Q Are heat pumps a good thing to have for heating or cooling in Minnesota?
A It depends. First, it's best to understand what we're talking about.
A heat pump is a device that transfers thermal energy from one location to another. The most common heat pump in a home is the refrigerator, which transfers heat from inside the refrigerator to the surrounding air by way of the coils on the back of the unit.
For heating and cooling homes, the two basic types of heat pumps are air-source heat pumps and ground-source heat pumps.
The air-source heat pump (ASHP) exchanges thermal energy with the outside air, while the ground-source heat pump (GSHP) exchanges thermal energy with a system of liquid-filled pipes buried underground.
Both systems work in Minnesota, but the financial and environmental benefits can vary greatly according to location, building type and available alternatives.
The ability of the ASHP to efficiently provide heat falls dramatically as the outside air temperature drops. According to the U.S Department of Energy, the efficiency factor for an ASHP in the Twin Cities is about 68 percent (compared with 93 to 95 percent for a high-efficiency gas furnace). For this reason, an ASHP requires secondary heating systems to provide adequate heat on cold days.
The efficiency of GSHP systems is primarily based on the size of the system. Although a system can be designed to provide all of the anticipated heating needs for a home, this will result in a system oversized for cooling and will greatly add to installation costs. That's why systems are typically sized to meet cooling needs; as with ASHP, additional heating is then required to provide indoor comfort in the winter.
In terms of cost-effectiveness, properly sized, sited and installed GSHP systems might be a good choice if you are replacing a high-cost fuel source such as fuel oil or propane. However, nearly 70 percent of Minnesota homes are heated by natural gas, which is comparatively low-cost.
Installation costs of GSHP vary according to the site, soil type and size of the system, but can range from $15,000 to $50,000. (For comparison, a high-efficiency gas furnace system might cost $5,000 to $15,000.)
As for environmental concerns, a recent study published by Minnesota's Office of Energy Security found that GSHP systems, in general, have higher CO2 equivalent emissions than other fuel sources, with the exception of all-electric resistance heating.
To read the complete report or to learn more about building efficiency or energy use, go to www.energy.mn.gov. Type "Ground Source Heat Pump" in the search box.
Phil Smith, energy specialist, Minnesota Office of Energy
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