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August 2011 Energy Solutions
Pumping up efficiency
 by James Dulley
Engineering Consultant
Heat pumps efficiently heat and cool your home
Q: I have an old central air conditioner and electric furnace. I want to install a heat pump but cannot afford a geothermal one. What are the newest designs of standard heat pumps, and how do they work?
A: Heat pumps are becoming a more common alternative to central air conditioners no matter what type of existing heating system you have. This is because a heat pump can also heat, as well as cool, your house efficiently.
The cost of electricity for heating and cooling a house, although it gradually increases as most prices do over time, is much less volatile than natural gas, oil or propane.
A geothermal heat pump is one of the most energy-efficient heating and cooling systems for any climate. Even though it provides a good long-term payback over its life on the investment, particularly in very hot or cold climates, the initial installation costs are considerably higher than for standard air-source models. Also, depending upon the yard and soil type, it may not applicable for every house.
A standard air-source heat pump is basically a central air conditioner with a few extra parts. The outdoor unit looks exactly the same as a central air conditioner. It is called a heat pump because it literally pumps heat out of your house (cooling mode) or into your house (heating mode) to or from the outdoor air around the outdoor compressor/condenser unit.
During the summer in the cooling mode, it draws heat from the indoor air as it passes through the indoor evaporator coils. Through a refrigeration cycle identical to an air conditioner, it expels this heat outdoors. The cooling efficiency is rated by its SEER (seasonal energy efficiency ratio). A heat pump’s cooling efficiency is only slightly less than its similar central air conditioner model.
During the winter, a reversing valve inside the heat pump outdoor unit switches position. This reverses the flow of the refrigerant throughout the entire system. Instead of running the cool refrigerant through the indoor coil, it runs the hot refrigerant indoors.
The cold refrigerant is run outdoors where it draws heat from the outdoor. Since the refrigerant is colder than the outdoor air, is absorbs heat even though the outdoor air may feel cold to you. Heating efficiency is rated by HSPF (heating seasonal performance factor).
As it gets colder outdoors, it becomes more difficult for the heat pump to draw heat from the cold outdoor air just as the heating needs of your house increase. At a certain point, the heat pump can no longer provide enough heat to keep your house warm, and the backup heating source comes on. Depending upon the type of backup heat and relative energy costs, your heating/cooling contractor can set the temperature at which the backup takes over.
There have been many recent developments in standard air-source heat pumps. The modulating, multistage output rotary compressor design, which was first introduced in central air conditioners, is now available in heat pumps. This design produces extremely high efficiencies for both heating and cooling (HSPF — 10, SEER — 22). You can get $2 to $3 worth of heat for each $1 on your utility bills.
This heat pump uses a rotary compressor with inverter technology to allow it to vary its heating or cooling output from about one-third to full capacity output. This not only saves electricity, but it also produces great comfort, quiet operation, and even room temperatures. Two-stage heat pumps also improve efficiency and comfort over standard single-stage models.
Another new two-stage heat pump design couples a solar panel with the outdoor unit. On a sunny day, this solar panel produces enough electricity to operate the condenser fan for up to 8 percent electricity savings. When it is not sunny or at night, the outdoor condenser fan runs on electricity like any other heat pump.
A standard low-cost, single-stage heat pump with a scroll compressor will still be efficient and provide comfort, especially when it is coupled with a variable-speed blower. This is the system type I use in my own home.
Even if your indoor air handler seems to be working well, it should be replaced with one compatible with the new efficient outdoor unit. At the very least, the indoor evaporator coil should be replaced with a matching one.
No matter what type of new heat pump you select, make sure your duct system is compatible with it. There should typically be from 400 to 500 CFM (cubic feet per minute) of air flow per ton of cooling through the unit for the best efficiency. Your old duct system may have to be modified.
James Dulley is a nationally syndicated engineering consultant based in Cincinnati.
If you have a question about energy use or energy-efficient products, send it to: James Dulley, Electric Consumer, 6906 Royalgreen Drive, Cincinnati, OH 45244; or visit www.dulley.com.
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