Controlling a Heat Pump – Your Outside AC Unite
Like combustion heating systems, you control heat pumps by using thermostats. If you leave and return at regular times everyday, you’ll save money by using automatic thermostats, which minimize energy use during the times the home is unoccupied. However, choosing an automatic thermostat’s reactivation time requires considering the duration of heat-pump operation necessary to restore a comfortable temperature. During the heating season, some homeowners also set their thermostats back 10°F, manually or automatically, when they leave home or go to bed.
A two-stage thermostat controls the heating. The first stage activates the refrigeration system. If it’s too cold outside for the refrigeration system to counteract the home’s heat loss, then the thermostat’s second stage activates the electric resistance coils. An outdoor thermostat will prevent the less efficient electric resistance heat from coming on until the outdoor temperature falls below 40°F. An outdoor thermostat also will prevent auxiliary heat from activating when an automatic thermostat is warming the house after a set-back period. Use setback thermostats that are only for heat pumps.
A defrost control tells the reversing valve when to send hot refrigerant outdoors to thaw the outdoor coil during the winter. During the 2-to-10-minute defrost cycle, auxiliary heat takes over, reducing the heat pump’s overall efficiency up to 10 percent. The two most common types of defrost controls are time-temperature and demand-defrost. Time-temperature defrost controls activate defrost at regular time intervals for set time periods, whether there is ice on the outdoor coil or not.
A demand-defrost control senses coil temperature or airflow through the coil, and only activates defrost if it detects the presence of ice. Obviously, choosing a heat pump with demand-defrost will pay a significant efficiency dividend.
For greater efficiency, don’t locate a thermostat near a heat source or cold draft because they can cause a heat pump to operate erratically. This includes shading thermostats from direct sunlight. Also, do not turn the thermostat beyond the desired temperature. It will not make the heat pump heat or cool your home any faster. It will only waste energy. Residents who duel one another over the thermostat settings, moving it up and down to suit their different comfort levels, cause heat pumps to operate erratically and inefficiently.
Heat Pump Basics
There are two common types of heat pumps: air-source heat pumps and geothermal heat pumps (GHPs). Either one can keep your home warm in the winter and cool in the summer. An air-source heat pump pulls its heat indoors from the outdoor air in the winter and from the indoor air in the summer. A GHP extracts heat from the indoor air when it’s hot outside, but when it’s cold outside, it draws heat into a home from the ground, which maintains a nearly constant temperature of 50° to 60°F.
An air-source heat pump can provide efficient heating and cooling for your home, especially if you live in a warm climate. When properly installed, an air-source heat pump can deliver one-and-a-half to three times more heat energy to a home compared to the electrical energy it consumes. This is possible because a heat pump moves heat rather than converting it from a fuel, like in combustion heating systems.
How They Work
You might be wondering how an air-source heat pump uses the outdoor winter air to heat a home. Believe it or not: heat can be harvested from cold outdoor air down to about 40°F. And this can be accomplished through a process you’re probably already familiar with refrigeration.
Basically, a heat pump’s refrigeration system consists of a compressor, and two coils made of copper tubing, which are surrounded by aluminum fins to aid heat transfer. The coils look much like the radiator in your car. Like in a refrigerator or air-conditioner, refrigerant flows continuously through pipes, back and forth from the outdoor coils. In the heating mode, liquid refrigerant extracts heat from the outside coils and air, and moves it inside as it evaporates into a gas. The indoor coils transfer heat from the refrigerant as it condenses back into a liquid. A reversing valve, near the compressor, can change the direction of the refrigerant flow for cooling as well as for defrosting the outdoor coils in winter.
When outdoor temperatures fall below 40°F, a less-efficient panel of electric resistance coils, similar to those in your toaster, kicks in to provide indoor heating. This is why air-source heat pumps aren’t always very efficient for heating in areas with cold winters. Fuel-burning furnaces generally can provide a more economical way to heat homes in cooler U.S. climates.
The efficiency and performance of today’s air-source heat pumps is one-and-a-half to two times greater than those available 30 years ago.
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