HVAC Design Credit for Title 24 Compliance
The 2016 Energy Code is one of the toughest energy codes yet. It is helpful to know of other types of credits available that are often not thought of when pursuing energy compliance. This article addresses the compliance credits available with regards to HVAC design, equipment selection, and zoning.
The following credits are available for Title 24 Energy Compliance modeling (as shown in the 2016 Residential Compliance Manual [PDF, 22.3MB]):
Duct Location [§184.108.40.206]
There are three ways to achieve credit for favorable duct location when using the performance compliance method:
- Credit is available if no more than 12 LF (linear feet) of duct are outside the conditioned space and the user chooses the high-performance attic (HPA) as explained in Section 3.6.2. This total must include the air handler and plenum lengths. This credit results in a reduction of duct surface area in the computer compliance programs. This option requires certification by the installer and field verification by a HERS Rater.
- The second alternative applies when 100 percent of the ducts are located in conditioned space and the user chooses high-performance attic (HPA) as explained in Section 3.6.2. This credit results in eliminating the conduction losses associated with both the return and supply ducts; however, leakage rates still apply. This option requires field verification of the duct system by means of a visual inspection by a HERS Rater.
- Credit for a high-efficiency duct design is available through the diagnostic duct location, surface area, and R-value compliance option, which are described below. This option requires field verification of the duct design layout drawing(s) by a HERS Rater. Verified duct design, when required, will be included in the HERS Required Verification list on the certificate of compliance (CF-1R). This approach provides energy savings credits for having shorter duct runs, fewer ducts, ducts in beneficial locations of ductwork, and other benefits of a well-designed duct system. This credit is available regardless of whether a high-performance attic (HPA) or ducts in conditioned space (DCS) option is chosen, as explained in Section 3.6.2.
There is no compliance credit provided for choosing a heating system such as a wall furnace, floor heater, or room heater, even though those systems typically have no ducts. For these cases, the standard design in the compliance calculation uses the same type of system and has no ducts. However, other systems, such as hydronic heating systems with a central heater or boiler and multiple terminal units, are considered central HVAC systems that are compared to a ducted system in the standard design. If the hydronic system has no ducts, there may be a significant energy credit through the performance method.
A High-efficiency Duct Design [§220.127.116.11]
This compliance option allows the designer to take credit for a high-efficiency duct design that incorporates duct system features that may not meet the criteria for the duct location and/or insulation compliance options described above. This method requires that the designer must enter the design characteristics of all ducts that are not located within the conditioned space. The information required for the input to the compliance software includes the length, diameter, insulation R-value, and location of all ducts. This method will result in a credit if the proposed duct system is better than the standard design.
To claim this credit, the duct system design must be documented on plans that are submitted to the enforcement agency and posted at the construction site for use by the installers, the enforcement agency field inspector, and the HERS Rater. The duct system must be installed in accordance with the approved duct system plans, and the duct system installation must be certified by the installer on the CF2R form and verified by a HERS Rater on the CF3R form. Details of this compliance option are described in the Residential ACM Reference Manual, and verification procedures are described in RA3.1 of the Reference Residential Appendix.
Buried and Deeply Buried Ducts [§18.104.22.168]
This compliance option also allows credit for the special case of ducts that are buried by blown attic insulation. For ducts that lie on the ceiling (or within 3.5 inch of the ceiling), the effective R-value is calculated based on the duct size and the depth of ceiling insulation as shown in Table R3-38 in the Residential ACM Manual. This case is referred to as "Buried Ducts on the Ceiling." For the case of deeply buried ducts, which are ducts that are enclosed in a lowered portion of the ceiling and completely covered by attic insulation, then the effective R-value allowance in the compliance calculations is R-25 when the attic insulation is fiberglass and R-31 for cellulose attic insulation. To take credit for buried ducts, the system must meet the verified duct design criteria described above, be diagnostically tested for duct sealing compliance by a HERS Rater according to Reference Residential Appendix RA3.1, and meet the requirements for high insulation installation quality described in Reference Residential Appendix RA3.5. Verified minimum airflow (350 cfm/ton or higher if higher is specified on the CF1R) is required when a measure is selected for compliance that has a verified duct design as a prerequisite.
Multi-speed Compressor, Low-leakage Air Handlers, Bypass Ducts & ZoningCredits are available for multi-speed compressors and low leakage air handlers even if the HVAC system is not zoned.
A multi-speed compressor is a type of equipment that runs at two or more speeds. Two-stage cooling means the air conditioner or heat pump has a compressor with two levels of operation: high for hot summer days and low for milder days. Since the low setting is adequate to meet household-cooling demands 80% of the time, a two-stage unit runs for longer periods and produces more even temperatures. [From Wikipedia]
A multi-speed compressor provides a more efficient cooling system. A cut-sheet from the manufacturer of the proposed HVAC equipment will be needed by the Title 24 Energy Documentation author.
Low-leakage Air Handlers
Here are the requirements to receive a credit for a low-leakage air handler:
JA9.2 Qualification Requirements
To qualify as a low leakage air-handling unit for use for compliance with applicable performance compliance credits, the air-handling unit shall be certified to the Energy Commission according to the following requirements:
- JA9.2.1 Method of Test
The air-handling unit shall be tested in accordance with the requirements given in ASHRAE Standard 193.
- JA9.2.2 Testing Laboratory Requirements
The Air-Handling Unit shall be tested in a laboratory that has demonstrated compliance with ISO Standard 17025, General Criteria for the Competence of Testing and Calibration Laboratories, and is accredited for the ASHRAE Standard 193 test methods. The accreditation body shall be a signatory to the International Laboratory Accreditation Cooperation Mutual Recognition Arrangement (ILAC MRA) www.ilac.org.
- JA9.2.3 Nominal Air-handling Unit Airflow
The nominal air-handling unit airflow used for determining the leakage criterion for qualification shall be as follows:
- For heating-only systems the nominal air-handling unit airflow shall be 21.7 cfm per kBtu/hr of rated heating output capacity.
- For systems that provide space cooling, the nominal air-handling unit airflow shall be 400 cfm per nominal ton of cooling capacity as specified by the manufacturer, or the heating-only value, whichever is greater.
- JA9.2.4 Leakage Criterion for Qualification
Allowable leakage for qualification as a Low Leakage Air-Handling Unit shall be equal to or less than 1.4 percent of the nominal air-handling unit airflow determined by Section JA9.2.3.
Information On Zonal Systems
Zonally Controlled Central Forced Air Cooling Systems [§22.214.171.124]
The primary purpose of zoning ducted air conditioners, heat pumps, and furnaces is to improve comfort. Increased comfort is attained by having the capacity of the HVAC system (cooling or heating delivered) follow the shift in load as it changes across the house. For example, it is common for two-story homes to be too hot on the second floor in both summer and winter. Zoning has the capability of diverting more of the HVAC capacity to the area with the increased load. Another common example is a home with a significant area of west-facing and east-facing windows. In the summer, the east rooms overheat in the morning, and the west rooms overheat in the afternoon.
Providing the most agreeable temperature to all the zones is comfortable, but it carries with it the distinct possibility of increased energy consumption. Since the most common home is single-zoned and has only one thermostat placed near the center of the house, temperatures in the rooms distant from that thermostat will vary, sometimes significantly. If zoning is added, the more distant rooms may be conditioned to a more comfortable temperature. This increased conditioning requires more energy. When designed correctly, zoning allows only the zones that need conditioning to be conditioned, thus potentially saving energy.
It is common for zonally controlled central forced air cooling systems to produce lower airflow through the returns thus lowering the sensible efficiency of the single-stage heating or cooling equipment. There are two primary methods by which the common multizoned dampered system lowers airflow: additional restriction of zoning dampers and recirculation through the air conditioner from a bypass duct. To avoid this efficiency problem, zonally controlled central forced air cooling systems using a single-speed air conditioner must simultaneously meet the following criteria:
- In every zonal control mode, the system shall provide airflow through the return grilles that is equal to or greater than 350 CFM per ton of nominal cooling capacity.
- In every zonal control mode, the fan watt draw must be less than or equal to 0.58 watts per CFM.
The airflow and fan watt draw must be HERS-verified. See Reference Residential Appendix RA3.3 for the HERS verification procedures.
Zonally controlled central forced air cooling systems with multispeed or variable speed compressors need to be verified only to meet the above 350 CFM per nominal ton and 0.58 watts per CFM criteria with the compressor on high speed and all zones calling for cooling.
Zoned Systems and Airflow 126.96.36.199 and Fan Efficacy Requirements [§188.8.131.52]
Recent studies have shown that zoned systems (multiple zones served by a single air handler with motorized zone dampers), with or without bypass dampers, usually do not meet the AF/FE requirements when fewer than all zones are calling. The energy penalty that results from this is greater than the benefit of having zonal control; therefore zonal control is no longer simply assumed to be a "better than minimum" condition, and there are special compliance requirements for them. Zonal control accomplished by using multiple single-zone systems is not subject to these requirements.
There are two choices for modeling zoned systems. One is for air conditioning condensers that have single-speed compressors, and the other is for condensers that have "multispeed" compressors. Two-speed and variable-speed compressors are considered multi-speed. Multispeed compressors allow the system capacity to vary to more closely match reduced cooling loads when fewer than all zones are calling for cooling. Therefore, multispeed compressor systems are given special consideration when used in zoned systems and are not required to verify performance in all zonal control modes. Instead, the airflow and fan efficacy testing is required to be performed only at the highest speed with all zones calling. Zoned systems with single-speed compressors must be tested and pass in all operating modes.
Because zoned systems, with or without bypass dampers, are less likely to meet the AF/FE requirements when fewer than all zones are calling, a way is provided in the performance compliance option to take this penalty and still allow use of zone dampers. Other energy features must offset the penalty. In the performance compliance software, if the system is modeled as a zoned system with a single-speed compressor, the default airflow drops to 150 CFM/ton. The standard house is assumed to have an airflow of 350 CFM/ton, so there is definitely a penalty unless the designer specifies a value of 350 or higher. Entering a value between 150 and 350 can lessen the penalty.
It is extremely important that the energy consultant model airflow and fan efficacy values are reasonable and obtainable; otherwise they will fail in the field and will need to be remodeled at actual values. Energy consultants should coordinate with the HVAC designer before registering the certificate of compliance.
Note: Bypass dampers may be installed only if the certificate of compliance specifically states that the system was modeled as having a bypass damper.
Zonal Control [§4.5.2]
An energy compliance credit is provided for zoned heating systems, which save energy by providing selective conditioning for only the occupied areas of a house. A house having at least two zones (living and sleeping) may qualify for this compliance credit. The equipment may consist of one heating system for the living areas and another system for sleeping areas or a single system with zoning capabilities, set to turn off the sleeping areas in the daytime and the living area unit at night. (See Figure 4-27)
There are unique eligibility and installation requirements for zonal control to qualify under the Energy Standards. The following steps must be taken for the building to show compliance with the standards under this exceptional method:
- Temperature Sensors. Each thermal zone, including a living zone and a sleeping zone, must have individual air temperature sensors that provide accurate temperature readings of the typical condition in that zone.
- Habitable Rooms. For systems using central forced air or hydronic heating, each habitable room in each zone must have a source of space heating, such as forced air supply registers, radiant tubing, or a radiator. For systems using a combination of a central system and a gas vented fireplace or other individual conditioning units, the zone served by the individual conditioning unit can be limited to a single room. Bathrooms, laundry, halls and/or dressing rooms are not habitable rooms.
- Noncloseable Openings. The total noncloseable opening area (W) between adjacent living and sleeping thermal zones (such as halls, stairwells, and other openings) must be less than or equal to 40 ft². All remaining zonal boundary areas must be separated by permanent floor-to-ceiling walls and/or fully solid, operable doors capable of restricting free air movement when closed.
- Thermostats. Each zone must be controlled by a central automatic dual-setback thermostat that can control the conditioning equipment and maintain preset temperatures for varying periods in each zone independent of the other. Thermostats controlling vented gas fireplace heaters that are not permanently mounted to a wall are acceptable as long as they have the dual-setback capabilities.
Other requirements specific to forced air-ducted systems include the following:
- Each zone must be served by a return air register located entirely within the zone. Return air dampers are not required.
- Supply air dampers must be manufactured and installed so that when they are closed, there is no measurable airflow at the registers.
- The system must be designed to operate within the equipment manufacturer's specifications.
- Air is to positively flow into, through, and out of a zone only when the zone is being conditioned. No measurable amount of supply air is to be discharged into unconditioned or unoccupied space to maintain proper airflow in the system.
An HVAC system design using software that meets all the requirements for ACCA Manuals J, D & S design calculations will aid to incorporate these energy credits available for Title 24 Energy Compliance.
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