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Heat Pump Basics: Part 4

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Published
July 9, 2009
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Are there different well configurations and setups?

Yes. Water-source heat pumps can be configured with a number of different well configurations. Heat pump systems can utilize a supply well and a return well, a one-well system, a supply well and a dry well or a dedicated geothermal well. The type of well system used is determined, in part, by the geographic location of the system.


How is a two-well (supply and return) system configured?

The two-well system utilizes one well to supply the water to the heat pump system and another well to receive the water that has already flowed through the heat pump. The configuration is shown in the following diagram. The supply well feeds water to the pressure tank, which feeds water to the heat pump. Once the water has passed through the heat exchanger in the heat pump, the water is then sent back to the ground via the return well. The wells should be at least 100 feet from each other to prevent the heat from the water in the return well from affecting the operation of the system.



A two-well system.


How is a one well system configured?

If the system is located near a pond, river or lake, the water that is discharged by the heat pump system can be directed to it. In this configuration, the water is brought to the system by the well and then discharged directly to the pond, river or lake. Since the water does not come in contact with any chemicals or other substances within the system, no environmental damage results. It is important, though, to check with local codes and guidelines relating to the installation of this type of heat pump system.


               
A one-well system utilizes a lake, pond or stream to accept the water discharged from the system.


How is a supply well/dry well system configured?

If the ground is comprised of sandy soil, the dry well configuration may be used. A dry well is little more than a large hole filled with a combination of gravel and/or sand. The line that carries the water from the heat pump is directed to the dry well. The water seeps through the gravel-filled well and returns to the ground.



A dry well is used to accept the water that is discharged from the heat pump system.


How is a dedicated geothermal well system configured?

A dedicated geothermal well is a one-well system that provides for both the supply of water to the heat pump as well as the for the return of water leaving the heat pump system. The geothermal well has both supply water and return water piping connections. The water that is returning to the well from the heat pump is introduced to the well approximately three to four feet below the point where water is pumped from the well to the heat pump system. A cutaway of a dedicated geothermal well is shown here.


             
A dedicated geothermal well.


How is heat transferred from the water in the Earth to the heat pump system?

The water in Earth and the refrigerant in the heat pump system pass through a tube-in-tube heat exchanger. The tube-in-tube heat exchanger is configured as a tube positioned within another tube. The water flows through the inner tube while the refrigerant flows through the outer tube.


                      
A tube-in-tube heat exchanger.

In the heating mode, heat is transferred from the water to the refrigerant, causing the refrigerant to boil to a vapor at low temperatures. In the cooling mode, heat from the hot vapor refrigerant is transferred to the water, rejecting heat from the system and allowing the refrigerant to condense into a liquid. In the following two diagrams, the outer circle represents the refrigerant path while the inner circle represents the water path. The light blue indicates the lower temperature fluid and the red represents the higher temperature fluid.


                                             
In the cooling mode, the high temperature refrigerant transfers heat to the cooler water, which is then transferred back to the Earth.


                                                            
In the heating mode, the higher temperature water transfers heat to the cooler refrigerant, which is then transferred to the medium being heated.

A cutaway of an actual tube-in-tube heat exchanger is shown here. It should be noted that the inner tube is not smooth, but grooved to allow for greater heat transfer.


     

Cutaways of tube-in-tube heat exchangers.


If the heat pump is being used to heat and cool air, how does the heat from the heat pump get to the air?

Just like any other air conditioning unit, there is a fin and tube coil through which the refrigerant flows. When the heat pump is operating in the heating mode, the hot gas from the compressor is routed to the indoor coil. A blower then passes air from the conditioned space through the coil and heat is transferred from the refrigerant to the air. When the heat pump is operating in the cooling mode, the refrigerant that flows through the indoor coil is a low temperature liquid. As the blower passes air from the conditioned space through the coil, the air, being warmer than the refrigerant, transfers heat to the refrigerant, thereby cooling the air.



Refrigerant flowing through the fin and tube coil transfers heat to the air for heating or absorbs heat from the air for cooling.


If the heat pump is being used to heat water, how does the heat from the heat pump get to the water?

Instead of using a fin and tube heat exchanger that was used to transfer heat to air, another tube-in-tube heat exchanger is used. This type of system will use on tube-in-tube heat exchanger to transfer heat between the Earth water and the heat pump and another to transfer heat from the refrigerant in the heat pump to the water being used for heating purposes.


What would the piping diagram look like for a water to air heat pump?

A water-to-air heat pump uses water as the heat source to ultimately heat air. Since we are heating air, the indoor coil is of the fin-and-tube variety. Also, since the source of the heat is water, the water-to-refrigerant heat transfer surface will be of the tune-in-tube variety. Here is a simplified piping diagram of this type of system.


        
A water-to-air heat pump system piping diagram.


What would the piping diagram look like for a water to water heat pump?

A water-to-water heat pump uses water as the heat source to ultimately heat water. Since we are heating water with water, both heat exchangers will be of the tube-in-tube variety. Here is a simplified piping diagram of this type of system.



A water-to-water heat pump system piping diagram.