The reversing valve, show in this illustration as a small gold cylinder, allows the heat pump to switch from cooling to heating mode.
Question: It’s time to replace my central air conditioning unit and the old electric heater in my home. Is a heat pump a smart investment? How do they work?
Answer: A heat pump can be a very smart investment because a single unit can both heat and cool your house efficiently.
There are two types of heat pumps to consider—geothermal and air source. A geothermal heat pump exchanges
heat with the ground while air-source systems exchange heat with the surrounding air. Geothermal systems are one of the most energy efficient heating and cooling systems for any climate, but the initial installation costs are high and not all homes have the right yard and soil type to make them work.
Air-source heat pumps are a more common choice and they look almost like a standard central air conditioner, but they have a few extra parts. In cooling mode, the refrigerant inside the heat pump’s coils draws heat from the indoor air and expels it to the outdoors. The efficiency of this cooling process, as rated by the seasonal energy efficiency ratio (SEER), is comparable to that of a modern central air conditioner.
In heating mode, a reversing valve changes the flow of the refrigerant through the system, allowing it to draw heat from the outdoor air and warm the inside of your home. The efficiency of this process is rated by the heating seasonal performance factor (HSPF). As long as the refrigerant is colder than the outdoor air, it will absorb and transfer heat. However, as outdoor temperatures drop, it becomes more difficult for a heat pump unit to draw sufficient heat. When the outdoor air falls below about 30 degrees, a backup heating system may be required.
There have been many recent developments in standard air-source heat pumps. Some newer units have modulating, multistage-output rotary compressors. This new design is extremely efficient for heating and cooling (10 HSPF and 22 SEER), allowing consumers to get $2 to $3 worth of heat for each $1 of electricity. They also run very quietly. Other units use solar panels to generate some of the electricity required to operate the system, but it’s not necessary to invest in the latest and greatest units. Even a standard, low-cost, single-stage heat pump with a scroll compressor (like the one I have in my own home) can be an efficient option and provide year-round comfort.
When choosing between units, consult a qualified contractor to find the right unit for your home, weather conditions and budget. And no matter what type of new heat pump you select, make sure your air handling and duct system are compatible. For the greatest efficiency, there should typically be from 400 to 500 cubic feet per minute of airflow per ton of cooling. Your old duct system may have to be modified to match.
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