Despite their advantages, contractors often overlook geothermal systems when installing new or replacing old systems, simply from lack of experience. With more information, however, contractors can see why geothermal systems can be an excellent choice for the future.
A geothermal system includes three main components:
1. An earth connection, usually polyethylene (PE) pipe in a closed loop, to transfer heat between the water in the pipe and the earth.
2. Circulating pumps to move water in and out of the building.
3. A water-source heat pump in each zone to deliver quiet and efficient heating and cooling.
For contractors, this means much less equipment to install and maintain. There are no chillers, air handling units, boilers, cooling towers, fuel tanks, smoke stacks, blower units, or radiators. Most equipment is not visible, so it's not exposed to weather or vandalism. According to the Geothermal Heat Pump Consortium (Geoexchange), a geothermal system requires about one-third the space of a traditional boiler room.
Key considerations for a geothermal system are budget, space, and building loads. Energy savings and reduced maintenance deliver the largest cost benefits. Geothermal systems can deliver a payback in two to five years.
Additional factors to establish include the building's peak heating and cooling loads and the condition of the soil. A thermal conductivity test prior to designing the loop can help determine the most effective loop design based on the heat transfer characteristics of the soil. Dedicated loop-design software can help determine all of these factors.
The building site will determine the most suitable geothermal system. There are three closed-loop systems to choose from: vertical loops, horizontal loops, and surface water loops.
Vertical loops are best for projects that have limited surface area available for the ground loop. Commercial and institutional buildings often use vertical designs, even though they can be the highest first-cost of the ground-loop options. Vertical loops generally require approximately 250 sq ft of ground surface/ton. Vertical wells that are 200 ft deep will use 1-in. diameter pipe, whereas 400-ft wells may use 1-1/4-in. diameter pipe.
Horizontal loops are best for projects that have large amounts of land available that won't interfere with future building expansion. Advancements with installation trench boxes have improved installation time and horizontal loops are being considered in many school applications due to the cost savings over a vertical loop.
Horizontal loops may require over 1,500 sq ft of ground surface/ton. Horizontal trench piping is usually installed using coils of 3/4-in. or 1-in. PE pipe with circuit lengths are usually no greater than 600 ft.
Surface water loops are considered the most cooling-efficient, closed-loop option. It is common to use existing retention ponds to act as the heat sink for heating and cooling modes. Software design services are available to verify that the building design loads match the available body of water.
If the pumps are not energized, flow through the heat pump will simply be shut off and the water-source heat pump will be de-energized. This is ideal for spaces that don't require continual heating and cooling. As heating or cooling is needed, the valve opens and begins circulating water through the pump, heating or cooling depending on individual zone requirements.
The water-source heat pump unit chosen should yield maximum flexibility in design, installation, and operation, while maintaining the ideal system for the building. High-efficiency units that use R-410A refrigerant helps provide the most efficient system available.
Look for these features in seven different types of units:
Configurations can be left- or right-return and straight or end discharge. Interchangeable airside panels on horizontal units allow you to modify heat pumps on site. For easy maintenance, look for easy-access panels to the blower motor and compressor section, as well as filters and control box.
Bob Koschka is a water-source heat pump systems applications engineer for McQuay International. He can be reached at Robert.email@example.com.
Publication date: 11/14/2005