© Copyright CalCERTS, Inc. 2019
Code Cycle: 2016 & 2019 Standards
Reference: CalCERTS Alterations Training Manual Chapter 7, Section 5
Before performing a total duct leakage verification test, the HERS rater must calculate the duct leakage target for the verification test. The duct leakage target is the “pass/fail criteria” or the “not to exceed loss” for the total duct leakage test.
Total system airflow used to calculate a duct leakage percentage can be taken from the:
- Measured system airflow, or
- Nominal system airflow (Predominately used)
The duct leakage target criteria are expressed as a percentage (≤5%, ≤10%, or ≤15%) of the total HVAC system airflow during normal operation.
RA3.3.3 - Measuring System Airflow
To identify total system airflow, it can be measured. There are three primary methods for measuring airflow:
- Flow capture hood
- Flow grid, and
- Plenum pressure matching
Any of these methods can be used to measure the actual airflow of the HVAC system.
RA220.127.116.11.2 - Nominal System Airflow
Instead of using the actual measured HVAC system airflow, the nominal system airflow may be used by the HERS Rater to identify a duct leakage target value. It is important to remember that this is not measured airflow, and may only be used to express duct leakage as a percentage of the system airflow.
Generally, the nominal system airflow is greater than the actual, measured system airflow. Therefore, using the nominal system airflow typically results in a higher duct leakage percentage or a larger duct leakage target.
Note: For heat only systems, use the heating calculation only. For heat pump furnaces, use the cooling calculation. If a heat pump is present and there is no cooling, then measured airflow would have to be used to calculate the duct leakage target.
Nominal system airflow can be used from the heating or cooling capacity of the system and is specified below:
Heating Capacity of an HVAC System
The heating capacity is the output capacity of the furnace. This is unique for each system being tested. The heating output capacity is stamped on a plate on the furnace and is listed in thousands of BTU’s, such as 48,000 or 60,000. The HERS Rater can usually find this information on the nameplate inside the furnace cabinet. Once this information is obtained, it can be calculated using the following formula:
The formula for calculating the nominal airflow of the heating capacity is:
21.7 cfm x heat output of the furnace in KBTU’s
If a furnace had an output capacity of 60,000 BTUh, then 60 is plugged into the formula for calculating the nominal system airflow:
21.7 X 60 = 1302 cfm. This is the nominal system airflow.
Cooling Capacity of an HVAC System
The cooling capacity of the HVAC system is the second method that can be used to calculate the nominal system airflow. The cooling capacity is determined by the size of the HVAC system condenser. Most residential A/C condensers come in the following nominal sizes: 1.5 tons, 2 tons, 2.5 tons, 3 tons, 3.5 tons, 4 tons and 5 tons. Whereas a “ton” of cooling is 12,000 BTUh.
Unfortunately, a condenser’s cooling capacity is not stamped on HVAC equipment like the heating capacity output. However, a condenser’s cooling capacity can usually be determined from the model number. The model number will usually include 2 or 3 digits that reflect the tonnage of the unit divisible by 12. For example:
The formula for calculating the nominal airflow of the cooling capacity:
400 cfm x nominal tonnage of the condenser, or
250 cfm x nominal tonnage of the condenser for SDHV systems
If an A/C system has a 3-ton condenser, three is plugged into the formula for calculating the nominal system airflow:
400 cfm x 3 tons = 1200 cfm. This is the nominal system airflow.
In summary, the two methods for calculating nominal system airflow are:
Furnace Capacity = 21.7 cfm x output of the furnace in KBTUh
Cooling Capacity = 400 cfm x nominal tonnage of the condenser
The larger of these two methods would be used to calculate the duct leakage target for the HERS verification test.
Note: Nominal system airflow is used for establishing a duct leakage target only! Other HERS verification require taking actual system airflow measurements.
Below are two Duct leakage worksheets, each is applicable to the specified code cycle.
Special Case Scenario (Inverter driven compressors)
With new equipment coming to market it has been recognized that a line of Bosch compressors and possibly other manufacturers have inverter driven compressors with dip switches to dictate tonnage. These can be set up for a variety of capacities. This means that if it was a 5-ton system it could be configured to be a 3-ton system.
If the 5-ton system is configured as a 3-ton, can i use the 3-tons for calculating my duct leakage target even though it is a 5-ton system?
The protocols currently dictate “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.” So, it boils down to what is the manufacturer calling this system. For that, we can rely on the AHRI ratings. There is some question as to whether the scalable package equipment has been AHRI tested at the reduced capacities. Raters should make sure that the installer can provide AHRI ratings for the equipment as installed. Then, determine the tonnage based on that. It may require confirming dip switches if that is indicated by the AHRI ratings. There is a chance that the AHRI listing may only show the higher capacity, in which case the system will need to be treated as such until the CEC tells us otherwise.