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Ground source heat pump at 21 Acres Farm

Case Study by Eliott Frank- intern and Chris van Daalen- project supervisor
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Ground source heat pump at 21 Acres Farm
Heating, Ventilation & AC
King County
21 Acres Farm
Evitavonni Architecture
LEED Platinum


The ground source heat pump used at 21 Acres farm helped to earn the building a LEED Platinum certification. A geothermal system is the most efficient and healthy way to heat and cool a building, with minimal environmental impact and long lasting performance. The heating/cooling system uses 1/4th the amount of energy as a conventional system, and when coupled with radiant floor heating, delivers even an dispersion of heat across the floor, making it suitable for buildings with high ceilings, leaving no cold spots.

Permitting Process

The permitting process for a ground source heat pumps is reportedly easy in King county. In the case of 21 Acres, no additional grading permit was needed, as grading was covered by the building’s permit. The IRC states that heat pump installation and engineering are to be installed according to manufacturer specifications and meet IRC/IBC performance standards.  In some cases it may require a SEPA review to acquire permission for excavation to install the ground source loop. A key to successful permitting is hiring a qualified installation company with adequate field expertise and complex modeling software to size and install the system properly.

Code RequirementCompliance Path
IRC M1301 General Mechanical System Requirements Prescriptive path was followed
IRC M 1401.1 Installation.  Heating and cooling appliances must be installed per manufacturers instructions
Variation of ground loop depth was allowed due to high water table
IRC M1403.1 Heat Pumps Prescriptive path was followed

Project Description

21 Acres is a non-profit, community focused agricultural organization that aims to educate community members of the land’s agricultural history, as well as provide fresh, organic, local foods that are sustainably grown. The 21 Acres main building contains many sustainable technologies that have recently earned them recognition as LEED platinum certified building. One of which is a ground source heat pump, also known as a geothermal heat Ground source heat pump: vertical wells or horizontal fieldloop. This is a mechanism that utilizes the laws of thermodynamics, absorbing heat from the ground to heat a water/glycol mixture, which is then transferred to the building using a reverse refrigeration process.  The system produces four time the amount of energy it uses, making it an extremely efficient heater for radiant floor systems.  The operating costs are not volatile like natural gas or propane, instead, the earth’s heat is relatively constant, providing a dependable long term heat source, especially when coupled with photovoltaic cells, as is this system. The ground source system has the option of heating both radiant floor water systems and domestic hot water systems.

Zonal heating manifold - control system for hydronic heatAt 21 Acres this system was used in conjunction with a radiant floor heating system to provide even and long lasting heat for the building. Heat rises from the warmed concrete floors, heating the space from the floor up without forced air stirring dust and pollens. This method of heating was chosen for the 21 Acres main building due to its environmentally clean function, efficient heat return, and longevity. The system has a lifespan close to double or triple that of a conventional system. 21 Acres facilities manager Patrick Park emphasized this in an interview, reporting no maintenance problems, and a projected 50 year lifespan. This same system also functions as an air conditioner, taking heat from the building in the summer and putting the heat in the ground, cooling the building.

In the process of reviewing the site to install the loop, it was found that 21 Acres has a higher-than-normal water table. This resulted in the pipe depth to rest at two to three feet, opposed to the conventional four to six feet. This, however,  was a benefit, as water is more thermally conductive than soil, making the system more efficient with less excavation.

Design / Build Process

A ground source heat pump geothermal loop has two different configurations that can be used, a series of vertical wells, or horizontal field making up the underground loop.  The choice between these depends on the amount of property one chooses to dedicate to the loop. Horizontal systems are buried down approx. 6 feet in the ground, and coil across a larger footprint. A vertical loop can go down to 150 ft. and requires less large of a footprint. This allows a ground source heat pump to be installed in a variety of sites. The ground loop is sized depending on the square footage of the building, averaging about 1,500 ft. of pipe for a 2,000 square foot home. This depends on multiple factor, and requires a computer modeling program to account for the many variables.

Cost / Benefit

For the facilities manager at 21 Acres, Patrick Park, a long lasting system (3-5 times longer than traditional HVAC systems) that requires few updates or services makes his job much easier . He informed me that after tracking the performance of the system through a series of individual energy meters, that the system was 300-400% efficient, meaning for every 1kW of electricity, 3-4kW of heat is produced. This results in as much as a 70% decrease in energy bill. Conventional systems are at most 98% efficient. However, the system itself is rather costly. “It is a trade off” Park explained, “in the long run it is much cheaper, and pays for itself, up front it is quite expensive.” As the caretaker of this large building, he recommended ground source heat because it was the healthiest, most cost effective, and longest lasting choice for the building and surrounding environment. Although, finding the right company that offers computer modeling and sizing is crucial to not over taxing the ground’s heat. This especially applies in the winter, as the earth needs breaks to replenish it’s heat. To keep a warm house in the winter, it takes an appropriately sized system to allow for periodic breaks and still provide consistent indoor temperatures.

Lessons Learned

To make a truly efficient and environmentally sensitive building, it requires a multifaceted plan of functional systems to work in concert. This includes the climate conscious shape of the building and proper development of the site to accommodate all of the functions together. One innovative technology may save you money over time, but with a more comprehensive plan, can cut that time in half. The combined use of PV cells, a ground source heat loop, radiant floor heat delivery, an airtight envelope, and a preheated earth-tube ventilation system accomplished the goals of 21 Acres, earning them the highest rank of environmental design.


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