Heat Pumps in the Hill Country
Heat pumps are gaining popularity as the most cost effective systems available today to heat your home. One of the major factors for this is the skyrocketing and unpredictable price of natural gas. Though there are many different types of systems available, the common air-to-air split system heat pump is the best suited to our Hill Country environment. Heat pumps are used throughout the entire United States but are well suited to our moderate climate. Ground source, earth coupled, or geo-thermal heat pumps that use some sort of loop system, either directly buried or connected through a heat exchanger, can be more efficient, but have limited application here, due to our rocky terrain and the high cost of installation.
The design and installation of a new heat pump system must take into account several factors. Heat pumps perform better in well-insulated, well-sealed homes. This is mainly because the heat pump does not produce a "hot" heat. Supply air temperatures between 100ºF and 120ºF are typical for heat pumps. A typical gas or oil fired furnace heats up to about 150ºF before the blower turns on to circulate the air. Strip heat furnaces also discharge fairly hot air. There are other special considerations when installing a new heat pump system. Warm air rises and cool air, being heavier, sinks to the floor. For maximum comfort and air distribution, cooling systems should have the return duct at the ceiling level and the supply ducts also at the ceiling level. The cold supply air sinks and cool the entire room. Conversely, for heating systems, the best design would have the return duct located low to pull in the cool air and then discharge the warm air at the floor level where it would rise and heat the entire room. Since our climate requires much more time spent in the cooling mode than the heating mode, the duct systems in most local homes are designed for optimum airflow for cooling. For some customers, duct systems need to be tailored to better meet their needs. For older, or less active homeowners, more heat is needed at floor level. The situation can be worsened by house design as many modern homes have very high ceilings. As a compromise from the usual high return/high supply cooling system, the heating performance can be greatly enhanced by simply providing for a low return. If possible, dual returns, both high and low, can be installed. In the cooling cycle, the low return can be blocked and in the heating cycle the upper return can be blocked.
Heat pumps, like any other air conditioning systems, should be properly sized to match the cooling load of the home. A properly sized heat pump system will normally be slightly under-sized for the heating load of the house. Auxiliary, or supplemental resistive heat-strips will have to be installed to make up the difference in the heating capacity of the heat pump and the design heat loss of the home. The balance point is the point at which the heat pump cannot produce enough heat to maintain the inside temperature of the house at the thermostat setting. The actual balance point of each house depends on several factors, which include: building envelope, insulation levels, air infiltration and thermostat settings. ; The balance point may be between 15ºF and 35ºF. At the balance point, the supplemental heat strips installed in the air handler, controlled automatically by the thermostat, will turn on to help the heat pump maintain the inside temperature at the thermostat set-point. Homeowners must realize that at the balance point, the heat pump will run continuously. However, even with continuous operation, the heat pump is more economical than a strip heat furnace. Most thermostats have small indicator lights, usually blue, green or red, to show when the supplemental heat is on. Also, there may be an indicator light, usually red, to show when the system is manually switched to "Em Heat" or "Emer Heat". That setting is called emergency heat and is to be used only in the case of outside unit malfunction. Some thermostats will also have another indicator light to show if the heat pump has malfunctioned, requiring a call to your service technician. When set in the emergency heat position, the thermostat switches off the heat pump and uses only strip heat to provide heat to the home. This provides heat until repairs can be made. When the supplemental heat or emergency heat light is on, the energy usage increases tremendously.
The most efficient way to operate a heat pump is to find a comfortable thermostat setting and do not change it. The normal practice of setting the thermostat lower at night and then raising it in the morning is not recommended with heat pumps. Unless you purchase a special thermostat with "intelligent recovery", the morning warm-up that brings on the supplemental heat will use up any savings gained by setting back the temperature at night. Thermostats are available that will lock out the strip heat above a certain outdoor temperature or will allow thermostat setting changes without bringing on the strip heat. Also, programmable thermostats that regulate the temperature to differing settings at different times or days are available. Your air conditioning service company can install these different types of thermostats.
The normal operation of an air source heat pump is basically the same as a regular air conditioner. A compressor circulates refrigerant, usually called "Freon" (which is a brand name for DuPont) through inside and outside coils with fans blowing air over them to transfer the heat. During summer operation the inside coil is cold and the outside coil is hot. The heat pump has a special valve, called a 'reversing valve", that allows the flow of refrigerant to be reversed in the winter. During the heating mode the inside coil gets hot and heat is transferred into the home. The outside coil gets cold so it can pick up heat from the outside air. Since the coil temperature can be below 32ºF, any moisture in the outside air may freeze on the coil. The frost on the coil is normal and will be removed periodically by an automatic defrost controller. During the defrost cycle, the outside fan will stop and the reversing valve will switch to send the hot refrigerant to the outside coil to melt the ice. During this defrost cycle the supplemental heat strips are normally energized to keep the inside air from getting cold. As the ice melts from the outside coil the unit will appear to be overheating and steam may rise from the unit making it look like it is smoking. This is a normal cycle and will be repeated as necessary depending upon the outside air temperature and humidity. Older units used a clock-type defrost control much like a household refrigerator. Newer systems utilize solid-state controls to defrost only as needed. Our normal weather patterns are such that very little time is spent in the defrost cycle. If your outside unit continually looks like a little igloo, the defrost system needs to be repaired as the system operates at less than peak efficiency while covered in ice.
Heat pumps are becoming more efficient and more reliable than ever; but still require periodic maintenance to keep them operating at maximum efficiency. Regular air filter changes are the best and simplest thing the homeowner can do. Periodically having the entire system professionally serviced assures that potential problems are noted and corrected.
Heat pumps are continually gaining acceptance as most large manufactures offer specialized training to field installers and service technicians. Proper installation by a qualified technician is needed for efficient operation. A poorly installed "high efficiency" system will be less efficient than a properly installed "standard" system. It is also necessary to install matched equipment, both inside and outside to get the rated performance and efficiency. In times past, air conditioning dealers would routinely replace a defective outside unit without replacing the inside unit. That would often result in decreased life expectancy for the outside unit and operation at less than advertised efficiency ratings.
The Air-Conditioning & Refrigeration Institute (ARI) rates all residential air conditioning systems. The efficiencies of various types of units can be compared for heat output and energy usage. The output of heat pumps is rated at 47ºF and at 17ºF. Typical heat pumps are three times more efficient at 47ºF and more than twice as efficient as regular strip heat (also called resistance heat) at 17ºF. Strip heat can be thought of as a giant toaster element that glows red-hot with a fan blowing over it. The ARI ratings show that the efficiency of a heat pump does drop as the outside temperature goes down; but, even at 17ºF, which is about the normally lowest temperature seen here, the heat pump is still producing more heat output per energy used than strip heat.
Residential air conditioning and heat pump systems are rated according to Seasonal Energy Efficiency Ratio (SEER). The minimum rating now available is 10 SEER. That means that each watt of electricity, as measured by your electric meter, produces 10 Btu's of cooling. (A Btu, British thermal unit, is an industry standard measure of cooling capacity.) A 12 SEER is basically 20% more efficient than a 10 SEER in that it produces 12 Btu's per watt. SEER ratings as high as 14 to 16 are available, but the increased cost is substantial. Homeowners must decide if the potential savings in energy consumption within their payback period will offset the increased cost. Even a 10 SEER system can be 10% to 25% more efficient than an existing system if it is older than 3 to 5 years. Systems that are 10 years old or older are probably in the 6 to 8 SEER range. Customers with older systems can substantially decrease the heating & cooling portion of their electrical consumption by installing a new heat pump system. Along with lower utility bills, customers can also expect a cleaner, more reliable, quieter, more comfortable system with increased resale value added to the home.