Extracting comfort from the air and ground
Don’t let the name fool you. A heat pump is not just for heating your home. When the weather is warm, it provides cooling and, in the case of ground source heat pumps (GSHP), hot water as well.
There are two kinds of heat pumps, both powered by electricity. The traditional type is an air-source heat pump, which extracts heat from the outside air during the heating season and releases it to the outside air during the cooling season. The second type, the ground-source heat pump (GSHP), uses the relatively constant temperature of the ground as its heat source and sink.
Heat pumps have been evolving ever since they were first invented over fifty years ago. Since 1993, the Department of Energy (DOE) and the Environmental Protection Agency (EPA) consider GSHPs to be one of the most effective and environmentally friendly way to heat and cool buildings. A GSHP system can be over 500% efficient, can reduce annual energy consumption for a home by up to 75% and can, depending on many variables, cut carbon emissions by 70%+.
If GSHPs can perform so well, why is their use not more widespread? Three main reasons: I) initial costs; II) lack of public awareness; III) inconsistent regulatory and incentives regimes. Until now, support from the federal government has been rather limited but things are improving in this area. Because the ground is a steady state, warmer than the air in the winter and cooler than the air in the summer, GSHPs are more than twice as efficient than air-source heat pumps, especially in heating mode. They are also more expensive to install because they require many hundreds of feet of piping buried in shallow trenches in the ground or looping through vertical boreholes. Sometimes the pipes are run through a pond or other body of water (these are sometimes called water-source heat pumps). Water or a water/antifreeze mix circulates through the pipes. During the winter, the heat pump extracts heat from the liquid and transfers it to the home using a compressor to increase the temperature of the energy from the ground, a fan and ducts. The cooled water is pumped back through the pipes, where it picks up heat and returns again to the heat pump. During the summer, the system works in reverse and produces hot water as a byproduct.
GSHP overall performance—in terms of cost savings and emissions reduction—varies greatly by location and is tied to two factors: I) climate and II) the type and cost of fuel they are replacing, i.e., home heating oil, natural gas, electric resistance heat, etc. Essentially, the more heating and cooling that is required and the more expensive the energy prices are, the greater the economic savings are. As an example, two similar houses:
- Berkeley, CA – natural gas furnace – no AC – annual savings: $700, projected payback 8.25 years
- Boston, MA –heating oil – central AC – annual savings: $4,000,, projected payback: 2.67 years
- Reduce demand. When it comes to heating and cooling energy, the best approach for your wallet and the planet is to use less of it. Before shelling out money for a heat pump or any other HVAC equipment, invest in energy efficiency improvements to your home: add insulation, tighten up air leaks around the house, and consider replacing single-pane windows with efficient double-pane windows. Once you’ve made those improvements, you may find that you can get away with much smaller, less expensive heating and cooling equipment.
- Take care of the ducts. Heat pumps distribute heated and cooled air through the home via ducts. As with any ducted heating or cooling system, efficiency will plummet if the ducts are leaky or uninsulated.
- Replacements, renovation, and new construction. Environmental and financial objectives vary from household to household, but for existing buildings, GSHPs tend to make more sense when the existing furnace and / or AC units need replacing, or the home is undergoing a renovation. When building from scratch, the cost of GSHP can be amortized across the mortgage thereby shortening payback periods.
When shopping, look for
- An Energy Star. Always choose government-approved Energy Star heat pumps. Air-source ones are about 8% more efficient than non-Energy Star models. Energy Star GSHPs use 30% less energy than non-Energy Star air-source heat pumps.
- Water heating too. If choosing a GSHP, look for a model equipped with a “de-superheater.” Heat removed from the house is used to heat your household water instead of being discharged back to the ground.
- Ground-source heat pumps cost more than an air-source heat pump or other conventional heating and cooling system, mainly because they require an extensive network of pipes for circulating liquid through the ground or groundwater. A typical 2,000-square-foot home might need 1,800 feet of pipe laid out horizontally in shallow trenches dug in the ground or vertically in wells bored hundreds of feet into the earth. Before tax rebates, expect to pay $15,000 to $20,000 for a GSHP system, or more if extensive drilling or digging is required.
- Installation costs vary considerably, depending on soil conditions, labor costs, and other factors. Air-source heat pumps are no more difficult to install than a conventional air conditioning system, but it’s usually a job for the pros. GSHPs, with their extensive network of underground or underwater pipes, must be professionally installed.
- The IRS offers a full, dollar for dollar income tax rebate for 30% of the total system cost. If a GSHP system costs $20,000 to install, you would have to pay $6,000 less in Federal income tax. Additionally, many cities, utilities and states offer local incentives. Check here: DSIRE Database
- Older heat pumps use R-22, a hydro-chlorofluorocarbon (HCFC) refrigerant that causes ozone depletion if the refrigerant leaks from the equipment. HCFC manufacturing also results in the creation of significant greenhouse gases. Under international treaty, HCFCs are being phased out of production and will be replaced by non-ozone depleting refrigerants (after 2010, manufacturers can use R-22 to service existing heat pumps but not in new equipment). GSHPs use much less refrigerant than air-source heat pumps.
- Installing the pipes for GSHPs can disturb the local environment although the long-term effects aren’t clear. Installers should follow the industry’s best practices guidelines to prevent groundwater contamination.
- Ground-source heat pumps change soil and groundwater temperatures, making the area around the pipes colder in the winter and warmer in the summer. It’s not clear what subterranean ecological effects this “thermal pollution” may have, especially if use of GSHPs becomes more widespread.
Unlike furnaces that burn natural gas or heating oil, heat pumps are powered by electricity, so there’s no combustion or air pollution produced at your home. They are well-regarded to maintain a highly comfortable and consistent indoor air temperature and, amongst those who have them installed, enjoy a 95% satisfaction rating.
…to your wallet
Air-source heat pumps are not recommended for very cold climates. But in an area with moderate heating needs, an air-source heat pump can reduce heating costs by 30% to 40% if you heat with electricity. If you heat with natural gas or fuel oil, on the other hand, an air-source heat pump may not deliver savings. For cooling, air-source heat pumps and standard central air conditioners offer similar efficiencies; the main advantage of heat pumps over standard AC is that they can be reversed in the winter to provide heat.
Depending on the situation, a GSHP may save between $350 and $4,500 per year in heating and cooling energy. If you currently heat your house with conventional electric heating (such as electric baseboard heating) and standard air-conditioning equipment, a GSHP can reduce your energy use and greenhouse gas emissions by as much as 72%.
Also in many cases, the systems are guaranteed for 25 years and require little or no maintenance.
…to the Earth
Heat pumps can save energy and associated greenhouse gas emissions and air pollution. In fact, in a recent report published by the DOE, it was estimated that widespread adoption of GSHPs within the US could save 3.9 quadrillion BTU’s per year, which translates into an annual savings of about $38 Billion and would reduce new energy consumption by 30% to 40% by 2030.
- You say tomato, I say tomahto. Ground-source heat pumps are often called geothermal heat pumps, but that’s a misnomer. GSHPs take advantage of solar heat stored by the ground and water. Geothermal heat, on the other hand, utilizes heat in Earth’s interior, six miles beneath the earth’s surface. In some parts of the world, utilities tap into the geothermal energy of geysers and hot springs to produce electricity, but that’s an entirely different energy source.
- Pumping and dumping. Some GSHPs are open-loop systems. Instead of continually recirculating liquid, an open-loop GSHP pumps water out of the ground and then returns it to the groundwater via another well. Other types of open-loop systems, known as “pump and dump,” discharge the water to a drainage field on the ground. Some states prohibit pump-and-dump systems because they can deplete aquifers and cause a build-up of salts and minerals in the topsoil.
- A new heat pump–or any new HVAC system–is a major investment. Allow plenty of time to interview contractors and evaluate their proposals. Keep in mind that in the long run, the lowest bid isn’t always the most economical choice. The Air Conditioning Contractors of America (ACCA) provides a detailed checklist to help you evaluate the proposals you receive from HVAC contractors.
- To determine the right size for a new heat pump or other HVAC system, the contractor must do a “Manual J residential load calculation.” This ACCA-approved procedure involves measuring your home, noting the size, type, and number of windows, evaluating levels of insulation and air infiltration, and assessing internal heating loads from lights and major appliances. This data gets factored into computer-based calculations that help ensure the new heating system is neither too small nor too large. Don’t let the contractor get away with skipping the Manual J calculations or basing the new system size on the old equipment. The old equipment may have been incorrectly sized, or the home may have been remodeled or its energy efficiency improved since the old system was installed.
- As part of the estimating process, make sure the contractor evaluates the design and condition of existing ducts and recommends any necessary repairs or changes.
- Air-source heat pumps are no more difficult to install than conventional air conditioning equipment, but the design and installation of GSHPs is much more complex. Since a GSHP system includes loops of pipe buried in trenches or wells, the contractor will need to carefully evaluate your site, including geological and hydrological conditions.
- For general advice on what questions to ask contractors and other tradespeople, see our “What to Ask Your Contractor” article.
- And don’t forget to check the Directory of State Incentives for Renewables and Efficiency, DSIRE, to find out about rebates for purchasing energy-efficient heat pumps.