat source for the days when the outside air temperature drops below a threshold temperature (described as a “balance point” –usually somewhere between 30 and 45 degrees Fahrenheit) or to raise heat quicker than it does in its primary heat transfer mode.
Heat pumps have some features in common with conventional air conditioning.
- The majority of heat pumps utilize a compressed gas refrigerant. They can also use alternative working fluids of various types.
- The main components of an air source heat pump are the compressor, two heat exchangers (often referred to as the condenser for the outside and evaporator inside) and an expansion or reversing valve.
- The inside air handler components consist of the following: housing cabinet, the blower, the evaporator coil, and the controls.
- Heat pumps are available in package units without ducts or split systems with ductwork.
- The warmed or cooled air can be distributed indoors through a forced air system with ductwork. Other types of systems use radiator or floor type distribution.
Much less common than the compressed refrigerant version of heat pumps is the absorption method. An absorption design uses thermal energy rather than mechanical drive of compressed gas to power the system.
Heat Pump Efficiency
When manufacturers rate the performance of heat pumps, the equivalent of “SEER” (in A/C equipment,) and “AFUE” (for furnaces) is COP, or coefficient of performance. This term describes the ratio of heat output (in BTUs) to electricity input in Kilowatts (KW). Like the other two ratings, a higher COP rating is better.
Another rating for heat pumps is heating seasonal performance factor (HSPF). Like COP, the higher the HSPF rating, the less electricity the unit will use to heat your home.
Strengths and Limitations of heat pumps
- Depending on available local resources, heat pumps can use renewable natural or waste heat sources and temperature differentials from the surrounding air, ground, or water as their primary energy source.
- This reduces fossil fuel usage, costs, and emissions.
- In areas where natural or propane gas is not available, all electric heat pumps are one of the primary choices for heating and are usually more efficient than using resistance heating for all heat production.
- The optimum situation for a heat pump requires a climate (or available source of temperature differential) that is suited to its specific characteristics. For air source heat pumps, this usually discourages climates with a significant number of sub-freezing days in a row (unless using auxiliary heating mode is not a lot more expensive than the other heat sources that are available to that location.)
- In heating mode, the air coming from the vents is often not as hot as that from a furnace (unless the auxiliary heater is on). Some people cannot get accustomed to this fact.
- In defrost mode, some heat pumps blow cooler air from the vents before returning to heating temperature air.
- Because of their cooler heat source, heat pumps cycle on and off slower –thus run longer—than a gas furnace.
- For those who live in areas of heavy snow build up, you must make or find a safe way to ensure that snow and ice do not pile up around the outdoor components.
- New equipment installations, particularly ground source heat pumps, tend to cost more than the equivalent central air conditioning and furnace system.
- Due to having to ice melting for coils and other equipment particular to their design, some types of heat pumps have higher maintenance costs than other types of heating and air conditioning systems. (While researching the topic, we noted that advocates of different types of heat pumps, such as air source vs. ground source units, each claim their type of systems to have lower maintenance costs).
Brief History of Heat Pumps
Since heat pumps work like a refrigerator in reverse, the concept has been around a long time. Here are some of the more important dates and events:
1824: Sadi Carnot, an officer in the French army, described the concept for the heat pump.
1852: Irish Professor William Thomson (a.k.a. Lord Kelvin) proposed what was later named “heat pump”. Thompson called it the “heat multiplier”.
1920s: Krauss and later Morley expanded on Thomson’s work.
1927: The first patent for this device was awarded to T. G. N. Haldane, an English inventor. He had built an experimental version of the heat pump for indoor heating and hot water for his house in Scotland.
1930: The first commercial sized heat pump was in Los Angeles. The Southern California Edison Company used heat from refrigerating equipment for heating their offices.
Heat Pump Maintenance
There are sometimes higher maintenance costs with heat pumps. For example, the compressor in a heat pump works during cooling and heating, as opposed to during A/C only on a split furnace and air conditioning system. As described in the section on air conditioning maintenance yearly service agreements is usually a good value. For those elements in common with air conditioning, maintenance would be similar: filter changes per the manufacturers directions, checking the refrigerant levels, checking mechanicals such as fans, making sure electrical connections are working properly, cleaning of coils.
Heat Pump Repairs
If you are new to heat pumps, there are a few characteristics you should know about. These are normal for many heat pumps but may be different than you are accustomed to. So these don’t get confused with the need for repair, new heat pump owners should become familiar with them:
- The air coming from the vents is cooler than air from gas furnaces and other types of central heating
- Because of this, the run cycles are longer than you may be accustomed to
- In colder damp weather, a moderate coating of frost sometimes builds up on outdoor equipment.
- Many heat pumps have a defrost cycle for the outside equipment during heating mode. As the frost melts, his can produce steam from the coils. A moderate amount of steam or fog can be normal. Be sure you can differentiate a normal amount of steam or fog from any smoke, because the latter would not be normal.
When it does come time for repair, most heating and air conditioning contractors can service all brands of heat pumps.
Ductless Heat Pumps
Some situations call for heat pumps that do not require ductwork. Examples of suitable applications include one-room additions, offices, or garage apartments, and in a commercial setting, motel rooms. Although the equipment costs more than a portable space heater and needs professional installation, fixed ductless heat pumps offer some distinct advantages over portable space heaters. Because of their design, ductless heaters:
- are safer than unvented gas portable space heaters
- are more efficient that electric portable (“strip” heat source only) space heaters, depending on the climate.
- provide heating and air conditioning with one unit
To fill these needs, ductless heat pumps have options as to where the equipment has to be located, what equipment is inside the room or outside the wall, what equipment has to penetrate the wall, and more.
Except for the absence of ductwork, the main components of the ductless heat pumps work very much like the other units we described. Because of the variables and technical nature of the equipment, it needs professional installation. The same contractors and companies that sell the ducted variety usually stock or can order ductless equipment.
Some of the manufacturers of ductless fixed heat pumps include Mitsubishi, Sanyo, Fujitsu, Carrier, Panasonic, Friedrich, and Goodman.
Decisions On Purchasing A Heat Pump
The locally available utilities (natural or propane gas), the climate you live in, and the temperature differential available from the air, ground, or water, will help determine if a heat pump is right for you. Your contractor will help with this decision, along with the load calculation and the best heat pump features for your situation. For these warmer climates, air conditioning efficiency (SEER) is more important than the heating efficiency (HSPF) of any heat pump you are considering. If you live in a colder climate, this ratio could be reversed. There you would shop for the highest HSPF you can practically afford. Often the efficiencies of the two sides will be matched.