1、Page 1 of 2 2017 ASHRAE. All Rights reserved. Interpretation IC 170-2013-14 of ANSI/ASHRAE/ASHE Standard 170-2013 Ventilation of Health Care Facilities Date Approved: June 27, 2017 Request from: Erick Phelps, Smith Seckman Reid, Inc., 301 North Cattlemen Rd., Suite 300, Sarasota, FL 34232. Reference
2、: This request for interpretation refers to the requirements in ANSI/ASHRAE/ASHE Standard 170-2013, Section 7.1 and Table 7.1, regarding system minimum outdoor air calculation. Background: In Section 7.1.a.6 of ASHRAE/ASHE Standard 170 (S170), two methods are given to calculate the system minimum ou
3、tside air quantity. Sub paragraph (i) states, “ it shall be the sum of the individual space requirements as defined by this standard Table 7.1.” Sub paragraph (ii) allows the use of, “the Ventilation Rate Procedure (multiple zone formula) of ASHRAE Standard 62.1. The minimum outdoor air change rate
4、listed in this standard shall be interpreted as the Voz (zone outdoor airflow) for purposes of this calculation.” This first table presents a hypothetical multiple-zone VAV system that only serves spaces described in Table 7.1, where the two methods of calculating minimum required system outdoor air
5、 intake are used: Variable Abbr. Value Units Reference Room Volume RV 2500 cu. ft. Outdoor air change rate OACH 2 AC/hr S170 Table 7.1 Zone Outdoor Airflow Voz 83 cfm RV x OACH / 60 Zone Discharge Airflow Vdz 250 cfm Critical Zone Discharge Outdoor Air FractionZd 0.33 Voz / Vdz Uncorrected Outdoor A
6、ir Intake Vou 2000 cfm Sum of Vozs System Primary Airflow Vps 10000 cfm Average Outdoor Air Fraction Xs 0.20 Vou / Vps Zone Ventilation Efficiency Evz 0.87 1 + Xs - Zd System Ventilation Efficiency Ev 0.87 Min(Ev) Outdoor Air Intake Flow Vot 2308 cfm Vou / Ev Required OA fraction Y 0.23 Vot / Vps In
7、 this example, the method in Sub-paragraph (i) results in 2000 cfm being required, equivalent to a System outdoor air fraction of 20%. Using the method in sub-paragraph (ii) results in 2308 cfm being required, with a System outdoor air fraction of 23%. However, we understand that there are some in t
8、he industry that interpret the standard as requiring the System OA fraction to be no less than the Discharge Outdoor Air Fraction of the systems critical zone, being the zone with the highest ratio of required outdoor air to supply air (Max(Zd), as described in this second table: Interpretation IC 1
9、70-2013-14 Page 2 of 2 2017 ASHRAE. All Rights reserved. Variable Abbr. Value Units Reference Alternate Req. OA Fraction Y_alt 0.33 Max(Zd) Alt. Outdoor Air Intake Flow Vot_alt 3333 cfm Vps x Y_alt Interpretation No.1: For the hypothetical system described above, the first table correctly uses the t
10、wo methods described in the standard to calculate the Minimum System Outdoor Air Intake Flow. Question No.1: Is this interpretation correct? Answer No.1: Yes. Interpretation No.2: For the hypothetical system described above, the second table presents an alternate acceptable method of using the stand
11、ard to calculate the Minimum System Outdoor Air Intake Flow. Question No.1: Is this interpretation correct? Answer No.2: No. Comments No.2: The second table is not an alternate method to calculate the Minimum System Outdoor Air Intake Flow. Due to the use of the critical zone fraction (max (Zd), thi
12、s proposed calculation method will always yield a result at least equal to and likely in excess of the minimum OA flow required by Standard 170. The multi-space equation recognizes that the unused portion (unvitiated) of outdoor air in recirculated systems has value to the ventilation within the ove
13、rall system. Furthermore, Standard 62.1, Appendix A specifically advises that a designer may increase supply zone air flow, particularly to the critical zones requiring the highest fraction of outdoor air, and thereby reduce the system outdoor air intake flow requirement determined in the calculation, sometimes dramatically.
