Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
I'm not familiar with IECC, so I only mention ASHRAE90.1.
Appropriate fan curve coefficients: ASHRAE 90.1 Appendix G Table G3.1.3.15 Method 2 as you already mentioned. I personally have no doubt about this.
Fan Power Minimum Flow Fraction: That's what I also want to know the answer. We might be able to refer to Section 6.5.3.2.1, but I have another suggestion. ASHRAE90.1 does not specify all the parameters for the Baseline case. Some parameters are not found such as Fan Total Efficiency, Pump Total Efficiency, Chiller Minimum Part Load Ratio, Chiller Minimum Unloading Ratio, etc. When I come across such unspecified parameters, I often refer to ANSI/ASHRAE/IES Standard 90.1 Performance Rating Method Reference Manual. The Table below is extracted from the Reference Manual. (b) is the curve of G3.1.3.15 Method 2.
Please note the rightmost column. %PowerMin is 20%. %PowerMin = Fraction of Full Load Fan Power * 100.
The chart below is what I plotted the curve (b). The Minimum Fraction of Full Load Fan Power = 0.2. The Fraction of Full Load Airflow Rate corresponding to the Minimum Fraction of Full Load Fan Power of 0.2 is 0.393198 ≈ 0.4. So, I vote for Min Flow Fraction = 0.4, but I'm not that confident. Please take this as just one opinion.
| | 2 | No.2 Revision |
I'm not familiar with IECC, so I only mention ASHRAE90.1.
Appropriate fan curve coefficients: ASHRAE 90.1 Appendix G Table G3.1.3.15 Method 2 as you already mentioned. I personally have no doubt about this.
Fan Power Minimum Flow Fraction: That's what I also want to know the answer. We might be able to refer to Section 6.5.3.2.1, but I have another suggestion. ASHRAE90.1 does not specify all the parameters for the Baseline case. Some parameters are not found such as Fan Total Efficiency, Pump Total Efficiency, Chiller Minimum Part Load Ratio, Chiller Minimum Unloading Ratio, etc. When I come across such unspecified parameters, I often refer to ANSI/ASHRAE/IES Standard 90.1 Performance Rating Method Reference Manual. The Table below is extracted from the Reference Manual. (b) is the curve of G3.1.3.15 Method 2.
Please note the rightmost column. %PowerMin is 20%. %PowerMin = Fraction of Full Load Fan Power * 100.Minimum Fraction of Full Load Fan Power.
The chart below is what I plotted the curve (b). The Minimum Fraction of Full Load Fan Power = 0.2. The Fraction of Full Load Airflow Rate corresponding to the Minimum Fraction of Full Load Fan Power of 0.2 is 0.393198 ≈ 0.4. So, I vote for Min Flow Fraction = 0.4, but I'm not that confident. Please take this as just one opinion.
| | 3 | No.3 Revision |
I'm not familiar with IECC, so I only mention ASHRAE90.1.
Appropriate fan curve coefficients: ASHRAE 90.1 Appendix G Table G3.1.3.15 Method 2 as you already mentioned. I personally have no doubt about this.
Fan Power Minimum Flow Fraction: That's what I also want to know the answer. We might be able to refer to Section 6.5.3.2.1, but I have another suggestion. ASHRAE90.1 does not specify all the parameters for the Baseline case. Some parameters are not found such as Fan Total Efficiency, Pump Total Efficiency, Chiller Minimum Part Load Ratio, Chiller Minimum Unloading Ratio, etc. When I come across such unspecified parameters, I often refer to ANSI/ASHRAE/IES Standard 90.1 Performance Rating Method Reference Manual. The Table below is extracted from the Reference Manual. (b) is the curve of G3.1.3.15 Method 2.
Please note the rightmost column. %PowerMin is 20%. %PowerMin = Minimum Fraction of Full Load Fan Power.
The chart below is what I plotted the curve (b). The Minimum Fraction of Full Load Fan Power = 0.2. The Fraction of Full Load Airflow Rate corresponding to the Minimum Fraction of Full Load Fan Power of 0.2 is 0.393198 ≈ 0.4. So, I vote for Min Flow Fraction = 0.4, but I'm not that confident. Please take this as just one opinion.
UPDATE on 20 Jun 2025
COMNET provides Appendix H - Equipment Performance Curves which includes very similar data as the above Table 50 Fan Curve Default Values. The Appendix H suggests that %PowerMin in the above table may be a mistake for %FlowMin. Now I don't trust ANSI/ASHRAE/IES Standard 90.1 Performance Rating Method Reference Manual that much.
I would like to mention a few other sources of the Baseline VAV Fan Min Flow Fraction for ASHRAE 90.1 Appendix G PRM.
None of the above three are definitive. At this time, I adopt the item 1 i.e. Min Flow Fraction = 0.5, which maximises the performance cost index. Hopefully the Baseline VAV Fan Min Flow Fraction will be clearly specified in ASHRAE 90.1-2025 Appendix G.
