ASHRAE Standard 90.1, Appendix G Fan Power Calculations

It’s safe to say that there’s a solid ten (or more) requirements in Appendix G that engineers, energy modelers, and rating authority reviewers consistently trip over. If I had $100 for every conversation I’ve had about correctly calculating the baseline system fan power per Appendix G, I’d be sipping a drink with an umbrella on an island somewhere.

In this first installment of “Wisdom from a TRACE Jedi” we’ll be discussing the baseline supply fan power calculations when exhaust fans (toilet, garage, etc.) are involved. The ECB subcommittee of 90.1 published a formal interpretation ruling on just this topic in the summer of 2010 and thus will be the basis of our discussion. The interpretation ruling was specifically written in regards to 90.1-2004 though the logic and foundation of the ruling does still apply to 90.1-2007 and 90.1-2010.

1. Should Table G3.1 No. 12 be used for determining fan power from equipment such as toilet, garage, or general exhaust fans?

No – If the space use classification for a zone or thermal block is semi-heated or unconditioned then hat space’s fan power from equipment such as toilet, garage, or general exhaust fans would be the same between the baseline and proposed per Table G3.1 No. 12. If the space use classification for a zone or thermal block is conditioned then G3.1.2.9 is used to determine baseline system total fan power.

Per Appendix G the total system fan power for baseline system(s) serving conditioned spaces is specified independently of the proposed building, enabling credit for more efficient fan systems. System fan electrical power for supply, return, exhaust and relief fans is calculated based on the formulas in G3.1.2.9. The power from this formula includes supply, return, relief, and exhaust fans, but excludes power to fan-powered VAV boxes when these are part of the baseline building HVAC system (systems 6 and 8). The calculated system fan power shall be distributed to supply, return, exhaust, and relief fans in the same proportion as the proposed design.

2. Should only supply air flow rate (excluding exhaust flow rate) be used in the Section G3.1.2.9 fan power equation to calculate fan power from supply and exhaust fans even if the exhaust fans are independent and unrelated system such as toilet exhaust, garage, or general exhaust fans?

No – It is correct that only the baseline supply air flow rate should be used in the Section

G3.1.2.9 fan power equation to calculate fan power from supply and exhaust fans. However, the baseline system maximum design supply fan airflow per G3.1.2.8 requires the system design supply airflow rates for the baseline to be based on a supply-air-to-room-air temperature difference of 20°F or the required ventilation air or makeup air, whichever is greater. Thus the calculated baseline supply air flow is a balanced air flow and accounts for any and all exhaust requirements when used to find total system fan power in G3.1.2.9.

3. Should supply and exhaust fans should be calculated separately when using the fan power equation in Section G3.1.2.9?

No – The total baseline fan power is based on the baseline supply air flow rate as calculated per G3.1.2.8.

The calculated system fan power shall be distributed to supply, return, exhaust, and relief fans in the same proportion as the proposed design. (This in fact now section G3.1.2.10.1 of 90.1-2010.)

If you need to calculate the 90.1 total supply power, check out the fan power calculator