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Compression Cooling

Scope

Choose the highest performing cooling equipment that project funding will allow, to meet the cooling load of the project.

If the design cooling load is low (below around 14,000 Btuh capacity) due to high insulation and air sealing levels or climate, consider alternative lower-load cooling sources such as ducted or ductless variable refrigerant flow heat pumps. In dry climates, consider direct/indirect evaporative coolers and ventilation cooling. Also consider passive cooling techniques such as shading with architectural and landscape features.

Confirm that the selected system is a matched system, as certified according to the Air-Conditioning, Heating, & Refrigeration Institute. AHRI assigns a certification number and efficiency ratings to specific combinations of equipment (outdoor unit, indoor unit, indoor coil, fan type, etc.) that have been tested by the manufacturer according to AHRI test procedures using AHRI-specified test conditions (AHRI 2012).

Install in accordance with the manufacturers’ instructions and relevant standards including ACCA Standard 5: HVAC Quality Installation Specification and the ACCA’s Technician's Guide for Quality Installations and ACCA Standard 9: HVAC Quality Installation Verification Protocols.

Properly size the cooling equipment and ducts for the design cooling load of the home, following the sizing guidelines in ACCA’s Manual S: Residential Equipment Selection. When determining equipment sizing per ACCA Manual S, use the original equipment manufacturer (OEM)’s expanded performance table to obtain performance data at design conditions, rather than using the performance data on the AHRI certificate, which lists heating and cooling capacity and SEER and EER cooling efficiencies at factory conditions of 90°F outdoor, 80°F indoor, and 67°F wet bulb.

The OEM-listed capacity at design conditions should be between 95% and 115% of the design total heating load calculated in the Manual J HVAC load calculation, or the next nominal size. The next largest nominal piece of equipment available may be used to satisfy the latent and sensible requirements. Single-speed systems generally have OEM nominal size increments of one-half ton. Multi-speed or multi-stage equipment may have OEM nominal size increments of one ton. Therefore, the use of these advanced system types can provide extra flexibility to meet the equipment sizing requirements.

If the whole-house ventilation system uses the air conditioner air handler, then the fan motor should be a variable speed electronically commutated motor (ECM) or an integral control motor (ICM) that includes a controller (e.g., a smart cycler) that reduces the ventilation run time by accounting for hours when the HVAC system is already operating the fan for heating or cooling the home.

Design an efficient air distribution system with a compact layout in accord with ACCA Manual D. Install ducts properly for maximum airflow and efficiency. Consider zoning for low-load homes (over 1,000 sq ft per ton of cooling) with thermostat-controlled dampers. 

If you are participating in an energy-efficiency program, select cooling equipment that complies with the requirements for your climate zone. To determine your climate zone, see the International Energy Conservation Code (IECC) 2012 climate zone map on the Climate tab.

To determine your climate zone, see the International Energy Conservation Code (IECC) climate zone map on the Climate tab. 

See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.

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Description

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. Residential compression cooling systems include any system that uses the refrigeration cycle for space cooling; this includes split-system air conditioners, 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. Noncompression cooling systems include evaporative cooling and absorption systems. However, compression cooling systems hold the lion's share of the U.S. housing market. 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 (EIA 2013). Other cooling options available to consumers that are worth considering because of their energy-saving potential include passive cooling techniques such as shading with architectural and landscape features, night ventilation cooling systems, and ceiling fans (Gilbride et al. 2011).

The compression cooling cycle, especially as it applies to split-system central air conditioners, is described here. 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.

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, 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 (Figure 1). Refrigerant is piped to the evaporator coil in the air handler unit where it cools the distribution air. 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 (the compressor, condenser, evaporator, and expansion device) in one unit that is located outside, typically mounted on the wall or on the roof (Figure 2). 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 are rarely used for single-family homes.

Traditional split-system

Figure 1. 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. (Image Courtesy of CalcsPlus)

Packaged unit

Figure 2. A packaged unit heat pump or air conditioner contains the compressor, condenser, evaporator coil, and metering device in one roof- or wall-mounted unit. (Image Courtesy of CalcsPlus)

Packaged air conditioners and heat pumps come from the factory ready to go as soon as they are connected to a duct system, thermostat, and power source. Split systems have to be plumbed, i.e., the indoor unit must be connected to the outdoor unit via refrigerant piping, and 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/hour of cooling capacity. The capacity is often indicated in the model number. Look at the name plate on the outdoor condensing unit and locate the model number (not the serial number). Look for two digits in the model number that match the numbers below to indicate tons or Btus/hour. For example, a model SSX160241 is a 2-ton (24,000 Btu/hr) air conditioner.

18 = 1.5 Ton (18,000 Btu/hr)
24 = 2 Ton (24,000 Btu/hr)
30 = 2.5 Ton (30,000 Btu/hr)
36 = 3 Ton (36,000 Btu/hr)
42 = 3.5 Ton (42,000 Btu/hr)
48 = 4 Ton (48,000 Btu/hr)
60 = 5 Ton (60,000 Btu/hr)

Proper sizing of air conditioners has become more important in recent decades as homes have become more air tight and better insulated. HVAC contractors can no longer rely on rules of thumb based on a rough estimate of square footage. Where an older two-story 3,000-ft2 home might have required two 3-ton units, a new 3,000-ft2 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. The HVAC contractor should use ACCA’s Manual J: Residential Load Calculation to calculate the home’s cooling load and ACCA’s Manual S: Residential Equipment Selection to correctly size the central air conditioning system. ACCA makes available Excel-based spreadsheets to help contractors with these calculations. 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, the updated version of ACCA Manual S, to be released soon, limits oversizing of two-speed and variable-speed units 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 ratings 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 using AHRI-specified test conditions (AHRI 2012). See the AHRI Directory of Certified Products. Proper matching of system components according to AHRI is one of the items that will be confirmed by a Residential Energy Services Network (RESNET) rater when the home is assessed for a Home ENERGY Rating System (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 (and one required by ACCA Manual S) is to use the original equipment manufacturer (OEM)’s expanded performance table to obtain performance data at design conditions. 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, 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 is important to design an efficient air distribution system with a compact layout in accord with ACCA Manual D. Good installation (with short straight runs and air-sealed and insulated ducts) is important for maximum airflow and efficiency. See the guides listed under HVAC/R 2 Duct Quality Installation, HVAC/R 3 Duct Insulation, and HVAC/R 4 Duct Leakage in the ENERGY STAR Certified Homes Checklist for more information.

For best performance, the ducts and air handler should be located within the home’s thermal boundary (this is a DOE Zero Energy Ready Home requirement).

Some new homes are so well air sealed and insulated, they can be considered low-load homes (over 1,000 square feet of floor space per each ton of cooling). While older, less air-tight, less well air sealed homes might require two or more cooling units or one large unit, for example a 5-ton unit, with well-insulated homes, one smaller unit, perhaps 2 or 2.5 tons, might do.

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 communicates with 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 cooling can be cut to less-used areas of the home. They also help provide more airflow 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 airflow available, so dampers that close some zones make more air flow available to the zones calling for cooling. 

The HVAC system sizing should be based on the heating or the cooling system, whichever is more in demand in your climate zone. Both the DOE Zero Energy Home Program and ENERGY STAR will allow designers to oversize furnaces by up to 150% to satisfy the airflow requirements of the cooling system. Cooling systems should not be oversized.

The Refrigerant Cycle

The vapor-compression refrigeration system uses a circulating liquid refrigerant as the medium that absorbs heat from the indoor air and rejects the heat outside. Figure 3 shows the path of the refrigerant as it cycles through a typical, single-stage vapor-compression air conditioner’s indoor and outdoor components. This figure depicts an air conditioner only. If this 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.

Liquid refrigerant cycles

Figure 3. 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)

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. 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 “superheated” vapor is routed through the condenser coil where it is cooled by flowing air and condensed back into a liquid, releasing heat which is carried away by the flowing air.
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 turn to vapor again and the vapor is again routed outside to the evaporator, beginning the cycle all over again.  

Measuring the Efficiency of Cooling Systems

The efficiency of compression cooling systems is measured in SEER, EER, and COP. 

Seasonal Energy Efficiency Ratio (SEER) – the SEER rating of a unit is the cooling output during a typical cooling-season divided by the total electric 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 (EER) – the EER of a particular cooling device is the ratio of output cooling (in Btu/h) 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 would. By better matching fluctuations in demand, the newer models can 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 SEER 14 if manufactured on or after January 1, 2015. Several states in the south are requiring split system air conditioners to also meet the minimum SEER 14 by January 2015 (DOE 2014).

To receive an ENERGY STAR label, a split system air conditioner must have a SEER of 14.5 or greater. 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 is 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 havoc with metering devices and check valves. The oil used for lubricant in refrigeration systems is highly hygroscopic, which means that the oil wants to absorb moisture. If the lubricant mixes with water or water vapor, it creates an acidic sludge that eats away at compressor windings, causing burnouts; the sludge can also block orifices and valve openings inside the system.

During the rough-in installation, the open ends of the copper tubing should be 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 (Figure 4).

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. Use the charging method recommended by the manufacturer. There are three methods for refrigerant charging: the subcooling method (typically for units with a thermal expansion valve), the superheat method (typically 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). Verify that you are using the correct method for the specific air conditioning model to be installed. Refrigerant charging must be done by an EPA-certified technician.

Vacuum pump removes air from lines

Figure 4. A vacuum pump removes air from refrigerant lines in a compression cooling system prior to start up. (Image Courtesy of CalcsPlus)

How to Select and Install Compression Cooling Equipment

  1. Choose the highest performing air conditioner project costs will allow, to meet the design cooling load of the project. If the design load is low (e.g., <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 your climate zone, as described in the Compliance tab.
  2. Install in accordance with the manufacturers’ instructions and relevant standards including ACCA Standard 5: HVAC Quality Installation Specification, the ACCA’s Technician's Guide for Quality Installations, and ACCA Standard 9: HVAC Quality Installation Verification Protocols.
  3. Properly size the cooling equipment for the design cooling load of the home. Use ACCA Manual J to calculate your cooling load and use ACCA Manual S to correctly size your system. This is especially important if you have done significant air sealing and insulating, which will reduce your heating and cooling load.
  4. Design an efficient air distribution system with a compact layout in accord with the ACCA Manual D. Install ducts properly for maximum airflow and efficiency in accord with ACCA Manual D. See also the Building America Solution Center guides on duct installation, insulation, and air sealing in the ENERGY STAR Certified Homes Checklist.
  5. 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.
  6. After connecting the indoor unit and the outdoor unit, vacuum the lines to 500 microns to remove air pockets.
  7. Follow the manufacturer’s recommendations for refrigerant charging. Verify that you are using the correct charging method for the specific air conditioning model to be installed. Refrigerant charging must be done by an EPA-certified technician.
  8. 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 air stream and contributing to indoor humidity. Set the fan on the central air conditioning systems to “Auto” rather than “On” for the most efficient operation. Set the compressor to start before the blower. Make sure the drain pans are correctly installed.
  9. Test air flow and duct leakage.

Ensuring Success

Choose the highest efficiency SEER rating product possible.

Verify that the air handler is correctly matched to the outdoor unit. Matched systems can be verified at the AHRI website, listed under "Air Conditioners and Air Conditioner Coils" and "Heat Pumps and Heat Pump Coils."

Install in accordance with the manufacturers’ instructions and relevant standards including ACCA Standard 5: HVAC Quality Installation Specification and the ACCA’s Technician's Guide for Quality Installations and ACCA Standard 9: HVAC Quality Installation Verification Protocols. These standards address quality installation and commissioning requirements for vapor compression cooling systems, heat pumps, and combustion and hydronic heating systems.

Install the air handler and ducts within the home’s thermal envelope.

Keep the refrigerant lines sealed and dry during construction and after connecting the indoor and outdoor units vacuum the lines to 500 microns to remove air pockets.

Follow the manufacturer’s recommendations for refrigerant charging.

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 air stream and contributing to indoor humidity. Set the fan on the central air conditioning systems to “Auto” rather than “On” for the most efficient operation. Set the compressor to start before the blower. Make sure the drain pans are correctly installed.

Climate

For ENERGY STAR and DOE Zero Energy Ready Home climate-specific guidance, see the Compliance tab.

In mild or cold climates, consider non-compression cooling options such as trees, awnings, pergolas, and porches to shade windows and walls; ceiling fans; and timer-controlled night-time ventilation cooling (in dry climates) (Gilbride et al. 2011).

In humid and mild or cold climates, consider adding a dehumidifier for indoor humidity control in the shoulder seasons and in locations with short summers as an alternative to compression cooling.

In humid climates, set the time-delay relay on the unit to 30 seconds or less to prevent moisture on the evaporator coil from evaporating back into the air stream and contributing to indoor humidity.

To determine your climate zone, see the International Energy Conservation Code (IECC) climate zone map.

IECC climate zones

IECC Climate Zone Map

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Compliance

The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

2009 IRC 2012 IRC,  and 2015 IRC

Comply with all relevant sections of the applicable International Residential Code, including Chapter 14: Heating and Cooling Equipment.\

2009 IECC

403.1 Each heating and cooling system should have its own thermostat. If the primary heating system is a forced-air furnace, at least one thermostat must be programmable and capable of controlling the heating and cooling system on a schedule to maintain different temperatures at different times of the day.

403.2 Ducts - Insulate supply ducts in attics to at least R-8 and all other ducts to at least R-6. Duct tightness shall be verified as described in 403.2.2 Sealing.

403.6 Heating and cooling equipment sizing shall be in accordance with Section M1401.3 of the 2009 International Residential Code.

2012 IECC

403.1 Each heating and cooling system should have its own thermostat. If the primary heating system is a forced-air furnace, at least one thermostat must be programmable and capable of controlling the heating and cooling system on a schedule to maintain different temperatures at different times of the day.

403.2 Ducts - Insulate supply ducts in attics to at least R-8 and all other ducts to at least R-6. Duct tightness shall be verified as described in 403.2.2 Sealing. The air handler shall have a manufacturer’s designation showing air leakage is no more than 2% of the design air flow rate when tested in accordance with ASHRAE 193.

403.6 Heating and cooling equipment shall be sized in accordance with ACCA Manual S based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methods.

ACCA Standards

ACCA Manual S. Residential Equipment Selection, ANSI/ACCA 3-Manual S-2004, provides information on how to select and size heating and cooling equipment to meet Manual J loads based on local climate and ambient conditions at the building site. Manual S covers sizing strategies for all types of cooling and heating equipment, as well as comprehensive manufacturers’ performance data on sensible, latent, or heating capacity for various operating conditions.

ACCA Manual D: Residential Duct Systems, ANSI/ACCA 1-Manual D-2011, provides ANSI-recognized duct sizing principles and calculations that apply to all duct materials; the system operating point (supply cfm and external static pressure) and airway sizing for single-speed and multi-speed (ECM) blowers; a method for determining the impact of duct friction and fitting pressure drop on blower performance and air delivery; and equivalent length data.

ACCA Manual J: Residential Load Calculation, ANSI/ACCA 2-Manual J-2011, provides information for calculating heating and cooling loads for equipment sizing for single-family detached homes, small multi-unit structures, condominiums, town houses, and manufactured homes.

ACCA Standard 5: HVAC Quality Installation Specification, ANSI/ACCA 5 QI-2010, details nationally recognized criteria for the proper installation of residential and commercial HVAC systems, including forced air furnaces, boilers, air conditioners, and heat pumps. The Standard covers aspects of design, installation, and distribution systems, as well as necessary documentation. The Technician’s Guide for Quality Installation, produced by ACCA, explains the HVAC Quality Installation (QI) Specification and provides detailed procedures for the steps technicians must complete and document to show compliance with the HVAC QI Specification.

ACCA Standard 9: HVAC Quality Installation Verification Protocols, ANSI/ACCA 9 QIVP-2009, specifies the protocols to verify the installation of HVAC systems in accordance with ACCA Standard 5. The protocols provide guidance to contractors, verifiers, and administrators who participate in verification efforts using independent objective and qualified third parties to ensure that an HVAC installation meets the requirements in Standard 5.

RESNET Mortgage Industry National Home Energy Rating Systems Standards

Procedures and technical standards by which home energy ratings are conducted including home energy audits.

U.S. Department of Energy Zero Energy Ready Home

The U.S. Department of Energy’s Zero Energy Ready Home program allows builders to choose a prescriptive or performance path. The DOE Zero Energy Ready Home prescriptive path requires builders to meet or exceed the minimum HVAC efficiencies listed in Exhibit 2 of the National Program Requirements, as shown below. The DOE Zero Energy Ready Home performance path allows builders to select a custom combination of measures for each home that is equivalent in performance to the minimum HERS index of a modeled target home that meets the requirements of Exhibit 2 as well as the mandatory requirements of Zero Energy Ready Home Exhibit 1.

DOE Zero Energy Ready Home Notes:

(8) State energy code specifications that exceed the DOE Zero Energy Ready Home National Program Requirements always take precedence and shall be used instead of DOE Zero Energy Ready Home specifications to determine DOE Zero Energy Ready Home compliance.

(20)  Use the 2012 IECC Climate zone map.

(22) DOE recommends, but does not require, that cooling systems in hot-humid climates utilize controls for immediate blower shutoff after condenser shutoff, to prevent re-evaporation of moisture off the wet coil.

(23) Air source heat pumps with electric resistance backup cannot be used in homes qualified in Climate Zones 7 & 8 using the Prescriptive Path. 

ENERGY STAR Certified Homes

The ENERGY STAR Certified Homes program, Version 3 Rev 08, allows builders to choose a prescriptive or performance path.

The ENERGY STAR prescriptive path requires builders to meet the minimum HVAC efficiencies listed in Exhibit 1. The ENERGY STAR performance path allows builders to select a custom combination of measures for each home that is equivalent in performance to the minimum HERS index of a modeled reference home that meets the requirements of Exhibit 1 as well as the mandatory requirements of ENERGY STAR Exhibit 2.

Exhibit 1: ENERGY STAR Reference Design Home

Follow the criteria in the ENERGY STAR HVAC System Quality Installation Contractor and Rater Checklists.

ENERGY STAR Footnotes:

(1) This Checklist applies to ventilation systems, split air conditioners, unitary air conditioners, air-source heat pumps, and water-source (i.e., geothermal) heat pumps up to 65,000 Btu / h with forced-air distribution systems (i.e., ducts) and to furnaces up to 225,000 Btu / h with forced-air distribution systems (i.e., ducts). All other permutations of equipment (e.g., boilers, mini-split / multi-split systems) and distribution systems are exempt. If the ventilation system is the only applicable system installed in the home, then only Section 1 shall be completed. One Checklist shall be completed for each system and provided to the Rater.

(8) Heating and cooling loads shall be calculated, equipment shall be selected, and duct systems shall be sized according to the latest editions of ACCA Manuals J, S, & D, respectively, 2009 ASHRAE Handbook of Fundamentals, or other methodology approved by the Authority Having Jurisdiction. The HVAC system design shall be completed for the specific configuration (e.g., plan, elevation, option, and orientation) of the home to be built except as permitted herein.

For each house plan with multiple configurations (e.g., orientations, elevations, options), the loads shall be calculated for each potential configuration. If the loads across all configurations vary by ≤ 25%, then the largest load shall be permitted to be used for equipment selection for all configurations, subject to the over-sizing limits of ACCA Manual S. Otherwise, the contractor shall group the load for each configuration into a set with ≤ 25% variation and equipment selection shall be completed for each set of loads.

For each house plan with multiple configurations, the room-level design airflows shall be calculated for each potential configuration. If the design airflows for each room vary across all configurations by ≤ 25% or 25 CFM, then the average room-level design airflow shall be permitted to be used when designing the duct system. Otherwise, the contractor shall group the room-level design airflow for each configuration into a set with ≤ 25% or 25 CFM variation and the duct design shall be completed for the average airflow of that set.

(13) Design airflow is the design value(s) for the blower in CFM, as determined by using the manufacturer’s expanded performance data to select equipment, per ACCA Manual S procedures.

(14) Design duct static pressure shall account for the installation of a MERV 6 or higher filter.

(15) The load calculation for the home shall be provided, documenting all design elements and all resulting loads, including but not limited to the values listed in Items 2.1 through 2.17.

(16) All evaporators and condensing units shall be properly matched as demonstrated by an attached AHRI certificate. If an AHRI certificate is not available, a copy of OEM-provided catalog data indicating acceptable combination selection and performance data shall be attached.

(17) If the whole-house ventilation system utilizes the HVAC air handler, then the fan speed type shall be ECM / ICM and variable speed, or include a controller (e.g., smart cycler) that reduces the ventilation run time by accounting for hours when the HVAC system is heating or cooling the home.

(18) Listed system capacity at design conditions is to be obtained from the OEM expanded performance data.

(19) For cooling systems, the next largest nominal piece of equipment may be used that is available to satisfy the latent and sensible requirements. Single-speed systems generally have OEM nominal size increments of ½ ton. Multi-speed or multi-stage equipment may have OEM nominal size increments of one ton. Therefore, the use of these advanced system types can provide extra flexibility to meet the equipment sizing requirements.

(21) For warm air heating systems, the output capacity must be between 100% and 140% of calculated system load unless a larger size is dictated by the cooling equipment selection.

(22) Either factory-installed or field-installed TXV’s may be used. For field-installed TXV’s, ensure that sensing bulbs are insulated and tightly clamped to the vapor line with good linear thermal contact at the recommended orientation, usually 4 or 8 o’clock.

(23) Examples of return or supply duct static pressure measurement locations are: plenum, cabinet, trunk duct, as well as front, back, left or right side. Test hole locations shall be well marked and accessible.

(24) Ducts shall not include coiled or looped ductwork except to the extent needed for acoustical control. Balancing dampers or proper duct sizing shall be used instead of loops to limit flow to diffusers. When balancing dampers are used, they shall be located at the trunk to limit noise unless the trunk will not be accessible when the balancing process is conducted. In such cases, Opposable Blade Dampers (OBD) or dampers located in the duct boot are permitted.

(25) Condensate pan shall be made of corrosion-resistant materials, to include galvanized steel and plastic. Drain pan shall drain condensate to a conspicuous point of disposal to alert occupants in the event of a stoppage of the primary drainage system; and shall be equipped with a backflow prevention valve when drained to a shared drainage system, such as a storm water management system.

Many states have adopted state- or region-specific ENERGY STAR Certified Homes criteria - Please see the ENERGY STAR Certified Homes website for regional specifications.

This Retrofit tab provides information that helps installers apply this “new home” guide to improvement projects for existing homes. This tab is organized with headings that mirror the new home tabs, such as “Scope,” “Description,” “Success,” etc. If there is no retrofit-specific information for a section, that heading is not included.

Additional Scope Information for Retrofit Applications

Assess the need for replacing or upgrading the HVAC system. See Pre-Retrofit Assessment of Existing HVAC Systems.

For more information on compression cooling, see the U.S. Department of Energy’s Standard Work Specifications regarding compression cooling equipment.

Additional Description Language for Retrofit Applications

Assessment

The typical lifespan of HVAC equipment is 15 to 20 years. New equipment has much higher efficiencies, safety, control flexibility, and performance capabilities. Existing equipment should be carefully assessed to determine if investment in repairs, upgrades, or expansion is warranted or if replacement is the better option. See the following BASC guides and resources for information to aid in making this determination. The guides provide economic guidelines to help determine if this investment is warranted, and also contain important safety and health information for dealing with older construction and equipment.

Replacement

Although replacement of HVAC equipment can be costly and labor-intensive, it often reaps large rewards in energy cost savings and comfort. Table 1 below gives examples of what energy cost savings can be expected when upgrading from existing HVAC equipment to equipment with higher rated efficiencies. Use your summer cooling bill and the before and after SEERs to estimate potential savings.

Table 1. Annual Estimated Savings for Every $100 of Cooling Costs. (Source: www.energysavers.gov)

Because A/C units typically provide only space cooling and dehumidification, the demise of the current A/C system is an opportunity to re-assess the homeowner’s needs. In mild to temperate climates, if the home also has a furnace nearing the end of its expected life, an air-source heat pump could replace both pieces of equipment because a heat pump can operate the refrigeration cycle in reverse to provide heating. In warm-humid climates with moderate space heating needs, a heat pump may be a good option for comfort, control, and operating cost, since many homes in these climates focus on air conditioning, and space heating  is is typically provided by an electric coil that heats the airstream. An evaporative cooler can improve thermal comfort by adding moisture to very dry air; evaporative coolers are often much less expensive to buy and operate than A/C, but they are only effective in dry climates. The delta T of evaporative coolers is limited, so in very hot, dry climates A/C may be the right equipment to meet peak load. The point is to consider all options when recommending a replacement. The guide Pre-Retrofit Assessment of Existing HVAC Systems provides additional guidance.

If a decision is made to replace your current equipment, it can be replaced in-kind or with a different type of system. See the following Solution Center guides for more information on other types of HVAC systems that provide space cooling:

For the highest efficiency and best performance with split-system central A/Cs, always replace the indoor and outdoor units at the same time. Also, replace the entire refrigerant line set when installing new cooling equipment. If any water vapor were to get into the old refrigerant line during replacement of the indoor and outdoor components, the water vapor would mix with the lubricant in the refrigerant system, causing an acidic sludge that would foul the new equipment; see the Description tab for more information. Follow the manufacturer’s directions and the suggestions in the Description tab of this guide for proper sizing, selection, and installation.

Repair/Upgrade

Start-up commissioning and full system maintenance are often overlooked by homeowners who do not understand the strong relationship between these precautions and actual performance. The Air Conditioning Contractors of America Association, Inc. (ACCA), is one of the best sources for guidance on the installation, commissioning, and maintenance of HVAC equipment. Their free Quality Standards can be found on the ACCA website.

Useful documents available for download from the ACCA website include the following:

Faulty ductwork can cause poor performance in central forced air heating and cooling systems. Duct blower testing and inspection by a certified technician may reveal leaky, uninsulated, constricted, or even disconnected ducts that prevent heated or cooled air from reaching its destination. See Ducts in the Solution Center for more on improving ducted distribution systems.

Additions

If additional rooms will be added to the home, or if an attic, basement, or garage will be converted to living space and the home has an existing furnace, the forced-air HVAC equipment may have sufficient capacity to extend conditioning to the additional space. This should be confirmed by performing an accurate load calculation (ACCA Manual J) for the entire house including the addition. If the current system doesn’t meet the additional needed capacity (and/or for increased efficiency or for zone control) you may decide to add a new, dedicated HVAC system to serve the new space independently or you may choose to replace the existing system with a new more efficient system capable of serving the whole house.

When installing new ducts for the addition, consider the following:

  • New ducts should be properly sized in accordance with ACCA Manual D.
  • If using the existing furnace, simply extending the nearest existing supply branch ducts into an addition is unlikely to deliver sufficient air flow to the addition because that takeoff was not designed for the new (combined) flow requirement. This will result in poor comfort for all spaces dependent on the modified branch, both existing and new.
  • Ideally, install a separate, dedicated supply air trunk duct to serve the addition. This trunk should be run back as close to the furnace/air handling unit as practical. This will ensure that air flow to the addition is removed proportionally from the total system air flow and therefore will not affect the air flow balance in the existing house. Install manual balancing dampers in the supply trunks serving the addition and existing house to allow fine-tuning of the system.
  • If there are interior doors separating the addition - or portions of the addition - from the main house, install transfer grilles, jump ducts, or ducted returns as required to ensure a return air path to the central air handler unit.

Additional Compliance Language for Retrofit Applications

Check with the Authority Having Jurisdiction to determine if upgrade or expansion to existing HVAC equipment requires compliance with current codes.

More Info.

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Case Studies

  1. Author(s): BSC
    Organization(s): BSC
    Publication Date: February, 2009
    Case study describing a building project in the hot-humid climate zone.
  2. Author(s): PNNL
    Organization(s): PNNL
    Publication Date: November, 2011

    Case study about a new building project that partnered with Habitat for Humanity.

References and Resources*

  1. Author(s): ICC
    Organization(s): ICC
    Publication Date: January, 2009

    Code containing 2009 ICC language for mechanical draft systems.

  2. Author(s): Air Conditioning Contractors of America
    Organization(s): Air Conditioning Contractors of America
    Publication Date: December, 2013
    Standard outlining industry procedure for sizing residential duct systems.
  3. Author(s): Air Conditioning Contractors of America
    Organization(s): Air Conditioning Contractors of America
    Publication Date: January, 2011

    Standard covering equipment sizing loads for single-family-detached homes, small multi-unit structures, condo­miniums, town houses and manufactured homes.

  4. Author(s): Air Conditioning Contractors of America
    Organization(s): Air Conditioning Contractors of America
    Publication Date: April, 2013

    Standard covering sizing strategies for all types of cooling and heating equipment, as well as how to use comprehensive manufacturer’s performance data on sensible, latent, or heating capacity for various operating conditions. 

  5. Author(s): Air Conditioning Contractors of America
    Organization(s): Air Conditioning Contractors of America
    Publication Date: January, 2015

    Standard providing a universally accepted definition for quality installation for residential and commercial heating, ventilating, and air conditioning applications.

  6. Publication Date: January, 2016

    Document detailing the requirements, roles, and obligations for participants in an organized effort, ensuring that HVAC installations comply with the ANSI/ACCA 5 QI – 2010 (HVAC Quality Installation Specification) QI Standard.

  7. Author(s): Gilbride, Baechler, Hefty, Hand, Love
    Organization(s): PNNL, ORNL
    Publication Date: August, 2011
    Report providing information about energy-efficient heating, ventilation, and cooling (HVAC) equipment options to help homeowners cut their energy use, reduce their carbon footprint, and increase their homes comfort, health, and safety.
  8. Author(s): EIA
    Organization(s): EIA
    Publication Date: January, 2009
    Federal statistics about national energy consumption in residential homes.
  9. Author(s): RESNET
    Organization(s): RESNET
    Publication Date: January, 2013

    RESNET standards aimed to ensure that accurate and consistent home energy ratings are performed by accredited home energy rating providers through their raters nationwide.

  10. Author(s): Burdick
    Organization(s): IBACOS, NREL
    Publication Date: February, 2012
    Report describing the equipment selection of a split system air conditioner and furnace for an example house in Chicago, Illinois, as well as a heat pump system for an example house in Orlando, Florida.
  11. Author(s): Air Conditioning Contractors of America
    Organization(s): ACCA
    Publication Date: January, 2010

    The Technician's Guide equips practitioners with the knowledge to properly implement all of the measurement procedures required in the HVAC QI Specification.

Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Last Updated: 08/16/2017