Air Conditioner - Typically, equipment that cools the living space by transferring heat from the air inside the building to air outside of the building using a vapor compression cycle.
Heat Pump for Space Heating and Cooling - A heat pump heats the living space in cold weather and cools the living space in hot weather by transferring heat between the living space and the outside air using a vapor compression cycle. A heat pump for space heating and cooling is an air conditioner with a reversing valve.
Reversing Valve - In a heat pump, a reversing valve in the refrigeration loop changes the direction of heat transfer into or out of the living space, in response to a signal from the thermostat.
Heat Exchanger - In heat pumps, there are two heat exchangers: the evaporator and the condensor. The evaporator produces a cooling effect as the refrigerant evaporates. The condensor produces a heating effect as the refrigerant condenses. In a heat pump each heat exchanger serves alternately as the evporator or condensor depending on whether the heat pump is cooling the living space or heating the living space. For space heating and cooling, typically each heat exchanger includes a refrigeration coil and a fan to move air through the refrigeration coil for heat transfer. These fans are called the "evaporator" fan and the "condensor" fan. Typically the bigger the heat exchanger, the more efficient the heat transfer. A variable speed control on the fan also tends to increase efficiency.
Condensor - See "heat exchanger".
Evaporator - See "heat exchanger".
Compressor - In a heat pump or air conditioner, the compressor
increases the pressure of the refrigerant so that it will condense (turn
back into a liquid) at a temperature high enough to be useful for the
application.
Refrigerant - A refrigerant is a substance used in a vapor compression cycle to transfer heat from a cold area to a hot area by evaporating at a low temperature and condensing at a high temperature. There are many refrigerants. The goals used in selecting a refrigerant are the following: appropriate pressure/temperature relationships for the application, low operating pressures (for safety and ease of leakage reduction), high energy efficiency, low cost, low toxicity, low flammability, low global warming potential, low ozone depletion potential. Over time, the range of refrigerants used in new equipment shifts depending on discovery of previously unknown problems, and depending on the evolution of societal goals. As a consequence the refrigerants that were legal in one generation of heat pumps and air conditioners may not be legal in the next generation. Currently, for space heating and cooling, there seems to be no magic bullet; proposed new refrigerants balance tradeoffs rather than meeting all goals. Exception: For hot water heat pumps, use of CO2 as a refrigerant seems very promising, satisfying all goals other than low pressure. This page includes technical descriptions that are not accurate for equipment using CO2 as a refrigerant (also called R-744). The CO2 vapor compression cycle is fascinating. It uses a higher pressure, and different refrigerant phases than simple gas and liquid. The concepts are the same, but the descriptions of what happens in the evaporator, condenser, and expansion valve would be different.
Mini-split - A ductless heat pump for space heating. It comes in two units: the outside unit and the inside unit, connected by refrigerant piping, hence the term "split". Like all ductless heat pumps, it's designed to serve a single room.
Split - A split heat pump comes in two pieces. The compressor, heat exchange and fan are located outside. The other heat exchanger and fan are located inside. Small-diameter refrigeration pipes (and insulation and insulation cladding) go through the wall to connect the outside and inside units.
Unitary - A unitary heat pump combines all parts (heat exchangers, compressor and fans) in one package with one nameplate. If located outside, the hot water pipes (for a heat pump hot water heater or hot water space heat system) or ducts go through the wall into the living space. If located inside the building, the space in which the heat pump is located (such as a garage or basement) will be heated if the heat pump is used for air conditioning, or cooled if the heat pump is used for space heat or water heat.
Ductless - A ductless heat pump uses a small supply fan typically located high on a wall in the living space to transfer heat directly from the refrigerant loop into or out of the living space. A variable refrigerant flow (VRF) system runs refrigeration pipes from one or more outside units to a larger number of inside units within the building. Each inside unit has a thermostat. Ductless heat pumps gain efficiency by lowering the required fan energy.
Ducted - A central heat pump will use a larger, central supply fan to deliver hot or cold air to several rooms through ducts. The system has a single thermostat in one of the rooms served.
Duct - Ducts are passageways for air being moved by a fan from one place to another. For a given air flow rate, the larger the ducts, the quieter the operation, and the less fan energy required.
Latent Energy - Latent energy is released (heating the surroundings) when a substance condenses from a gas to a liquid or transforms from a liquid to a solid. Latent energy is absorbed (cooling the surroundings) when a substance goes from a solid to a liquid or from a liquid to a gas (evaporation). The temperature at which a substance evaporates or condenses depends on the chemistry of the substance itself, and the pressure. The chemistry of a refrigerant is selected in part to obtain the pressure/temperature conditions under which it condenses and evaporates, going back and forth between the gas and liquid state to move heat from cold to hot areas. In day to day life, we are familiar with latent energy when we boil water. At normal atmospheric pressure the temperature of the water goes no higher than 212F, because that's the temperature at which it "evaporates". Once the water gets that hot on the stove, the stove is no longer raising the temperature of the water, it's just converting the water to steam.
Sensible Energy - Sensible energy is easier to understand than latent energy. It's the energy stored as temperature. Electric resistance heat such as a baseboard heats the room using sensible heat. If you swim in cold water or put your hand on a cold surface, you feel the effect of sensible energy.
Combinations of Sensible and Latent energy transfer - When air flows over a cold surface, water will often condense out of the air as the air cools down. This can happen in cooling systems at the cooling coil, adding some latent heat to the cooled air and requiring a drain. For this reason, air conditioners typically dehumidify (latent) the air in the process of cooling it (sensible). In heat pumps condensation may occur in the outside air cooled by the outside coil when the heat pump is in the heating mode. In that case the outside coil can develop ice if the condensed water freezes. In the exhaust air flows through an air-to-air heat recovery during cold weather, condensation may form in the heat exchanger, requiring a drain, and possibly running the risk of forming ice during very cold weather. Energy recovery ventilators try to reduce this problem by using porous membranes to transfer humidity out of the warm air into the cold entering air.
Capacity - The capacity of the heat pump is the amount of heating or cooling it can perform. It will have two capacities: one for heating, the other for cooling. Each is determined by multiple factors, primarily the outside air temperature, but also the cleanliness of the heat exchangers and the filters. Be sure to find out what the heating capacity will be at the cold temperatures typical of your climate. Heat pump capacities fall off at low outside air temperatures, while the amount of heat needed by your building goes up at low outside air temperatures. Heat pumps with variable speed drives on the compressors are better able to handle operation at a range of outside air temperatures, including low outside air temperatures. The capacity may drop off when freeze protection strategies are employed to protect the outside heat exchanger from ice build up in cold weather. Find out the outside air temperature below which freeze protection will be employed. Find out whether your heat pump capacity is being selected to meet your cooling needs or your heating needs, or both. Depending on your climate, these two criteria may produce different selections. If all you need for cooling is a tempering effect, not full cooling down to 70F in the hottest weather, let the contractor know.
Cooling Efficiency - An efficiency is the amount of useful work performed divided by the amount of energy consumed. Heat pumps have two types of efficiency ratings, one for the heating mode, the other for the cooling mode. The two efficiencies are different in part because the energy consumed by fans and compressors generates friction that introduces heat, and motors generate heat, some of which is useful in the heating mode while all of it is a form of inefficiency in the cooling mode. In the cooling mode, there's potential inefficiency if the heat pump dehumidifies the space to the point that it's too dry for comfort. The Energy Star lists of equipment are useful for comparing efficiency ratings. Select a high efficiency to help the environment and lower operating costs.
Heating Efficiency - Heat pumps should have heating efficiencies well above 100%, in contrast to electric resistance heat, which is only 100% efficient. Some of the heat delivered by a heat pump comes from the equipment itself (heated motors, and friction in compressors and fans), but some of the heat is simply transferred out of the cold outside air. Unfortunately there are many units of measure used in the standard ratings, making it difficult sometimes to compare different kinds of systems. Look at the manufacturer's product literature to check the efficiencies at low temperatures typical of your climate. Generally speaking, the heating efficiency drops off as the outside air temperature drops, but this effect is less dramatic with some types of heat pumps than others. When looking for a high efficiency heat pump, look for larger heat exchangers, low noise ratings, and variable speed drives on the fans and compressor. Once the heat pump is installed, check that refrigerant pipe insulation doesn't have gaps, and if outside check that the outside piping insulating has water proof cladding that won't get damaged over time.
Sones - Sones are a measure of noise level. Because all heat pumps contain fans and compressors, They have the potential of being noisy. However, some are noisy and some are quiet. The Energy Star website offers easy comparisons for noise as well as efficiency ratings. Generally the higher quality equipment (durability and energy efficiency) the quieter the operation. Once the equipment is installed, if the system has a noise problem, you may also be able to get the contractor to lower the noise levels by adjusting the fan speed settings. Neighbors may be affected by the noise from outside heat pump units, so take their interests into consideration as well.
Vapor Compression Cycle - A vapor compression cycle uses a compressor and a refrigerant to transfer heat from a cold area to a hot area through use of latent heat transfer by manipulating the refrigerant pressure to be high in the hot area and low in the cold area.
Expansion Valve - In a vapor compression cycle, after turning into a liquid in the condensor, the refrigerant goes through an expansion valve to reduce the pressure to a point that the refrigerant will evaporate (even at a relatively low temperature) in the evaporator.
Ozone Depletion Potential (ODP) - The ozone in the atmosphere reduces the rate of skin cancer by absorbing some of the damaging solar radiation. When an "ozone hole" was discovered over Antarctica, legislation was introduced to eliminate the types of refrigerants that had contributed to the problem. Since then, manufacturers are required to determine and publish the Ozone Depletion Potential (ODP) of each refrigerant so that the legislated ODP limits can be enforced. The rapid and effective response to the discovery of atmospheric ozone depletion is considered one of the major successes in international cooperation around environmental challenges.
Global Warming Potential (GWP) - When global warming became a concern, it was noticed that some refrigerants, particularly some of those that had been developed to lower the ODPs, had much higher global warming potential than CO2. It has also been noticed that some refrigerant systems, such as variable refrigerant flow (VRF) heat pump systems, leak more refrigerant than expected. There is currently legislation in place to phase out high GWP refrigerants, but the characteristics of the replacement refrigerants proposed thus far are not inspiring. (A strong exception is successful use of CO2 itself as a refrigerant for heating domestic hot water.) There are also efforts to reduce refrigerant leakage to the atmosphere. Each of these topics--reduction of leaks and search for replacement refrigerants--is highly complex, and worthy of attention.
Electric heat. There are two common types of electric heat; electric resistance heat, and heat pumps. Sometimes the term "electric heat" is used to mean electric resistance heat, but such usage can lead to ambiguity or confusion.
Electric resistance heat. Electric resistance heat is inexpensive, highly reliable, and very inefficient. It's inexpensive and reliable because it has no moving parts and is very simple. When a current runs through a metal, it heats up the metal. In electrical distribution systems, the wires are sized to avoid excess heat. In electrical resistance heating devices, the design is one that intentionally creates a lot of heat. Examples of electric resistance heat include baseboards, toasters, traditional electric hot water heaters, traditional electric stoves, and electric furnaces.
Payback Period. When an investment is motivated in part or in whole by a desire to save energy, the term "payback period" refers to the length of time it takes to recover the cost of the project through savings on the energy bills. If the cost of the project is 10 times the annual reduction in the energy bills, the payback is 10 years. Low flow hot water plumbing fixtures such as showerheads normally have a payback of less than a year, whereas heat pump hot water heaters normally have a payback of several years. The cost of energy may go up, and the cost of the installation may be offset by utility rebates or federal or state tax incentives, so the calculation can have some twists and turns. If you need to buy a hot water heater anyway, the cost of energy efficiency used in the payback calculation is the difference between the cost of one option and the cost of the other.
7/17/24