Heat pumps
Heat Pumps Use Heat in the Air, Earth, or Water to Heat a House
Bird's Eye View
An air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps. is like a window air conditioner
Running one way, it cools, running the other, it heats. When outside air is cold enough, electric resistance heaters kick in. Most air source heat pumps work better in moderate climates than cold ones, although there are specialized models for cold areas.
Heat pumps can produce more energy than they use
A heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump. draws heat from the air, earth, well water, or pond water and uses that energy to heat or cool a house.
All heat pumps work on the same principle, moving heat from one location to another with the help of a closed refrigerant loop, a compressor, and a heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank.. In winter, heat is extracted from air, water, or the earth; this heat is used to heat the house. In summer, the process can be reversed so the heat pump pulls heat from the inside and dumps it outside. A heat pump can be used with either a forced-air or hydronic distribution system.
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MORE ABOUT HEAT PUMPS
FURTHER RESOURCES
Heat Pump Types
The ground can heat and cool houses
Coils buried in the ground typically contain a refrigerant gas which in it's vapor form is about 5 degrees F. The gas absorbs heat and goes through a compressor, which raises the temperature before going into a heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank.. This heat can be forced through ductwork in a house.
Ground-source systems more expensive
Ground-source heat pumps can collect heat from the ground, from well water, or from a pond and use it to heat a house through forced air or hydronic systems. They can be either open or closed systems. Open systems pump water directly from a pond or well, extract the heat, and dump the cooled water. Open systems are usually cheaper than closed systems, which circulate fluid through a buried loop of PEXCross-linked polyethylene. Specialized type of polyethylene plastic that is strengthened by chemical bonds formed in addition to the usual bonds in the polymerization process. PEX is used primarily as tubing for hot- and cold-water distribution and radiant-floor heating. tubing.
Direct-exchange systems use a buried copper loop circulating refrigerant. Ground-source heat pumps can also heat domestic water.
Air-source heat pumps are like window air conditioners running backwards. They work best in mild climates, but some very efficient models are made for cold areas. Air-source heat pumps are an excellent option in tightly built and well insulated houses because they're small, inexpensive, and provide cooling too.
The refrigerant can be the weak link. In the past, refrigerants included chemicals that attacked the earth’s ozone layer; these chemicals have gradually been replaced by a succession of newer formulations that are more benign. Some manufacturers have already switched to R-410A, a non-ozone depleting alternative to R-22, scheduled for phaseout in 2010.
Design Notes
IT MAY BE FREE HEAT, BUT IT USES ELECTRICITY TO OPERATE. A ground-source heat pump uses vapor compression to extract heat from a closed, underground loop of water. Inside the heat pump, refrigerant is the heat-exchange medium. During summer, the direction of the refrigerant is reversed, allowing the pump to extract heat from the air inside a house.
Size the system correctly
The decision on what heating system to employ is a critical step. It's a challenge to keep up with all of the mechanical and technical aspects of a construction project. Many suppliers/installers will calculate heating requirements (heat loss efficiencies) based on typical construction or standard defaults. This calculation may result in suggesting a larger heating system than necessary. A third-party energy consultant can help to analyze your specific site and house system.
Careful load calculations are especially important with a ground-source heat pumpHome heating and cooling system that relies on the mass of the earth as the heat source and heat sink. Temperatures underground are relatively constant. Using a ground-source heat pump, heat from fluid circulated through an underground loop is transferred to and/or from the home through a heat exchanger. The energy performance of ground-source heat pumps is usually better than that of air-source heat pumps; ground-source heat pumps also perform better over a wider range of above-ground temperatures. system, since the equipment involved is so expensive. Designing a system that is 50% oversized can cost thousands of dollars more than necessary. Ground-source systems usually have to move large quantities of water. Choosing the right size pumps can play a big part in the system's efficiency.
Use air-to-air heat pumps in moderate climates
Air-to-air heat pumps can do a good job of meeting both heating and cooling needs. But in a cold climate they often need to be supplemented with an inefficient electric resistance heating element or use costly dual stage compressors. Ground-source heat pumps might be better in northern regions because their efficiency doesn't depend on outside air temperatures.
Noise can affect comfort too
Heat pumps typically have large fans and compressors. The vibration of concrete mounting pads or the echo off of adjacent masonry foundations and retaining walls can create quite a nuisance. Other pumps in mechanical rooms can also cause vibration and heat. Try to separate mechanical equipment from living spaces, or consider soundproofing the walls in between.
Builder Tips
The distribution system must be first-rate
A well designed heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump. system will barely heat the house on the coldest day of the year. If the designer of the system is willing to depend on electric resistance heat during extremely cold weather, the system may even be sized to handle less than the design heat load. To be sure that the precious heat gathered by the system is not wasted, it's important for the home's distribution system to be impeccable.
In a home with a heat pump and a forced-air distribution system, the duct system must be well designed and as tight as possible. All duct seams should be sealed with mastic, and the HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. installer should verify duct tightness with a Duct Blaster test.
The Code
Equipment must be accessible for service
Heat pumps are covered primarily in Sections 1401 and 1403 of the 2006 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.. Equipment sizing should be based on ACCA Manual J or other approved calculation method (1401.3). The equipment must be located so it can be serviced and replaced as necessary (1401.2). The outdoor components must be installed on a solid, level base at least 3 in. above grade (1403.2), except in flood-prone areas, where it must be installed above the design flood elevation outlined in Table 301.2(1). The total area of outside and return ducts should be sized so there’s a minimum of 6 sq. in. per 1000 Btus of output.
IRC Section 1103.1.1 requires that heat pumps with supplementary electric-resistance heat have automatic controls to prevent resistance heating when the heat pump compressor can meet the heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load..
Requirements for ground-source heat pumpHome heating and cooling system that relies on the mass of the earth as the heat source and heat sink. Temperatures underground are relatively constant. Using a ground-source heat pump, heat from fluid circulated through an underground loop is transferred to and/or from the home through a heat exchanger. The energy performance of ground-source heat pumps is usually better than that of air-source heat pumps; ground-source heat pumps also perform better over a wider range of above-ground temperatures. piping are found in Section M2105.1 of the IRC. Ground loops must be tested at 100 psi for 30 minutes.
ABOUT HEAT PUMPS
Heat pumps don’t create heat in the same way that a fuel-burning appliance does. They do need electricity (or another fuel source) to operate, but they can produce several times as much heat energy as the electricity they consume. However, a heat pump’s ability to extract heat is not unbounded; at some point it can get cold enough outside that air-source heat pumps become unable to maintain the indoor temperature (balance pointBalance point is the outdoor temperature at which the amount of heating provided by an air source heat pump just equals the amount of heat lost from the house. Below this point, supplementary heat (typically inefficient electric resistance heat or “strip heat”) is required. Typical balance point temperatures are in the range of 27 - 35 degrees Fahrenheit.). Although the relevant outdoor temperature may vary depending on the heat pump design, this rule applies to every air-source heat pump.
Ground-source heat pumps are among the most expensive mechanical systems to install, but they provide heating and cooling, so the higher cost is a little easier to swallow. Air-source heat pumps are more affordable than ground-source heat pumps.
Air-source versus ground-source
There are two basic kinds of heat pumps used for residential space heating: air-source heat pumps and ground-source (sometimes called geothermal) heat pumps. Ground-source heat pumps can be open-loop or closed-loop systems.
ABOUT GROUND-SOURCE SYSTEMS
Water loops are open or closed
Open systems draw water from a pond or well, circulate it through a heat exchanger and then dump it back. These are the least expensive type of heat pump.
Closed systems circulate fluid through a loop
Loops of plastic tubing are filled with water or an antifreeze solution, and the loops are buried in horizontal trenches, dropped into vertical wells, or submerged in a pond. Horizontal-loop systems typically require hundreds of feet of trenches and are generally not feasible in urban areas. Vertical loops are more expensive but fit in much tighter sites.
A variation on the closed system is the direct-exchange system that uses a buried copper loop filled with circulating refrigerant instead of a plastic loop filled with water or an antifreeze solution.
Ground-source heat pumps are expensive and efficient
More expensive than most other heating and cooling systems, they have low environmental impact and can be very efficient. The most efficient systems claim to have COPEnergy-efficiency measurement of heating, cooling, and refrigeration appliances. COP is the ratio of useful energy output (heating or cooling) to the amount of energy put in, e.g., a heat pump with a COP of 10 puts out 10 times more energy than it uses. A higher COP indicates a more efficient device . COP is equal to the energy efficiency ratio (EER) divided by 3.415. (Coefficient of Performance) values as high as 5, but these number are unrealistic. With perfect installation, a COP of 3 is more realistic.
Ground-source heat pumps can be equipped with desuperheaters, which are auxiliary heat exchangers used for heating domestic water.
Air-source heat pumps are smaller, less expensive
Most air-source heat pumps are best for relatively mild climates where both heating and cooling are needed but winters aren't extreme. According to the U.S. Department of Energy, switching from electric resistance heat to an air-source heat pump can trim energy costs by up to 40%. Their weakness is sharply reduced heating efficiency at low temperatures. Most air-source heat pumps switch to backup heating — usually electric resistance coils — when air temperatures drop to about 40°F.
There are exceptions: some units use a gas-fired furnace as a backup, for example, and more advanced air-source pumps like the Hallowell cold-climate heat pump are efficient in temperatures as low as 0°F.
Designers of near-zero-energy homes are beginning to specify air-source heat pumps, even in cold climates. A recently completed near-zero-energy house in Massachusetts with a peak heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load. of only 10,500 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /h uses a Mitsubishi ductless mini-splitA type of small-capacity heat pump (as little as a ton or even less) with a closely-associated outside compressor and inside evaporating coil (often through-the-wall in design). These heat pumps often come with variable-speed compressors and blowers,giving them excellent modulation for thermal comfort. These features also contribute to COPs of around 4 for ductless min-split heat pumps. They are also well-suited for ultra-high performance, small-volume homes. heat pump installed at a total cost of only $5,250. As a bonus, the system provides air conditioning at no additional cost.
On the Horizon: Absorption heat pumps
A variation on the air-source heat pump, these devices are driven by a heat source other than electricity — natural gas, propane, solar hot water, or even geothermal-heated water. They use an ammonia-water absorption cycle for both heating and cooling. Absorption heat pumps are used mostly in commercial and industrial settings but are under development for residential use. Their advantage is they can make use of any heat source and don’t need electricity.
FURTHER RESOURCES
Building Science Corporation:
Ground Source Heat Pumps: Carbon Emissions and Efficiency
Green Basics Index
Related Topics in the Green Building Encyclopedia
Image Credits:
- John Hartman / Fine Homebuilding
- Christopher Clapp / Fine Homebuilding 107
- Paul Perrault/Fine Homebuilding 133
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