Conquering Heat Pump Phobia
Conquering Heat Pump Phobia |
Knowing how to size, select and install an air-to-air heat
pump can make the ‘panic' reaction obsolete for installers
By Jack D. Bartell
Heat pumps are efficient, reliable alternatives to the high cost of straight electric heat – not to mention the escalating cost of systems that run on fossil fuels. However, some installers and service technicians are intimidated by these units and are reluctant to install them. Why? Because heat pumps are not as well understood as other systems.
In reality, the fundamentals of sizing, selecting and installing a heat pump are similar to those of a standard air conditioner. Technicians just need to use the correct tools. This article will attempt to shed some light in the hope of erasing that empty feeling the next time the boss says, “This one's a heat pump.”
Heat pumps are efficient systems that work year round to supply cool air in summer and warm air in winter. During summer, an air-to-air heat pump uses refrigerant to remove heat from the indoor air and exhaust it to the outdoors.
The procedure reverses in the winter, with refrigerant removing the heat present in outdoor air and transferring it to the indoor air, thus warming the space. Even at 32° F there is plenty of heat in the outdoor air to warm a home using a heat pump.
As is true with all comfort systems, success is tied to a correctly applied unit. An undersized unit will not provide the capacity needed to heat and/or cool the space. In heating mode, an oversized heat pump will be short on efficiency, resulting in higher energy costs.
In cooling mode, an oversized unit quickly will lower the temperature in a space, satisfying the thermostat. However, these short run cycles prevent the unit from running long enough to remove the moisture or latent heat from the air. This “short cycling” can lead to early equipment failure and unhealthy indoor environments, such as mold and mildew.
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Design Calculations |
Regardless of whether the unit will go into a new home or it is a retrofit, the system designer must carefully calculate the heat loss and heat gain (both sensible and latent) for the space. ACCA Manual J Version 8, which has recently been approved as an ANSI standard, is an excellent choice to complete this vital task.
Manual J looks at a wide number of variables. They include:
• Construction materials.
• Installation techniques.
• Physical size of the structure.
• Difference between the inside and outside temperatures.
• Ductwork location and leak rate.
• Amount and location of insulation.
• Number of occupants.
• Number of appliances inside the structure that produce heat.
• Ventilation patterns and areas of the structure that are exposed to the outside (windows, doors, roof, cracks or joints in construction).
With an accurate load calculation, technicians can use the ACCA Manual S guidelines for selecting equipment to meet their needs. All manufacturers provide equipment-rating data at Air-Conditioning and Refrigeration Institute (ARI) design conditions.
However, most applications have different design criteria than ARI's 95° F outdoor temperature with 80° F dry bulb and 67° F wet bulb temperatures across the evaporator. Manual S explains how to use the extended data supplied by a manufacturer to correctly identify how much sensible capacity, latent capacity, heating capacity and airflow the equipment can provide per the technician's design conditions.
The size of the duct system and selecting terminations are also important. ACCA Manual T offers guidelines on properly selecting diffusers, grilles and registers, based on performance data supplied by the manufacturer. ACCA Manual D uses the heat loss and gain, along with the performance information from the selected hvac equipment and terminations, to calculate the air volume (cfm) needed in each room.
When selecting terminations, give careful consideration to location. Discharge or supply air temperatures from heat pumps are typically lower than fossil fuel systems. If the diffusers or registers are not carefully selected, homeowners often complain about “cold blow” or drafting. Even if the equipment is providing the capacity required to maintain the thermostat set point, cool air from drafting reduces comfort, leading to a dissatisfied customer.
New construction provides the designer with the best opportunity to deliver maximum comfort. Retrofit applications will require evaluating the existing ductwork and the location of existing supply and return air registers.
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Heat Pump Placement |
After selecting the heat pump that best suits a particular application, a technician should give careful consideration to where the equipment is placed. Air-to-air heat pumps have an outdoor unit and an indoor unit.
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Proper installation procedures must go hand-in-hand with design calculations to obtain optimum comfort in the home
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Locating the outdoor unit on the north side of the structure or in a shady area that is free of obstructions will improve the efficiency, especially for homes located in warmer climates. The unit also should be placed at a distance from heat generating equipment or materials and in an area where noise will not impact building occupants or neighbors.
It is normal for frost to build up on the outdoor heat pump coil. When this occurs, the unit's defrost function reverses the heat pump cycle to redirect hot gas to the outdoor coil to melt the frost. On some air-to-air heat pumps, a demand defrost control activates the defrost function only when it detects frost on the coil.
Other units use a time-temperature control that activates the defrost function at scheduled intervals during a certain time period, whether or not there is frost on the coil. In either case, it is important to place the outdoor unit where it will not be subject to run-off from a gutter or eave that brings the potential for abnormal frost buildup on the outdoor coil.
Refrigerant lines also must be sized appropriately for the application. Technicians should always follow the equipment manufacturer's recommended maximum pipe lengths. Here are a few things technicians should consider when sizing the vapor line:
• Avoid pressure drops that exceed 3 pounds because every pound of pressure drop results in a 1 percent capacity loss.
• For R-22 systems, maintain a minimum velocity on horizontal runs of 700 fpm.
• For R-22 systems, maintain a minimum velocity on vertical risers of 1,000 fpm.
• Don't exceed 3,000 fpm to avoid noise and vibration.
• On vertical risers between 3 and 50 feet, a single oil trap at the bottom will aid with oil return.
With heat pumps, horizontal runs should be level, as the vapor will travel in both directions. On cooling only systems, pitch the line toward the compressor.
When sizing the liquid line, consider these suggestions:
• Don't exceed 30 pounds of pressure drop. Every 3 pounds of pressure drop will result in a 1° F loss of subcooling. Many high efficiency units only produce 10° F of subcooling at the condenser, so exceeding 30 pounds of pressure drop would result in flash gas at the metering device.
• Don't exceed 300 fpm of velocity, which may result in damage to a solenoid valve.
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An Installation Checklist |
When brazing the refrigerant lines, technicians should deliver a small amount of nitrogen through the lines to purge any oxygen in the refrigerant lines. This action prevents oxidation of the copper and the formation of scale that could damage the system.
After brazing the lines and carefully checking for leaks in the system, technicians should evacuate the lines down to at least 500 microns. Remember, a properly maintained vacuum pump, with clean oil, will greatly reduce the time needed to obtain an acceptable vacuum level.
Some heat pumps tolerate small variations in refrigerant charge and continue to provide satisfactory heating or cooling. Eventually, however, an incorrect refrigerant charge takes its toll on the efficiency and the effectiveness of the system. Therefore, it is necessary to verify the refrigerant charge when the system is up and running and make adjustments based on the length of the refrigerant lines.
Lines longer than those recommended by the manufacturer may require an additional charge, while refrigerant may need to be removed from shorter lines. To change the amount of refrigerant in the lines, technicians should use a set of refrigerant scales. In doing so, they will increase both the efficiency and the life of the heat pump.
Technicians should complete the installation by checking their work. Things to look for include:
• Is the equipment properly insulated?
• Do you have good airflow through each of the registers? If not, adjust the registers to create patterns that are comfortable and quiet.
• Use thermometers to perform a delta T or temperature rise across the indoor and outdoor coils.
• Record the refrigerant pressures, temperatures, amperage, superheating and subcooling temperatures, suction pressure, and the length and diameter of refrigerant lines. These findings should be compared to the numbers the manufacturer recommends for the heat pump that was installed. The measurements should be close to those of the manufacturer.
• Be certain the installation is in compliance with all codes, standards, ordinances and regulations.
Please note: Read the manual. No matter how much care the installer takes during the installation of the heat pump, mistakes are more likely to occur if the manufacturer's installation manual is ignored.
Training programs and materials are available from manufacturers and their authorized representatives. Use them to provide step-by-step installation instructions for specific models and refer to them to check the numbers that are associated with sizing the heat pump and measuring its efficiency.
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Educate The Homeowner |
Finally, the installation process should include homeowner education, including proper operation of the system and maintenance procedures. It would be helpful, for example, if homeowners inspected the air filters on a monthly basis and regularly checked their outdoor units to be certain they are free of debris (including leaves, grass, paper, snow and anything else that can obstruct the airflow) and that coils are clean.
It is also important for homeowners to understand that the discharge temperature of a heat pump (unlike that of a gas furnace) is determined by how much heat is in the outside air. As temperatures drop, the heat pump's discharge temperature also drops and supplemental electric heat is added to temper or heat the air.
Furthermore, make sure you are aware of advances in technology, such as the recent introduction of dual-compressor systems. These heat pump systems use two cylinders to produce discharge temperatures that are on average 10° F to 15° F warmer than discharge temperatures of standard heat pumps, resulting in greater comfort and less dependence on supplemental heating.
Heat pumps are efficient, reliable alternatives to electric heat. Although these units may intimidate some installers, heat pumps are very similar to the standard air conditioner.
Once technicians overcome their fear and learn more about heat pumps and how to use the correct tools, they will feel comfortable installing and maintaining these units. So, the next time the boss says, “This one's a heat pump,” the installer can respond, “No problem.”
Jack Bartell is manager of training services for York International's Unitary Products Group. He has worked in the hvac industry for more than 30 years as a service technician on the contracting side and as a distributor service manager.
By Jack D. Bartell
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