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|HVAC & Water
Home Heating > Heat Pumps & Furnaces
Introducing Heat Pumps
Heat Pump Types
|The two main types of heating systems used to provide consistent, even heating to all the various rooms and interior spaces of a home are furnaces and heat pumps. Understanding how these systems work and the differences and similarities between them is mandatory before you can make an informed decision regarding the purchase of a new system for your home. We provide an easy-to-follow guide below. It is written to be accessible to anyone, regardless of your level of knowledge.|
Heating systems that warm the entire home are referred to as central heating because heat is generated by a central system and then transported throughout the home, most commonly via a network of ducts. This is different from portable heaters, also known as space heaters, which provide localized heating to a single room and do not utilize ducts.
However, central heating systems are also significantly more expensive than portable heaters and use more energy. For small apartments and highly budget conscious consumers, portable heaters can provide a more affordable option. In addition, portable heaters can make a lot of sense for spaces which are only used occasionally, such as garages, dens, basements, and outdoor sheds. Heating such areas with central heating when they are not often utilized is a waste of money and energy.
The two most common types of central heating systems are furnaces and heat pumps. Although furnaces remain the most popular residential heating technology, heat pumps have been making significant inroads, largely due to their greater energy efficiency and ability to provide not only heating, but also cooling. While we explore furnaces and heat pumps in greater detail later in the text, their basic difference has to do with how they produce heat.
A furnace creates heat via a burning process that is used to warm the air and then distribute this warmed air throughout the home. By contrast, a heat pump creates heat by exchanging cool air in the home with warmer air in an outside location. Just as a refrigerator creates cold air by capturing the warm air inside the compartment and dumping it into the kitchen, so does a heat pump create warm air by capturing the cold air in the rooms of the house and dumping it outside of the home.
Central heaters work on the principle of utilizing a power source to run a process that increases the temperature of the surrounding air. However, both the power sources utilized and the heating processes used vary among heating systems. Elsewhere on the site, we explore the full range of available power sources and primary heating processes. In this section, we explain how furnaces and heat pumps work on a basic, fundamental level. That will provide a foundation for the discussion of the advantages and disadvantages of different power sources and heating processes which can be found in other sections of this site.
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A furnace is a major central heating appliance that is permanently installed inside the home with the purpose of providing heat to either all, or a majority of the interior spaces. The modern furnace is a standalone unit that does not require a chimney for its operation. A furnace warms the air in a home by using a burning process and extracting heat from the exhaust gases.
Generally, household furnaces are classified as either “condensing” or “non-condensing”, depending on how efficient they are in capturing heat from the exhaust gases. Furnaces with efficiencies of 90% or more are known as condensing furnaces because they are so efficient in extracting heat that they cause the water vapor inside the exhaust to condense. These furnaces can provide significant energy savings relative to more traditional non-condensing units.
A furnace can be broken down into three sub-systems: heat generation, heat distribution, and operational controls. The heat generation components typically include the burners, heat exchanger, draft inducer, and vents. The burners provide a flame that is then drawn into the heat exchanger as a result of pressure created by the draft inducer. In turn, the flame causes a combustion reaction that produces hot gases. As they pass through, heat is transferred from the hot gases to the heat exchanger and used to warm the air in the home. As a result of this process, the gases themselves cool and are then expelled through the vents. When hearing the word “furnace”, most people think of a large metallic box in the basement. This is the heat generation part of the furnace.
However, generating heat is of limited use unless it can then be effectively distributed to the individual interior spaces of the home. This is accomplished via the heat distribution sub-system of a furnace. Most typically, the heat distribution sub-system is comprised of a network of air ducts which are tubes made of either metal or plastic. A fan blows warmed air into supply ducts which bring it into the individual rooms of a home. Meanwhile, cool air is pulled into return ducts and circulated back to the furnace. After passing through the heat exchanger, the cool air becomes warm air and is pushed out through the supply ducts, re-circulating back into the rooms. This cycle continues until all of the air in the home is warmed to the necessary temperature. Such a heat distribution system is known as an “air convection” system.
Finally, there has to be a way to control the operation of the furnace’s heat generation and heat distribution systems. Most people are familiar with the thermostat, which is the external control that allows the user to set the minimum desired room temperature. In addition to the thermostat, the furnace has a number of internal controls, including the gas valve, igniter, ignition control, flame sensor, blower control, limit control, and flame rollout switch.
These are actually not as complicated as their names may make them seem. The gas valve controls the flow of gas, and opens when the thermostat instructs the furnace to begin operation. When the gas flows into the burners, it is ignited by the igniter. The ignition control is a simple circuit board that uses the flame sensor to determine whether a flame is present. If there is no flame, then the ignition control instructs the gas valve to shut off the flow of gas. Once the burners are operating, the blower control will turn on the blower motor and begin the process of blowing air through the heat exchanger. In case the furnace overheats, the limit control is activated and instructs the ignition control to stop the flow of gas. Similarly, in case the flame is not being drawn into the heat exchanger, the flame rollout switch will cause the gas to be shut off. These safety features are important, as an overheating furnace or improperly controlled flame can become potential fire hazards.
Generally, the different components of a furnace-based central heating system can be repaired and replaced independently of one another. For example, a new furnace (heat generation) can be installed utilizing old ductwork (heat distribution). A malfunctioning thermostat (operational controls) can be replaced without having to replace either the furnace (heat generation) or the ductwork (heat distribution). The most expensive components to replace or repair are generally the burners and the heat exchanger. In some cases, it may make more financial sense to replace the entire furnace unit rather than repair these components.
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There are two main types of furnaces, categorized by mode of heat distribution. The two types are: central air furnace systems and water- and steam-based hydronic systems. Central air furnaces are the most popular residential heating systems in North America and they work by delivering heated air from the furnace to every room in the home via a network of supply and return ducts. As mentioned earlier, this heat delivery mechanism is known as air convection.
Older furnaces used a gravity-feed system, in which the natural tendency of warm air to rise and cool air to fall was used to facilitate the exchange of heated and cool air. Newer furnaces tend to use a forced-air system, in which a fan pushes the warm air through a separate set of ducts than are used to return the cool air. This is a faster and more efficient way to heat the same space, and can support much thinner, longer ducts. In both types of systems, colder air is pulled into the furnace and heated air is pushed out from the furnace into the rooms and mixed in with the ambient air until the overall air temperature in each room is the same as the thermostat setting.
An alternative to air convection is provided by furnaces which use either steam or hot water as a heat distribution mechanism. Often called “boilers” and referred to within the industry as hydronic boilers, these furnace types supply either steam or water to radiators, convectors or pipes, depending on the specific setup. Most typically, the conduits carrying steam or hot water are embedded in the walls or ceilings of the interior spaces of the house. However, in some instances radiant heat is supplied through pipes that are inlaid in a concrete slab floor.
One benefit of a boiler versus an air convection furnace is that it can provide hot water to the faucets, showers, dishwashers, washing machines, and other appliances without requiring a separate water heater. However, this feature has a downside as well, which is that if there is a breakdown in the boiler’s operation, it will affect not only the heating of the home, but the availability of hot water as well.
Traditionally, hydronic boilers have been far more common in European homes than in North American homes. However, they are becoming increasingly more popular in North America because they offer a number of distinct advantages in addition to doing the double duty of warming the air and heating the water. Hydronic boilers are more efficient than air convection furnaces, leading to lower utility bills. The piping of a boiler-based system takes up less space than the ductwork of a forced-air system. Furthermore, hydronic boilers tend to provide a quieter, more even, and less volatile heating experience, as well as eliminate the risk of pollutants being introduced into the air if there is a malfunction in the furnace.
However, hydronic boilers are not necessarily better than forced-air furnaces in every respect. For example, an air convection furnace makes it easier to install a central air conditioning system. By adding a cooling coil at the exhaust of the furnace, the same network of ducts can be used to distribute cool air in the summer that is used to distribute warm air in the winter. Similarly, it is much easier to install air humidifiers and air purifiers with an existing air convection system, as they can be integrated with the current ductwork and air circulation systems. Additionally, forced-air furnaces are less expensive to purchase and install than hydronic systems and, unlike those systems, have no risk of water leakage. Finally, air convection systems remain the most popular and, by extension, most readily available systems in the North American market.
Generally, if a home already has one type of system or another, it will make sense to stay with the same type of system and simply repair and upgrade it as necessary. In most cases, the only time the question of whether to choose a forced-air furnace or a hydronic system is likely to arise will be with new construction or an older home that has no permanent heating system.
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Introducing Heat Pumps
A heat pump is a major appliance that moves heat from one location, known as the “source”, to another location, known as the “sink”. The net effect of this process is to exchange hot air for cold air. Because a heat pump can be used to pump heat in either direction, it can generally be used not only to heat, but also to cool an interior space. In this way, a heat pump can perform the work of both a furnace and an air conditioner.
The way a heat pump works is much the same as a refrigerator or an air conditioner. The difference is that unlike a refrigerator or an air conditioner, the direction of a heat pump’s operation can be reversed. The refrigerator cannot suddenly reverse direction and begin to cool the kitchen instead of cooling the inside compartment. Similarly, an air conditioner cannot be used to heat the room instead of cooling it. However, a heat pump can be made to either cool or heat.
The heat pump transfers heat via a three-step process that involves an evaporator, a condenser, and a compressor. Once again, the process is not as complicated as these terms may suggest. When the heat pump is in heating mode, the evaporator function takes place in the heat pump coils placed outside of the home. Heat is absorbed from the outside air as a result of an evaporation process. This heat is captured into a liquid called a refrigerant flowing through the evaporator coils. Once it picks up this heat, the refrigerant is compressed in the compressor and pushed into the condenser, where the heat is released and used to warm the air inside the home.
By contrast, when the heat pump is in cooling mode, the evaporator function takes place in the heat pump coils placed inside the home. Once again, heat is absorbed as a result of evaporation, but this time from the inside air. The refrigerant flows in the opposite direction, taking the heat into the condenser and the out into the outdoor coils, releasing it into the outside air.
At its most basic, a heat pump works by using evaporation to remove heat from the air and then utilizing condensation to release heat back into the air. When the evaporation takes place outside, this brings heat into the home and warms the interior spaces. When the evaporation takes place inside, this takes heat out of the home and cools the interior spaces. Since a heat pump can switch the evaporation and condensation functions back and forth between the indoors and the outdoors, it can either cool or heat, depending on the need.
Given that a heat pump warms an interior space by pulling heat from the outside air and bringing it inside the home, a natural question to ask is how exactly this works during the winter, when the outside air is cold. As it turns out, even at 32 degrees Fahrenheit, which is the temperature at which water turns to ice, the outside air still has heat energy. Consequently, a heat pump will work to warm the home in all but the coldest climates.
In terms of distributing warm air to each of the rooms of the home, a heat pump typically uses an air handler unit which blows the warm air through a set of supplier ducts. Thus, in terms of heat distribution, the heat pump is not very different from a furnace. The primary difference has to do with how the heat is generated, rather than how it is distributed.
Heat Pump Types
As with furnaces, it is possible to subdivide the vast majority of residential heat pump systems into two categories. However, whereas furnace systems were categorized based on their heat distribution mechanisms, heat pump systems are categorized based on whether they use the air or the ground as their heat source. As explained earlier, a heat pump operates by exchanging the cool air in the home with warmer air from an outside source. Thus, the two main types of heat pumps are air-source, which extract heat from the outside air, and geothermal, which extract heat from the earth.
While an air-source heat pump extracts heat from the outside air, a geothermal heat pump may be used to extract heat from a variety of ground sources, including a soil layer, a rock formation, or a body of water. Much of the heat extracted from the ground by a geothermal heat pump comes from stored solar energy that is found throughout the planet’s surface. For this reason, the term “geothermal” may be somewhat misleading. A geothermal heat pump is different from the geothermal energy associated with the high temperatures found deep within the earth. In fact, it may be more intuitive to refer to a geothermal heat pump by its other common name, which is a “ground source” heat pump.
Both an air-source and a geothermal heat pump are commonly used to transfer heat either to the air inside the home, or to a tank of water inside the home. Air-source heat pumps are known as “air-air” or “air-water”, depending on the element to which they transfer heat inside the home. Geothermal heat pumps have even more permutations, as both where they are transferring heat to and where they are transferring heat from can vary, so “ground-air”, “rock-air”, “water-air”, “ground-water”, “rock-water”, and “water-water” are all examples of geothermal heat pump configurations.
Air-source heat pumps are the most common and widely used type of heat pump for heating residential homes, largely owing to the relatively low cost and ease of installing them. However, air-source heat pumps are hamstrung in less temperate climates. Because they use the outside air as a heat source, if the weather outdoors is extremely cold the heat pump has a more difficult time capturing heat and its efficiency therefore drops considerably. In turn, this leads to greater energy usage for the same level of heating and causes utility bills to go up. Air-source heat pumps are still generally more efficient than furnaces at outside temperatures as low as 20 degrees Fahrenheit.
For extremely cold climates, several manufacturers offer specialized air-source heat pumps which are able to provide a better heat output than traditional air-source heat pumps. Relative to conventional air-source heat pumps, the low temperature optimized models can be more efficient than furnaces at temperatures as low as -10 degrees Fahrenheit. However, these models use additional energy to regularly defrost their outdoor components and may still be energy inefficient during the coldest months.
As the temperature of the earth is not as variable as the temperature of the outside air, geothermal heat pumps tend to have even higher energy efficiencies than air-source heat pumps. Unfortunately, this enhanced ability to capture heat year round is offset by the higher purchase price and installation cost of a geothermal heat pump. Installation in particular is more expensive as it requires the digging of outdoor trenches and placement of piping that facilitates heat capture from the earth. Still, homeowners that are willing to pay the higher initial cost of installing a geothermal heat pump will be able to capture significant energy benefits down the line and see substantially reduced utility bills. Moreover, a number of modern geothermal heat pumps facilitate the storing of heat in the ground from the summer months, which can then be tapped in the winter.
With respect to heat distribution, there are heat pumps that will work with either an air convection ductwork system or a hydronic piping system. An air-air air-source heat pump and ground-air, rock-air, and water-air geothermal heat pumps will provide heated air for an air convection distribution system, while an air-water air-source heat pump and ground-water, rock-water, and water-water geothermal heat pumps will provide heated water for a hydronic piping system.
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