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Air-source heat pumps, popular in some parts of the country, are basically
air conditioners that can work in reverse to deliver heat into the house in
winter. Heat pumps provide heating and cooling in one machine, with one duct
system and no combustion. Heat pumps are not subject to back-drafting, combustion
air requirements, and some of the off-cycle losses to which furnaces are prone.
Because they use ducts to deliver warm air to the house, heat pumps have
the same problems with duct leakage and insulation, and heat-pump ducts
must be sealed thoroughly and insulated. Heat pumps are more prone to airflow
problems — both indoor and outdoor — than furnaces are, and improper refrigerant
charging and electric-resistance backup heating can have a negative impact
on heating efficiency (see the drawing on the facing page).
Components of the air-source heat pump in diagram above:
Operating efficiency and resistance heating
Heat pumps are typically rated at efficiencies between 200% to 300%, which is expressed as a coefficient of performance, or COP, rating of 2 to 3. A COP of 2 means that for every kilowatt-hour of electricity you buy, you get two kilowatt-hours of heat delivered to your house. Newer heat pumps use a different rating system, called Heating System Performance Factor, or HSPF. An HSPF of 6.8 corresponds to a COP of 2. How can the system generate more energy than it consumes? The difference is made up by heat energy absorbed from the out door air. That energy goes into the system; it just happens to be free.
Heat pumps, by themselves, operate at reason able efficiencies until outside air temperatures drop to about 35°F to 40°EAs the outdoor air gets colder, less heat is available, and the heat pump output drops off just as the house needs more heat. So cold-weather performance is typically supplemented with electric-resistance backup heaters.
This auxiliary heat costs two to three times more per unit of heat than the compressor heating cycle. That may be fine in regions with long summers, mild winters, and relatively low electric rates, but it’s not in the Northeast, where electricity is expensive and winters are cold. One fairly inexpensive way to reduce the use of electric-resistance heating is to install an outdoor cutout thermostat. For about $100 to $150 installed, this device locks out the supplemental electric heat when the out door air temperature is above 30°F or 35°F.
One way to eliminate electric-resistance heating altogether is to remove or disable the electric coils and install a hydro-air coil. This coil is like a radiator installed in the supply plenum (about $500 to $1,000 installed) and is heated by a gas-, oil-, or propane-fired (not electric) domestic hot water heater. One potential pitfall of a hydro-air coil is that the installation may significantly cut the airflow through the heat pump. Hydro coils have much more resistance to airflow than electric-resistance coils do, so installers must be careful to select the right model. Depending on the heat pump’s existing airflow, additional duct modifications may also be necessary.
Heat pumps are even more sensitive to airflow than furnaces are. Heat pumps need to move a much larger volume of air, but installers who are used to working with furnaces often skimp on duct sizing, leading to low-airflow problems. Unfortunately checking the airflow of a heat pump is not as easy as checking that of a furnace. It can be done with special instruments or with the temperature-rise method used for electric-resistance backup heat.
HVAC technicians may use an anemometer or pitot tube to measure the air velocity in one of the main trunk ducts; the total cfm equals velocity (in feet per minute) times duct area (in square feet).Total airflow can’t be measured with a flow hood or anemometer at the registers, because duct leakage will be missed. If you have electric-resistance backup heat, the airflow can be calculated by measuring the temperature rise (as described earlier). Set the thermostat to “emergency heat,” so that the compressor doesn’t run; turn up the thermostat and let the heat run for 10 minutes. The total cfm equals the strip heat power (in watts) multiplied by 3.1 and divided by the temperature rise. Watts (amps X volts) are measured with an amp clamp and voltmeter at the service disconnect or breaker box. Don’t attempt to take this measurement unless you understand exactly what you are doing a service technician to do it for you if you aren’t sure.
For best efficiency, heat-pump airflow should be 375 to 425 cfm per ton of heating capacity. It should never be below 300 cfm/ton. With 1 ton equaling 12,000 btu/hour, equipment is typically rated in increments of 1/z ton, such as 24 (2 tons), 30 (2½ tons), 36 (3 tons), etc.
Heat pumps are also finicky about refrigerant charge. Many systems, even when serviced regularly, are either over- or undercharged. Both cases have a negative impact on efficiency. Unfortunately, many service technicians have a limited ability to diagnose or correct those problems. For one thing, airflow must be in the recommended range before refrigerant charge can be correctly diagnosed, and very few technicians test airflow as a matter of course.
There are very specific ways to measure refrigerant charge during unit operation. Depending on the type of unit, the superheat or sub-cooling must be measured carefully, as it is for an air conditioner. As an alternative, refrigerant may be removed with a vacuum pump, and the correct amount for the system may be weighed on a scale as it’s installed. The latter method is time-consuming but accurate.
iw_118-0.jpg A coil and a blower are the main components of a heat pump’s indoor unit.
iw_118-1.jpg Always keep material and debris away from the outdoor unit of a heat pump. Proper efficiency depends on good air circulation, so never cover up or build a deck over the unit.
Like furnaces, heat pumps should be serviced regularly. Basic service can be done by anyone. Air filters should be replaced monthly during the heating season, and the outdoor coil should be kept free of snow and debris. Because of the importance of indoor airflow, keep all registers open. Regular service calls typically every two to three years—should include testing the controls, cleaning the blower, cleaning both the indoor and the outdoor coils, and checking the insulation on the refrigerant lines.
An initial service appointment should include testing and fixing airflow problems, and then care fully measuring and correcting refrigerant charge.
Once that has been done, service technicians should not attach refrigerant gauges to the system unless the system performance drops off or there is other evidence that something is wrong. Refrigerant does not escape unless there is a leak or a technician attaches gauges.
Geothermal heat pumps
Geothermal heat pumps (also called ground source heat pumps) extract heat from the earth or from under ground water, rather than from outdoor air. Because temperatures are much more stable underground, geothermal systems can have much higher heating efficiencies than air-source heat pumps; COPs range from about 2.8 to 4.8.
There are two basic types of geothermal heat pumps. Closed-loop (ground-coupled) systems are more efficient but much more expensive to install. Open-loop (groundwater or water source) systems have a lower initial cost but are more expensive to run. Dealers may claim otherwise, but the power consumption of the large well pump that’s typically required for open-loop systems means that the higher operating cost often more than offsets any up-front savings. A third type of geothermal heat pump, called direct exchange, or DX, uses a copper pipe ground loop to circulate refrigerant underground. Those systems promise to be the most efficient, but long-term reliability is less clear than that of closed-loop water-based systems.
Like air-source heat pumps, geothermal heat pumps can be very cost-effective in regions with mild or hot climates and moderate electric rates.
However, in regions with cold climates and high electric rates, geothermal heat pumps do not pro vide enough savings to just if the high installation cost, particularly in a retrofit.
Geothermal heat pumps can also have very high cooling efficiencies. The efficiency and maintenance issues are the same as those for air-source heat pumps, except that there is no out door coil. Low airflow, refrigerant charge, and electric-resistance heat can all impact the operating efficiency. The heating capacity of geothermal heat pumps is much better in cold weather than that of air-source heat pumps, so there is typically less need for supplemental electric heat. In many cases, the resistance heat can be shut off completely closed-loop systems need little or no maintenance, but open-loop systems may have water-quality issues filters that need replacing, and well pumps with shortened life expectancies.
Next: Electric-Resistance Heat