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Heat Pump Basics: Part 3

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Published
July 9, 2009
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Are all heat pump systems the same?

Definitely not! There are many different types of heat pumps and are classified by the jobs they are to do as well as how they are designed to do these jobs. For example, heat pumps can be used to heat or cool water, air or other fluids.

For heat pumps to heat a substance, there must be a source of this heat, or heat source. This heat source can be water, air, or even the Earth. When water is used, it can come from a cooling tower or it can come from an underground well. When we use the Earth as a heat source, the heat of the Earth itself is used as a heat source when the system is operating in the heating mode and as the heat sink when the system is operating in the cooling mode.


What is a Geothermal Heat Pump?

The term geothermal is made up of two parts. The first portion of the word is “geo” and refers to the Earth and is the root of words such as geography, meaning the study of the Earth, and geology, which is the study of the origin, history and structure of the Earth., The rest of the word is “thermal”, which is related to the production, use, study or transfer of heat. So, getting away from the good old American Heritage College Dictionary, we can quickly conclude that the term geothermal refers to the heat that is contained in the Earth.

When we use the term geothermal heat pump, we are referring to heat pump systems that utilize the Earth as both a heat source and a heat sink depending on the mode in which the heat pump is operating. Geothermal heat pumps can use either the heat in the Earth/soil itself or it can use the water that is located underneath the Earth’s surface.


How are heat pumps classified?

Heat pumps are classified based on the fluid used for the heat source while the heat pump is operating in the heating mode. For example, a heat pump that uses air as its heat source when operating in the heating mode is referred to as an air-source heat pump. Also, a heat pumps system that uses water as its heat source when operating in the heating mode is classified as a water-source heat pump.


Can you give me some examples of an air source heat pump?

As just mentioned, air source heat pumps use air as the heat source when the system is operating in the heating mode. An other important thing to consider is the fluid that is ultimately being treated. For example, we can use the heat in the air to heat air, or water. When we use the heat in the air to heat air, we call that heat pump an air-to-air heat pump. When we use the heat in the air to heat water, we call that heat pump an air-to-water heat pump. The first part of the classification refers to the source of heat when the system is operating in the heating mode and the second part of the classification refers to the medium that is being treated. So, in general terms, a heat pump that is classified as X-to-Y uses the heat in “X” to ultimately heat “Y”.

An example of an air-to-air heat pump is a typical unit used for comfort cooling and heating. We use the heat contained in the outdoor air to heat the air inside the occupied space.

An example of an air-to-water heat pump would be a radiant heating system. In this case, the heat from the outside air is used to heat water that is ultimately circulated through the radiant heating system. Another example of an air-to-water heat pump system would be a pool heater. In this case, heat from the outside air is used to heat the water in a swimming pool.


Can you give me some examples of a water source heat pump?

Water source heat pumps use water as the heat source when the system is operating in the heating mode. An other important thing to consider is the fluid that is ultimately being treated. For example, we can use heat in the water to heat air, or water. When we use heat in water to heat air, we call that heat pump a water-to-air heat pump. When we use the heat in water to heat water, we call that heat pump an water-to-water heat pump. The first part of the classification refers to the source of heat when the system is operating in the heating mode and the second part of the classification refers to the medium that is being treated. So, in general terms, a heat pump that is classified as X-to-Y uses the heat in “X” to ultimately heat “Y”.

Water-to-air heat pumps are used for comfort heating and cooling. We use the heat in the water to provide for the heating of the occupied space.

Water-to-water heat pumps can be used in a variety of applications including radiant heating systems and pool/spa heating.


Where does the water supply come from for water-source heat pumps?

In most cases, the water that is used in conjunction with residential water-source heat pumps comes from wells. These wells are drilled on the property and must be deep enough to reach the water table. Commercial applications utilize a cooling tower to provide the water needed for proper system operation. Another popular heat pump configuration utilizes buries loops of piping that contain either water or a water/antifreeze mixture. Heat pump systems that utilize wells or cooling towers are referred to as open-loop systems while those that utilize buried, water-filled loops are referred to as closed-loop systems.


What is an open-loop heat pump system?

An open loop heat pump system is one that utilizes a water source that is open to the atmosphere or Earth. We can also say that open loop heat pump systems have, for the most part, “uncontained” water sources. During the heating mode, heat is transferred from the water in the Earth to the refrigerant circuit. This heat is then transferred from the refrigerant circuit to the air or water that is being heated. Open loop heat pump systems rely on a constant supply of underground water, so this must be taken into account before the decision is made about which type of loop system will be used. In addition, the mineral concentration in the water can affect system operation so this, too, must be evaluated by a profession trained to evaluate the quality of the water in a particular geographic region.


What is a closed-loop heat pump system?

A closed loop system utilizes loops or coils of buried tubing that contain water or a water/antifreeze mixture. These loops are sealed and, if there are no leaks, will remain completely filled all the time. The water contained in these loops is used over and over to facilitate the transfer of heat either into or out of the heat pump system. The closed loop system is a popular choice when the water table is far below the ground, the ground water temperature is too low, the water quality is poor or the mineral content is too high. Closed loop systems often require much more installation-related damage to the property as the loops that are buried are quite large and can, depending on the selected configuration, cover a large are of the property.


What does a well look like?

From the top, the typical well does not look like much. Probably the only thing you will see is the well cap, which is the only part of the well that is above ground. Underground, however, there is a lot more going on. The well must be drilled deep enough to allow the pump to be positioned below the static water line, or water table. There is a screen at the bottom of the well that prevents foreign matter from entering the well and the pump. There is a water line, often referred to as a drop pip, that connects the pump outlet to the underground water line. The underground water line carries the water from the well to the heat pump system.



Illustration of a well used for heat pump applications.


What is a Water Table?

The water table is the point or level where underground water is found. The exact location of the water table varies with geographic location, so experienced well-drillers are consulted to determine how deep a particular well must be drilled. This will help ensure that ample water will be available for the system. During periods of excessively dry weather, the water table can drop and, during periods of excessive rainfall, the water table can rise. Although a rising water table will not have a negative effect on heat pump system operation, a falling water table can dramatically reduce the amount of water available to the system and may, in severe instances, prevent the system from operating.


Why can’t we use domestic water in a water-source heat pump?

Large amounts of water are required to flow through water-source heat pumps in order to provide the needed capacity. For the most part, using the local municipal water supply for heat pump operations is illegal. Be sure to check the laws in your area before attempting to use your tap water supply for your heat pump.


How do wells provide water for the heat pump system?

Pumps are positioned at the bottom of the well to supply the needed water to the system. When the system is in operation and water is needed, the pump will turn on and pump water from the well to the water-refrigerant heat exchanger in the heat pump system.


Does the pump have to run whenever the system is operating?

Sometimes. If the system is not using a pressure tank, the answer is yes. This means that the pump will cycle on and off with the heat pump and can, in the event of system short-cycling, lead to premature pump failure. If the system utilizes a pressure tank, the pump’s run cycles will be less frequent.


What exactly is a pressure tank?

A pressure tank is a tank that stores pressurized water until it is needed by the heat pump system. The pressure in the tank is maintained at a desired level. When the tank pressure is low, the pump is cycled on and water is pumped into the tank. Once the tank pressure reaches the desired level, the pump is cycled off. As the heat pump system operates, water is fed from the pressure tank and through the heat pump. When the pressure in the tank drops below a predetermined level, the pump is cycled on again to increase the water pressure in the tank.



The pressure tank is located between the supply well and the heat pump.