Inspecting Compression Cooling Systems

According to InterNACHI’s Home Inspection Standards of Practice, a home inspector is required to inspect the cooling system using normal operating controls. Let’s review the process of how HVAC contractors install and test cooling systems for new construction and existing homes, which can give home inspectors some key facts for understanding and inspecting compression cooling systems.

Vapor-compression refrigeration (compression cooling) systems are the most common type of cooling equipment used to cool residential and commercial buildings. Compression cooling is often referred to as air conditioning, although, technically, any system used to intentionally heat, cool or ventilate the indoor air could be referred to as an air-conditioning system.

Home inspectors should note that residential compression cooling systems include any system that uses the refrigeration cycle for space cooling; this includes split-system air conditioners (Figure 1.), unitary air conditioners, air-source heat pumps, and ground-source (or geothermal) heat pumps, in system sizes up to 65,000 BTUH with forced-air distribution systems.

Non-compression cooling systems include evaporative cooling  and absorption systems. However, compression cooling systems are most common. According to the latest data from the U.S. Energy Information Administration (data collected in 2009 and released in 2013), 94 million out of 113.6 million households had some kind of cooling equipment – 69.7 million of those homes had central compression-based air conditioners (19% of these were heat pumps), 25.9 million had window or wall-unit air conditioners, and 2.8 million had an evaporative or swamp cooler. Because of their energy-saving potential, passive cooling techniques (such as night ventilation cooling systems, ceiling fans, and shading with architectural and landscaping features) are options worth considering.

The compression cooling cycle, especially as it applies to split-system central air conditioners, is described below. Traditional split heat pumps, mini-split heat pumps, and geothermal heat pumps are described in more detail in other guides, as are evaporative cooling and absorption cooling.

Figure 1. Split-system air conditioning

Home inspectors should remember that central air conditioners are typically installed with central furnaces and use the same blower and duct distribution system. Residential air conditioners are typically split systems (see Figure 1), which refers to the fact that there is an outside unit and an inside unit: the condenser and compressor are part of an outside unit, and the evaporator and expansion valve are located within the air handler in the inside unit (see Figure 2). Refrigerant is piped to the evaporator coil in the air-handler unit, where it cools the distribution air (see Figure 3).

Window or wall-mounted room air conditioners contain all of the elements in one box. Larger single-unit air conditioners, called packaged-unit air conditioners, contain all of the components in one unit (the compressor, condenser, evaporator, and expansion device), which is located outside, typically mounted on the wall or on the roof (see Figure 4). The conditioned air is vented inside either directly into a room or into a duct system for distribution throughout the building. Many small commercial buildings are equipped with packaged units, but they’re rarely used for single-family homes.

Figure 2. A traditional split-system air source central heat pump has an outdoor unit with a condenser and compressor, and an indoor air-handler unit with an evaporator coil, metering  device, and blower fan.

Figure 3. The refrigeration cycle

Figure 4. A packaged unit heat pump or air conditioner contains the compressor, condenser, evaporator coil, and metering device in one roof- or wall-mounted unit.

Packaged air conditioners (see Figure 4) and heat pumps come from the factory ready to go as soon as they’re connected to a duct system, thermostat, and power source. Home inspectors should make sure that split systems are plumbed, i.e., the indoor unit must be connected to the outdoor unit via refrigerant piping, and the electrical wiring must be connected to both units.

Air Conditioner Capacity and Sizing

Air conditioners are sized by their capacity in terms of tons. One ton equals 12,000 BTU per hour (or BTUH) of cooling capacity. The capacity is often indicated in the model number. Contractors will look at the name plate on the outdoor condensing unit and locate the model number (not the serial number) find the two digits in the model number that match the numbers below it to indicate tons or BTUH. For example, a model SSX160241 is a 2-ton (24,000 BTUH)-capacity air conditioner.

• 18 = 1.5 tons or 18,000 BTUH
• 24 = 2 tons or 24,000 BTUH
• 30 = 2.5 tons or 30,000 BTUH
• 36 = 3 tons or 36,000 BTUH
• 42 = 3.5 tons or 42,000 BTUH
• 48 = 4 tons or 48,000 BTUH
• 60 = 5 tons or 60,000 BTUH

Proper sizing of air conditioners has become more important in recent decades as homes are built to be more airtight and better insulated. HVAC contractors and home inspectors can no longer rely on guidelines based solely on an estimate of square footage. Whereas an older two-story 3,000-square-foot home might have required two 3-ton units, a newer 3,000-square-foot high-performance home might be adequately served by one 3-ton unit with zone dampers.

An overly large system will blast on quickly, bringing the air temperature below the thermostat set point, and shutting off before it has had time to remove moisture from the air, which can cause moisture problems in the home, especially in humid climates.

Figure 5. Natural Resources Canada Climate Zone Map of the U.S and Canada.

The HVAC contractor should calculate the home’s cooling load and correctly size the central air-conditioning system. Oversizing is less of an issue with cooling equipment that has variable speed motors and compressors, which can operate at lower speeds and capacities that better match low demand times, while having the ability to increase capacity when demand spikes. Even so, oversizing of two-speed and variable-speed units should be limited to 20% over the calculated size.

The HVAC equipment that is installed must be a matched system, as certified according to the Air-Conditioning, Heating, & Refrigeration Institute (AHRI). AHRI is an industry association that assigns a certification number and efficiency rating to specific combinations of equipment (outdoor unit, indoor unit, indoor coil, fan type, etc.), which have been tested by the manufacturer according to AHRI test procedures and specified test conditions. Home inspectors should note that proper matching of system components according to AHRI is one of the items that will be confirmed by a Residential Energy Services Network (or RESNET) rater when the home is assessed for a Home Energy Rating System or HERS score.

Many designers use the performance data listed on the AHRI certificate for selecting equipment to meet the home’s design cooling load. However, a more accurate method that professionals use to obtain performance data at design conditions (and one required by ACCA Manual S) is the original equipment manufacturer (OEM) expanded performance table. AHRI uses a specific set of conditions (95° F outdoor, 80° F indoor, and 67° F wet bulb) when determining the equipment performance data, such as heating and cooling capacity, and SEER and EER cooling efficiencies; these performance data are then listed on the AHRI certificate. The OEM expanded performance tables use design cooling conditions that include an indoor temperature of 75° F and 63° F wet bulb. Several manufacturers refer to this as TVA conditions because they were originally set by the Tennessee Valley Authority in the 1970s. Under these design conditions, cooling equipment will usually have a lower SEER than under AHRI conditions.

With a central air-conditioning system, the cooled air will be distributed by ducts, so it’s important for professionals to design an efficient air-distribution system with a compact layout in accordance with ACCA Manual D, which is something home inspectors can check for. Home inspectors should keep in mind that good installation (with short, straight runs and air-sealed and insulated ducts) will allow for maximum air flow and efficiency.

For optimum performance, home inspectors should make sure that the ducts and air handler is located within the home’s thermal boundary, which is a DOE Zero-Energy Ready-Home requirement.

Some new homes are so well air sealed and insulated that they can be considered low-load homes (for over 1,000 square feet of floor space per each ton of cooling). While older, less airtight, less well air sealed homes might require two or more cooling units or one large unit (such as a 5-ton unit), with well insulated homes, one smaller unit (perhaps 2 or 2.5 tons) might do. Whether the house is older or newer is something home inspectors should consider when inspecting compression cooling systems.

For larger homes with one unit, zone dampers are recommended. The dampers are located near the air handler unit at the base of each branch duct that will serve a zone. The dampers communicate electronically with a computer that is integrated with the thermostats located in each zone. Dampers are a good idea for several reasons. They save energy because different temperatures can be set for different zones, and cooling can be cut off at seldom-used areas of the home. They also help provide more air flow where needed. For example, while a larger 5-ton unit might have 2,000 cfm of air flow, a smaller 2-ton unit would have about 800 cfm of air flow available, so dampers that close some zones make more air flow available to the zones calling for cooling. 

Home inspectors should keep in mind that the HVAC system sizing should be based on the heating or the cooling system, whichever is more in demand in the climate zone (see Figure 5). Both the DOE Zero-Energy Home Program and ENERGY STAR allow designers to oversize furnaces by up to 150% to satisfy the air flow requirements of the cooling system. Home inspectors should familiarize themselves with cooling systems, as they should not be oversized.

The Refrigerant Cycle

The vapor-compression refrigeration system (see Figures 3 and 6) uses a circulating liquid refrigerant as the medium that absorbs heat from the indoor air and rejects the heat outside. Figure 6 shows the path of the refrigerant as it cycles through a typical single-stage vapor-compression air conditioner’s indoor and outdoor components. The figure depicts an air conditioner only. If the unit were part of a furnace, the furnace burner would be on the negative or return side of the blower, and the cooling evaporator would be on the positive or supply side of the blower.

Figure 6. Liquid refrigerant cycles through the evaporator coil inside the air handler, pulling heat from the air that circulates through the house as the refrigerant evaporates. The vapor is transferred outside, where it passes through the condenser and condenses, releasing heat to the outdoors, then the refrigerant returns to the inside unit as a liquid where it starts the cycle again (image courtesy of CalcsPlus).

Home inspectors should note that all compression cooling systems have four main components: a compressor, a condenser, a metering device known as a thermal expansion valve (TXV) or fixed orifice, and an evaporator coil (see Figure 3). Circulating refrigerant moves through the suction line and enters the compressor as a saturated vapor. In the compressor, it is compressed to a higher pressure and higher temperature. The now super-heated vapor is routed through the condenser coil, where it is cooled by flowing air, and condensed back into a liquid, releasing heat that is carried away by the flowing air.

Figure 7. Compressor

The liquid refrigerant is carried back to the indoor unit, where it passes through an expansion valve. The expansion valve (or metering device) causes the liquid refrigerant to experience an abrupt drop in pressure and temperature as it enters the evaporator coil. In the coil, the liquid absorbs heat from the circulating house air, thus cooling the house air that is passing through the air handler. The heat causes the refrigerant to return to vapor, and the vapor is again routed outside to the evaporator, and the cycle repeats.

Measuring the Efficiency of Cooling Systems

The efficiency of compression cooling systems is measured in SEER, EER and COP. Home inspectors should familiarize themselves with the following ratios:

• Seasonal Energy Efficiency Ratio or SEER:  The SEER rating of a unit is the cooling output during a typical cooling season divided by the total electrical energy input during the same period. The higher the unit's SEER rating, the more energy efficient it is. In the United States, the SEER is the ratio of cooling in British thermal units (BTU) per hour to the energy consumed in watt hours.
  
  
• Energy Efficiency Ratio or EER:  The EER of a particular cooling device is the ratio of output cooling in BTUH to input electrical power in watts at a given operating point.
  
  
• Coefficient of Performance (COP):  The COP (sometimes referred to as CP) of a heat pump is the ratio of the heating or cooling provided over the electrical energy consumed. The COP provides a measure of performance for heat pumps that is analogous to thermal efficiency for power cycles.

Technology improvements in recent years have made air conditioners much more efficient. These improvements include variable-speed fan motors, variable refrigerant flow technology, advanced compressors, and micro-channel heat exchangers. These changes enable the air conditioner to ramp up or ramp down, rather than just turning on or off like a single-speed split-capacitor motor does. By better matching the fluctuations in demand, the newer models improve efficiency, lower energy consumption, and increase comfort.

Since 2006, the federal government has required new air conditioners sold in the United States to have a SEER rating of 13 or higher. In 2011, these standards were amended to require split-system heat pumps and single-package air conditioners (but not split-system central air conditioners) to meet a SEER of 14 (if manufactured in 2015 or later).

To receive an ENERGY STAR label, a split-system air conditioner must have a SEER of at least 14.5. Home inspectors should note that the best available central air-conditioning units can have SEER ratings of over 20.

 Installation Concerns

How the installation and connection of the copper tubing for the refrigerant lines is performed is critical to the life expectancy of the compressor. During new construction, the copper tubing should be roughed in early on, before or during duct installation. The equipment (indoor unit and outdoor unit) is typically installed and connected to the copper tubing toward the end of construction. This means the copper lines can lay unconnected for quite some time.

Copper tubing used for refrigerant lines is sold in 50-foot rolls. The tubing is dried out (dehydrated) and sealed at both ends before shipping. Water is the enemy of the refrigeration system. If water vapor is allowed to enter the refrigeration lines during construction, it will greatly reduce the life of the compressor and create problems with metering devices and check valves. The oil used as lubricant in refrigeration systems is highly hygroscopic, which means that it wants to absorb moisture. If the lubricant mixes with water or water vapor, it creates an acidic sludge that eats away at the compressor’s windings, causing burnouts.  This sludge can also block orifices and valve openings inside the system.

During the rough-in installation, home inspectors can check to make sure that the open ends of the copper tubing are kept sealed at all times. When the installation is completed, the lines should be charged with dry nitrogen and soldered closed. After connecting the indoor unit and the outdoor unit, the lines should be vacuumed to 500 microns to remove air pockets, which can reduce heat transfer and cause erratic operation (see Figure 9).

Proper refrigerant charging is critical for maximizing the performance of compression cooling equipment. Too much or too little refrigerant can reduce the efficiency of the equipment and lead to premature component failures. Home inspectors should note that there are three methods for refrigerant charging: the sub-cooling method (typically for units with a thermal expansion valve); the super-heat method (for units with a fixed orifice); or the weigh-in method (using the refrigerant weight amount listed on the data plate on the outdoor unit). The charging method recommended by the manufacturer should be used. Refrigerant charging must be done by an EPA-certified technician.

 Figure 8. A vacuum pump removes air from refrigerant lines in a
 compression cooling system prior to startup.

How Professionals Should Select and Install Compression Cooling Equipment

Choose the highest-performing air conditioner that project costs will allow in order to meet the design cooling load. If the design load is low (<14,000 BTU), consider alternative lower-load cooling sources, such as ductless heat pumps with variable refrigerant flow technology. If you are participating in an energy efficiency program, select cooling equipment that complies with the requirements for the climate zone.

1. Install in accordance with the manufacturer’s instructions and relevant standards.
2. Properly size the cooling equipment for the design cooling load of the home. Calculate the cooling load to correctly size the system. This is especially important if there has been significant air sealing and insulating performed, which will reduce the heating and cooling load.
3. Design an efficient air distribution system with a compact layout. Install ducts properly for maximum air flow and efficiency.
4. Charge the copper tubing with dry nitrogen, seal the open ends with solder, and keep the tubing sealed at all times during the rough-in installation to prevent moisture from entering the lines.
5. After connecting the indoor unit and the outdoor unit, vacuum the lines to 500 microns to remove air pockets.
6. Follow the manufacturer’s recommendations for refrigerant charging. Verify that the correct charging method is being used for the specific air conditioner model to be installed. Refrigerant charging must be done by an EPA-certified technician.
7. Set the time-delay relay on the unit to 30 seconds or less in humid climates to prevent moisture on the evaporator coil from evaporating back into the airstream and contributing to indoor humidity. Set the fan on the central air conditioning systems to auto, rather than to on, for the most efficient operation. Set the compressor to start before the blower.
8. Make sure the drain pans are correctly installed.
9. Test for air flow and duct leakage.

Summary

Vapor-compression refrigeration (compression cooling) systems are the most common type of cooling equipment used to cool residential and commercial buildings. Compression cooling is often referred to as air conditioning, although technically any system used to intentionally heat, cool or ventilate the indoor air could be referred to as an air-conditioning system.

Home inspectors should remember that central air conditioners are typically installed with central furnaces and use the same blower and duct distribution system, and that residential air conditioners are typically split systems. Air conditioners are sized by their capacity in terms of tons, with 1 ton equaling 12,000 BTUH of cooling capacity. Proper sizing of air conditioners has become more important in recent decades as homes have become more airtight and better insulated. 

Technology improvements in recent years have also made air conditioners much more efficient, with the efficiency of compression cooling systems is measured in SEER, EER and COP. How the installation and connection of the copper tubing for the refrigerant lines is performed is critical to the life expectancy of the compressor. Home inspectors can familiarize themselves with these practices when they do in-progress inspections of new construction, as well as inspections of existing homes.