Basic concept of Geothermal Heat Pump or Ground Source Heat Exchanger System for Heating, Cooling and providing Hot Water.
Installing Geothermal Heat Pumps
Because of the technical knowledge and equipment needed to properly install the piping, a GHP system installation is not a do-it-yourself project. To find a qualified installer, call your local utility company, the International Ground Source Heat Pump Association or the Geothermal Heat Pump Consortium for their listing of qualified installers in your area. Installers should be certified and experienced. Ask for references, especially for owners of systems that are several years old, and check them.
The ground heat exchanger in a GHP system is made up of a closed or open loop pipe system. Most common is the closed loop, in which high density polyethylene pipe is buried horizontally at 4-6 feet deep or vertically at 100 to 400 feet deep. These pipes are filled with an environmentally friendly antifreeze/water solution that acts as a heat exchanger. In the winter, the fluid in the pipes extracts heat from the earth and carries it into the building. In the summer, the system reverses and takes heat from the building and deposits it to the cooler ground.
The air delivery ductwork distributes the heated or cooled air through the house's duct work, just like conventional systems. The box that contains the indoor coil and fan is sometimes called the air handler because it moves house air through the heat pump for heating or cooling. The air handler contains a large blower and a filter just like conventional air conditioners.
Most geothermal heat pumps are automatically covered under your homeowner's insurance policy. Contact your insurance provider to find out what its policy is. Even if your provider will cover your system, it is best to inform them in writing that you own a new system.
The ground heat exchanger in a GHP system is made up of a closed or open loop pipe system. Most common is the closed loop, in which high density polyethylene pipe is buried horizontally at 4-6 feet deep or vertically at 100 to 400 feet deep. These pipes are filled with an environmentally friendly antifreeze/water solution that acts as a heat exchanger. In the winter, the fluid in the pipes extracts heat from the earth and carries it into the building. In the summer, the system reverses and takes heat from the building and deposits it to the cooler ground.
The air delivery ductwork distributes the heated or cooled air through the house's duct work, just like conventional systems. The box that contains the indoor coil and fan is sometimes called the air handler because it moves house air through the heat pump for heating or cooling. The air handler contains a large blower and a filter just like conventional air conditioners.
Most geothermal heat pumps are automatically covered under your homeowner's insurance policy. Contact your insurance provider to find out what its policy is. Even if your provider will cover your system, it is best to inform them in writing that you own a new system.
Evaluating Your Site for a Geothermal Heat Pump
Because shallow ground temperatures are relatively constant throughout the United States, geothermal heat pumps (GHPs) can be effectively used almost anywhere. However, the specific geological, hydrological, and spatial characteristics of your land will help your local system supplier/installer determine the best type of ground loop for your site:
Geology
Factors such as the composition and properties of your soil and rock (which can affect heat transfer rates) require consideration when designing a ground loop. For example, soil with good heat transfer properties requires less piping to gather a certain amount of heat than soil with poor heat transfer properties. The amount of soil available contributes to system design as well — system suppliers in areas with extensive hard rock or soil too shallow to trench may install vertical ground loops instead of horizontal loops.
Hydrology
Ground or surface water availability also plays a part in deciding what type of ground loop to use. Depending on factors such as depth, volume, and water quality, bodies of surface water can be used as a source of water for an open-loop system, or as a repository for coils of piping in a closed-loop system. Ground water can also be used as a source for open-loop systems, provided the water quality is suitable and all ground water discharge regulations are met.
Before you purchase an open-loop system, you will want to be sure your system supplier/installer has fully investigated your site's hydrology, so you can avoid potential problems such as aquifer depletion and groundwater contamination. Antifreeze fluids circulated through closed-loop systems generally pose little to no environmental hazard.
Land Availability
The amount and layout of your land, your landscaping, and the location of underground utilities or sprinkler systems also contribute to your system design. Horizontal ground loops (generally the most economical) are typically used for newly constructed buildings with sufficient land. Vertical installations or more compact horizontal "Slinky™" installations are often used for existing buildings because they minimize the disturbance to the landscape.
Geology
Factors such as the composition and properties of your soil and rock (which can affect heat transfer rates) require consideration when designing a ground loop. For example, soil with good heat transfer properties requires less piping to gather a certain amount of heat than soil with poor heat transfer properties. The amount of soil available contributes to system design as well — system suppliers in areas with extensive hard rock or soil too shallow to trench may install vertical ground loops instead of horizontal loops.
Hydrology
Ground or surface water availability also plays a part in deciding what type of ground loop to use. Depending on factors such as depth, volume, and water quality, bodies of surface water can be used as a source of water for an open-loop system, or as a repository for coils of piping in a closed-loop system. Ground water can also be used as a source for open-loop systems, provided the water quality is suitable and all ground water discharge regulations are met.
Before you purchase an open-loop system, you will want to be sure your system supplier/installer has fully investigated your site's hydrology, so you can avoid potential problems such as aquifer depletion and groundwater contamination. Antifreeze fluids circulated through closed-loop systems generally pose little to no environmental hazard.
Land Availability
The amount and layout of your land, your landscaping, and the location of underground utilities or sprinkler systems also contribute to your system design. Horizontal ground loops (generally the most economical) are typically used for newly constructed buildings with sufficient land. Vertical installations or more compact horizontal "Slinky™" installations are often used for existing buildings because they minimize the disturbance to the landscape.
Economics of Geothermal Heat Pumps
Geothermal heat pumps save money in operating and maintenance costs. While the initial purchase price of a residential GHP system is often higher than that of a comparable gas-fired furnace and central air-conditioning system, it is more efficient, thereby saving money every month. For further savings, GHPs equipped with a device called a "desuperheater" can heat the household water. In the summer cooling period, the heat that is taken from the house is used to heat the water for free. In the winter, water heating costs are reduced by about half.
On average, a geothermal heat pump system costs about $2,500 per ton of capacity, or roughly $7,500 for a 3-ton unit (a typical residential size). ). A system using horizontal ground loops will generally cost less than a system with vertical loops. In comparison, other systems would cost about $4,000 with air conditioning.
Although initially more expensive to install than conventional systems, properly sized and installed GHPs deliver more energy per unit consumed than conventional systems.
And since geothermal heat pumps are generally more efficient, they are less expensive to operate and maintain — typical annual energy savings range from 30% to 60%. Depending on factors such as climate, soil conditions, the system features you choose, and available financing and incentives, you may even recoup your initial investment in two to ten years through lower utility bills.
But when included in a mortgage, your GHP will have a positive cash flow from the beginning. For example, say that the extra $3,500 will add $30 per month to each mortgage payment. The energy cost savings will easily exceed that added mortgage amount over the course of each year.
On a retrofit, the GHP's high efficiency typically means much lower utility bills, allowing the investment to be recouped in two to ten years. It may also be possible to include the purchase of a GHP system in an "energy-efficient mortgage" that would cover this and other energy-saving improvements to the home. Banks and mortgage companies can provide more information on these loans.
There may be a number of special financing options and incentives available to help offset the cost of adding a geothermal heat pump (GHP) to your home. These provisions are available from federal, state, and local governments; power providers; and banks or mortgage companies that offer energy-efficient mortgage loans for energy-saving home improvements. Be sure the system you're interested in qualifies for available incentives before you make your final purchase.
To find out more about financing and incentives that are available to you, visit the Database of State Incentives for Renewable Energy (DSIRE) Web site. The site is frequently updated with the latest incentives. You should also check with your electric utility and ask if they offer any rebates, financing, or special electric rate programs.
On average, a geothermal heat pump system costs about $2,500 per ton of capacity, or roughly $7,500 for a 3-ton unit (a typical residential size). ). A system using horizontal ground loops will generally cost less than a system with vertical loops. In comparison, other systems would cost about $4,000 with air conditioning.
Although initially more expensive to install than conventional systems, properly sized and installed GHPs deliver more energy per unit consumed than conventional systems.
And since geothermal heat pumps are generally more efficient, they are less expensive to operate and maintain — typical annual energy savings range from 30% to 60%. Depending on factors such as climate, soil conditions, the system features you choose, and available financing and incentives, you may even recoup your initial investment in two to ten years through lower utility bills.
But when included in a mortgage, your GHP will have a positive cash flow from the beginning. For example, say that the extra $3,500 will add $30 per month to each mortgage payment. The energy cost savings will easily exceed that added mortgage amount over the course of each year.
On a retrofit, the GHP's high efficiency typically means much lower utility bills, allowing the investment to be recouped in two to ten years. It may also be possible to include the purchase of a GHP system in an "energy-efficient mortgage" that would cover this and other energy-saving improvements to the home. Banks and mortgage companies can provide more information on these loans.
There may be a number of special financing options and incentives available to help offset the cost of adding a geothermal heat pump (GHP) to your home. These provisions are available from federal, state, and local governments; power providers; and banks or mortgage companies that offer energy-efficient mortgage loans for energy-saving home improvements. Be sure the system you're interested in qualifies for available incentives before you make your final purchase.
To find out more about financing and incentives that are available to you, visit the Database of State Incentives for Renewable Energy (DSIRE) Web site. The site is frequently updated with the latest incentives. You should also check with your electric utility and ask if they offer any rebates, financing, or special electric rate programs.
Heating and Cooling Efficiency of Geothermal Heat Pumps
The heating efficiency of ground-source and water-source heat pumps is indicated by their coefficient of performance (COP), which is the ratio of heat provided in Btu per Btu of energy input. Their cooling efficiency is indicated by the Energy Efficiency Ratio (EER), which is the ratio of the heat removed (in Btu per hour) to the electricity required (in watts) to run the unit. Look for the ENERGY STAR® label, which indicates a heating COP of 2.8 or greater and an EER of 13 or greater.
Manufacturers of high-efficiency geothermal heat pumps voluntarily use the EPA ENERGY STAR label on qualifying equipment and related product literature. If you are purchasing a geothermal heat pump and uncertain whether it meets ENERGY STAR qualifications, ask for an efficiency rating of at least 2.8 COP or 13 EER.
Many geothermal heat pump systems carry the U.S. Department of Energy (DOE) and EPA ENERGY STAR label. Ask your contractor about special financing or incentives for purchasing energy efficient products, including ENERGY STAR qualified products.
Manufacturers of high-efficiency geothermal heat pumps voluntarily use the EPA ENERGY STAR label on qualifying equipment and related product literature. If you are purchasing a geothermal heat pump and uncertain whether it meets ENERGY STAR qualifications, ask for an efficiency rating of at least 2.8 COP or 13 EER.
Many geothermal heat pump systems carry the U.S. Department of Energy (DOE) and EPA ENERGY STAR label. Ask your contractor about special financing or incentives for purchasing energy efficient products, including ENERGY STAR qualified products.
Significant savings with geothermal heat pumps
When it comes to heating and cooling homes, schools and even penguins at Woodland Park Zoo, a growing number of people are turning to a source as old as the Earth itself: geothermal heat.
"It seems like the floodgates have opened," said Gerard Maloney, owner of Earthheat, a Duvall company that has been installing commercial and residential geothermal systems for more than 10 years. "When [gasoline] hit $4.50 a gallon, the phone started ringing off the hook."
Significant savings
The main attraction of geothermal heat is the savings.
Maloney estimates the cost of running a geothermal system in a 3,000-square-foot house would be about $700 a year, with an equivalent natural-gas system costing as much as $6,000 a year.
But Dave Sjoding, a renewable-energy specialist at Washington State University, said measuring the efficiency of geothermal systems varies because comparisons are affected by differences between types of high-efficiency furnaces, as well as the costs of competing fuels.
One analysis used by Maloney, however, estimates a 70 percent drop in home energy costs.
The Department of Energy estimates ground-source heat pumps use 25 to 50 percent less electricity than conventional heating or cooling systems.
"We're really excited about this," said Monica Lake, project manager for the Woodland Park Zoo's penguin exhibit.
She said the zoo is using a $65,000 grant from Seattle City Light to help fund the $210,000 project. The zoo chose the geothermal system because it requires nearly zero long-term maintenance and because of the energy savings.
There is a catch. A geothermal system costs more to install. Maloney believes that may be the reason why geothermal systems haven't become widely popular.
"Our costs are usually about 50 percent more than conventional equipment," said Maloney, comparing a geothermal system with a high-efficiency furnace, hot-water heater and air-conditioner installation. "That 50 percent you'll generally see back in about five years."
He estimates the cost of providing a conventional natural-gas system, including a furnace, air conditioner and water heater, might be $10,000. A ground-source geothermal system probably would cost $15,000 to $20,000, he said.
Those high upfront costs also are cited as a major barrier by David Clement, Seattle City Light director of resource planning. Additional concerns include the space needed to install underground piping and a general ignorance that the systems even exist.
"Another thing that puts a damper on it is we enjoy relatively low-cost electricity," he said. "It's also partly a function of how they [potential buyers] just haven't seen much of it."
If I want to know more about geothermal heat - 50+ FAQs about Geothermal Heat Pumps
"It seems like the floodgates have opened," said Gerard Maloney, owner of Earthheat, a Duvall company that has been installing commercial and residential geothermal systems for more than 10 years. "When [gasoline] hit $4.50 a gallon, the phone started ringing off the hook."
Significant savings
The main attraction of geothermal heat is the savings.
Maloney estimates the cost of running a geothermal system in a 3,000-square-foot house would be about $700 a year, with an equivalent natural-gas system costing as much as $6,000 a year.
But Dave Sjoding, a renewable-energy specialist at Washington State University, said measuring the efficiency of geothermal systems varies because comparisons are affected by differences between types of high-efficiency furnaces, as well as the costs of competing fuels.
One analysis used by Maloney, however, estimates a 70 percent drop in home energy costs.
The Department of Energy estimates ground-source heat pumps use 25 to 50 percent less electricity than conventional heating or cooling systems.
"We're really excited about this," said Monica Lake, project manager for the Woodland Park Zoo's penguin exhibit.
She said the zoo is using a $65,000 grant from Seattle City Light to help fund the $210,000 project. The zoo chose the geothermal system because it requires nearly zero long-term maintenance and because of the energy savings.
There is a catch. A geothermal system costs more to install. Maloney believes that may be the reason why geothermal systems haven't become widely popular.
"Our costs are usually about 50 percent more than conventional equipment," said Maloney, comparing a geothermal system with a high-efficiency furnace, hot-water heater and air-conditioner installation. "That 50 percent you'll generally see back in about five years."
He estimates the cost of providing a conventional natural-gas system, including a furnace, air conditioner and water heater, might be $10,000. A ground-source geothermal system probably would cost $15,000 to $20,000, he said.
Those high upfront costs also are cited as a major barrier by David Clement, Seattle City Light director of resource planning. Additional concerns include the space needed to install underground piping and a general ignorance that the systems even exist.
"Another thing that puts a damper on it is we enjoy relatively low-cost electricity," he said. "It's also partly a function of how they [potential buyers] just haven't seen much of it."
If I want to know more about geothermal heat - 50+ FAQs about Geothermal Heat Pumps
Geothermal Heat Pump Installation Options
The ground heat exchanger in a GHP system is made up of a closed or open loop pipe system. Most common is the closed loop, in which high density polyethylene pipe is buried horizontally at 4-6 feet deep or vertically at 100 to 400 feet deep. These pipes are filled with an environmentally friendly antifreeze/water solution that acts as a heat exchanger.
There are four basic types of ground loop systems. Three of these — horizontal, vertical, and pond/lake — are closed-loop systems. The fourth type of system is the open-loop option. Which one of these is best depends on the climate, soil conditions, available land, and local installation costs at the site. All of these approaches can be used for residential and commercial building applications.
The installation of a GHP system is not for the do-it-yourselfer. Contact local utilities for references on licensed and experienced installers. In addition, many states have Heat Pump Councils which may provide additional referrals.
There are four basic types of ground loop systems. Three of these — horizontal, vertical, and pond/lake — are closed-loop systems. The fourth type of system is the open-loop option. Which one of these is best depends on the climate, soil conditions, available land, and local installation costs at the site. All of these approaches can be used for residential and commercial building applications.
The installation of a GHP system is not for the do-it-yourselfer. Contact local utilities for references on licensed and experienced installers. In addition, many states have Heat Pump Councils which may provide additional referrals.
GHP Open-Loop Systems
This type of GHPS uses well(s) or surface body water as the heat exchange fluid that circulates directly through the Geothermal heat pump system (GHP). Once it has circulated through the system, the water returns to the ground through the well, a recharge well, or surface discharge. This option is obviously practical only where there is an adequate supply of relatively clean water, and all local codes and regulations regarding groundwater discharge are met.
Vertical Closed-Loop Installation
Large commercial buildings and schools often use vertical systems because the land area required for horizontal loops would be prohibitive. Vertical loops are also used
where the soil is too shallow for trenching, and they minimize the disturbance to existing landscaping. For a vertical system, holes (approximately four inches in diameter) are drilled about 20 feet apart and 100 to 400 feet deep. Into these holes go two pipes that are connected at the bottom with a U-bend to form a loop. The vertical loops are connected with horizontal pipe (i.e., manifold), placed in trenches, and connected to the heat pump in the building.
Horizontal closed-loop type of installation
Horizontal closed-loop type of installation is generally most cost-effective for residential installations, particularly for new construction where sufficient land is available. It requires trenches at least four feet deep. The most common layouts either use two pipes, one buried at six feet, and the other at four feet, or two pipes placed side-by-side at five feet in the ground in a two-foot wide trench. Or, the Slinky method (shown) of looping pipe allows more pipe in a shorter trench, which cuts down on installation costs and makes horizontal installation possible in areas it would not be with conventional horizontal applications.
Pond/Lake ground loop systems
If the site has an adequate water body, this may be the lowest cost option. Asupply line pipe is run underground from the building to the water and coiled into circles at least eight feet under the surface to prevent freezing. The coils should only be placed in a water source that meets minimum volume, depth, and quality criteria.
Heat pump glossary
Heat pump glossary of Terms
Since the following pages will be devoted to the understanding of heat pumps and their applications it will be helpful to become familiar with the following terms:
Heat Pump--A heat pump is any device that moves heat from one place toanother.
Heat Source--The area where heat is taken from. (Water, air, etc.)
Heat Sink--The area where heat is deposited. (Inside a home, etc.)
Evaporator--The heat absorbing mechanism in a heat pump.
Condenser--The heat rejecting mechanism in a heat pump.
COP--The coefficient of performance of a heating system is a ratio of the heat we get out divided by the heat we put in electrically.
SCOP--The SEASONAL COEFFICIENT OF PERFORMANCE is the average COP over the entire heating season.
EER -- The ENERGY EFFICIENCY RATIO is the ratio of Btu's of cooling dividedby total watts used.
SEER -- Average EER over entire cooling season.
Degree day--the number of degrees that the mean temperature for that day is below 65° F. (eg. mean temp. of 40 for the day--65-40=25 degree days)
CFM--Cubic feet per minute of air flow.
KWH--Kilowatt hours
BTU--British thermal units (method of measuring a quantity of heat). The amount of heat required to raise one pound of water 1° F.
BTU-- WATTS * 3.413
1 WATT = 3.413 BTU'S
Since the following pages will be devoted to the understanding of heat pumps and their applications it will be helpful to become familiar with the following terms:
Heat Pump--A heat pump is any device that moves heat from one place toanother.
Heat Source--The area where heat is taken from. (Water, air, etc.)
Heat Sink--The area where heat is deposited. (Inside a home, etc.)
Evaporator--The heat absorbing mechanism in a heat pump.
Condenser--The heat rejecting mechanism in a heat pump.
COP--The coefficient of performance of a heating system is a ratio of the heat we get out divided by the heat we put in electrically.
SCOP--The SEASONAL COEFFICIENT OF PERFORMANCE is the average COP over the entire heating season.
EER -- The ENERGY EFFICIENCY RATIO is the ratio of Btu's of cooling dividedby total watts used.
SEER -- Average EER over entire cooling season.
Degree day--the number of degrees that the mean temperature for that day is below 65° F. (eg. mean temp. of 40 for the day--65-40=25 degree days)
CFM--Cubic feet per minute of air flow.
KWH--Kilowatt hours
BTU--British thermal units (method of measuring a quantity of heat). The amount of heat required to raise one pound of water 1° F.
BTU-- WATTS * 3.413
1 WATT = 3.413 BTU'S
Considering a HEAT PUMP?
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.
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.
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