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Make geothermal more affordable with propane

In hybrid heating systems, a backup gas furnace can make geothermal more comfortable and reliable and can even reduce operating costs.

By Jeffrey Lee

Staff Writer

For builders and their clients, deciding whether to invest in a geothermal heating system can be a tough choice.

Like most investments in expensive, energy-saving home technologies, it's a balancing act between high upfront costs and long-term energy savings. Although geothermal (also known as ground source heat pump or GSHP) systems can provide exceptionally low energy costs by utilizing thermal energy from the earth, the sky-high upfront costs for the heat pump and ground loop systems can result in payback periods of 15 years or longer.

But installing a geothermal system doesn't have to be an all-or-nothing proposition. Hybrid heating (also known as dual-fuel) systems offer a middle ground on installation costs while providing additional comfort and resilience benefits for homes. And today, heating equipment manufacturers are making it easier than ever to specify the optimal hybrid heating system for your home.

System setup

Hybrid heating systems combine a geothermal unit, which supplies a home's cooling load and part of the heating, with a propane or natural gas furnace. A home with only geothermal heating would switch to costly electric resistance heat during cold weather. But a home with hybrid heating would switch to the warmer and more affordable heat of the propane furnace.

The setup of a hybrid heating system is also different, says Kent Kuffner, geothermal business manager for Carrier, which manufactures both geothermal heating equipment and gas furnaces. "The geothermal unit used would normally be a split and not a package unit," he says.

A geothermal split consists of the compressor section with the coaxial heat exchanger for the loop and does not contain the air-handling section. Instead, the blower in the gas furnace moves the air through a home's duct system. An evaporator coil is mounted on the gas furnace and connected to the geothermal split with a traditional refrigerant line set to provide hot gas for heating and cool gas for cooling and dehumidification. In contrast, a complete geothermal package unit system would include the air handler and use electric resistance back up.


  Bosch Greenstar
  Buderus GB142 
Click to enlarge 
 

Downsize upfront

Adding a backup propane furnace to the GSHP provides a number of advantages in both system design and performance, Kuffner says. "The main benefits to this application are reduced installation cost, redundancy, additional comfort at extreme temperatures, and potentially reduced operating cost," he explains.

Because the geothermal system doesn't need to meet the home's entire heating load, both the heat pump unit and the earth loop can be smaller and more affordable, especially in colder climates. How much smaller depends on the climate and the desired outdoor temperature at which the unit would switch over, Kuffner says. A study performed by research firm Newport Partners found that geothermal systems could be sized to handle 50 to 75 percent of the heating load, reducing upfront costs by $5,200 (including the added cost of the furnace) in one design example.

Manufacturers like Carrier offer geothermal design software to help builders and contractors determine the most economical point for the switchover from geothermal heating to the propane furnace. "This software enables the designer to select different size units and determine at what outdoor air temperatures the gas furnace would be used, and calculate operating costs for both energy sources, electricity and gas," Kuffner says. "Then, installation costs of various sizing combinations can be considered to determine the best value for the homeowner."

Comfort and resilience

Value and energy savings aren't the sole benefits of hybrid heating systems, however.  "Geothermal systems deliver a very consistent comfort level," Kuffner says. "However, some consumers like to feel the hot delivery temperatures of a gas furnace, so when it's very cold outside, some homeowners may prefer the gas comfort." Propane furnaces deliver warm supply temperatures of 115 to 125 degrees Fahrenheit throughout the heating season, regardless of outdoor temperatures.

And ultimately, hybrid heating systems provide versatility and resilience because they utilize two different sources of energy. "If the system is powered by a generator during a power failure," Kuffner notes, "then the generator size requirement for a gas furnace is less than a package geothermal unit, providing an additional benefit." Hybrid heat systems are also used with air source heat pumps, providing similar advantages.

Geothermal systems may not be an option for every project. Because of the need for earth loops, space constraints are a significant issue in existing properties and urban areas. Higher heating or cooling loads may require deeper wells, more wells, or more trenches in limited spaces. For an in-depth discussion and a resource you can use when discussing the decision with your clients, download High-Efficiency Propane Furnaces vs. Ground Source Heat Pumps: A Comparison Guide(PDF).

 

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