How A Ground Source Heat Pump Works

Scott Houser is a maintenance foreman with the Fairbanks North Star Borough School District.  He was an integral part of Weller heat pump installation and is currently maintaining the system to ensure it works correctly. In this video he briefly discusses how the heat pump at Weller is used to heat ventilation air.

Read on to find out more about how the ground source heat pump at Weller School works…..

What do we use to help store our food and keep it cool?  A refrigerator!  A refrigerator works by taking heat from its interior (where the food is) and moving it outside.  The heat comes off the coils located behind the refrigerator and goes into the room.

A refrigerator moves heat from its interior, where food is stored, to the room where the refrigerator is located.

A heat pump is like a refrigerator, except instead of moving heat away from the food inside it, a heat pump moves heat into a building.  A Ground Source Heat Pump (GSHP) like the one at Weller gets this heat from the ground.  The soil outside has a fairly steady temperature all year (for information on the temperature of the soil around Weller, check out the Ground Temperature page).

Weller School

During the winter, the temperature underneath the ground surface is higher than the temperature of the outdoor air.

Pipes buried in the ground are filled with fluid that brings heat to the heat pump.  In September 2010, workers buried pipes underground at Weller School.  These pipes are arranged in slinky coils and make up the heat pump ground loop.

Weller Ground Loop

The ground loop at Weller is made up of 6 horizontal trenches with the ground loop pipes arranged in slinky coils. In three of the trenches the pipe is buried 8 feet deep, and in the other three the pipe is buried 12 feet deep.

Slinky Coils

Workers lay the ground loop at Weller School in September 2010

The ground loop at Weller consists of 6 trenches, which are about 100 feet long.  In each trench is nearly 1000 feet of pipe arranged in coils. That’s over a mile of pipe buried underground!  Three of the trenches are 8 feet deep, and the other three are 12 feet deep. All of them are buried behind the school on the south-facing hill. After the loop was buried, the pipes were filled with fluid to transfer the ground’s heat to the heat pump.  The fluid is a mix of water and ethylene glycol,  a chemical that keeps the water from freezing in the winter. A circulating pump located next to the heat pump forces the fluid to go through the pipes in the ground and then brings it back to the heat pump. As the fluid goes through the loops underground, it gets warmer because the temperature underground is higher than the temperature of the fluid when it leaves the heat pump.  When the fluid comes back to the heat pump, it brings the warmth with it.
There are pipes that connect the ground loop to the heat pump.  They travel through the ceiling of a classroom to a utility room on the second floor, where the heat pump is located.  It sits next to fans that bring in fresh air, air filters, and the storage area for Christmas decorations.

Pipes connect the ground loop to the heat pump.

The pipes in the classroom ceiling are white because they are covered in insulation. The temperature of the fluid is colder than the classroom, even after it has been warmed by the ground loop.  The insulation prevents condensation from occuring on the pipe surface.  There are 2 pipes-one takes fluid outside to the ground loop, and the other pipe brings fluid inside to the heat pump.

The heat pump transfers heat from the ground loop fluid to the ventilation air.  Every day, fresh air is brought into Weller School from outside.  In the fall and spring, it doesn’t take that much energy to heat the fresh air, but in the winter the air can be very cold.  The heat pump is there to warm up this ventilation air using the heat from the ground loop. There is also a boiler at Weller School that heats the building.  The heat pump heating the incoming air makes the boiler’s job much easier!
heat pump

Supply and return pipes transfer fluid between the ground loop and heat pump.

The heat pump is a very plain-looking grey box.  But inside the heat pump is a refrigerant loop that takes the heat from the ground loop fluid and transfers it to the ventilation air. A refrigerant is a fluid that evaporates at low temperatures. The two pipes going into the heat pump are the supply and return pipes for the ground loop.  The supply pipe brings the warmer fluid to the heat pump, and the return pipe takes the colder fluid back to the ground loop to warm up.   The refrigerant inside the heat pump gains heat from the ground loop fluid, and evaporates into a gas.  Then it goes to a compressor, which uses electricity to raise its temperature so it can transfer its heat to the incoming ventilation air. As it warms up the ventilation air, the refrigerant becomes a liquid again.  It then travels back to the beginning to gain more heat from the ground loop fluid.  The refrigerant continues to cycle around in a loop transferring heat every time the heat pump is turned on. The picture below shows what happens inside of the heat pump:
Heat Pump Graphic

This graphic was adapted from the article "Is a heat pump right for you?" by S. Gibson in the 2010 Fine Homebuilding magazine.

There is one more really cool feature of the Weller heat pump system: Weller School has solar panels that are used to help warm the soil.  Fairbanks doesn’t have very much sun in the winter time, but in the summer, there is lots of sun.  The Weller GSHP has extra pipes that can be opened to allow the ground loop fluid to travel through the solar panels on the roof of Weller School to gain additional heat in the summer.  When the GSHP is not needed to heat the ventilation air (such as during the summer) the ground loop fluid can transfer the heat it gains in the solar panels to the soil.  Engineers are hoping that this extra heat in the summer will help the ground to stay warm year after year as the heat pump operates.  Scientists at CCHRC are watching the ground temperatures to see if this will be true!

Weller Solar Panels

Solar panels on the roof of Weller School will help warm the soil around the ground loop.