Section G3.1.2.9 says that baseline "system fan electrical power for supply, return, exhaust, and relief (excluding power to fan-powered VAV boxes) shall be calculated" using the formulas given. This statement has origins in section 6.5.3.1.1 which requires that fan power not exceed a calculated amount including "supply fans, return/relief fans, exhaust fans, and fan-powered terminal units associated with systems providing heating or cooling capability."
The question I have (and please point me to any existing precedent on this) is how exactly do we define which fans are "associated" with systems providing heating and cooling? Should section G3.1.2.9 be interpreted as:
a) System-wide Area Interpretation - Only the main supply, return, relief, and/or exhaust fans which serve the entire area served by the supply system are to be included. This would mean toilet exhaust fans or other fans serving only part of the area (maybe one or two rooms) are considered miscellaneous and remain the equal in proposed and baseline cases. Transfer fans would also be equal in the proposed and baseline cases even though they provide indirect cooling effect since they do not serve the entire area of the system (example is an electrical closet with fan that trades are with an open plenum).
b) Tied Fan Systems Interpretation - Only fans that must run when the HVAC system is running are included in these calculations (examples are laboratory exaust fans tied to the supply unit, toilet exhaust fans tied to the HVAC unit serving the adjacent area, other fans that are tied to the supply system to operate simultaneously). This would mean any exhaust fans on a switch (switched toilet exhaust fans, switched kitchen grease exhaust fans not tied to a makeup air unit, other fans that operate independently of the main HVAC system) or switched transfer fans for indirect cooling would not be included in the calculations and would remain equal between proposed and baseline cases.
c) Fans Provided Only for Cooling Effect - All fans that are provided for cooling effect (either direct or indirect) must be included in the calculations. Toilet exhaust fans, fume exhaust fans, kitchen grease exhaust fans, etc. would not be included in the calculations and would be modeled equally in the proposed and baseline cases since they are provided for ventilation rather than solely for cooling effect. Transfer fans provided for indirect cooling would be included in the calculations since they do provide cooling. These fans may not be directly associated with any particular HVAC system (penthouse fans with no cooling system, etc.), so the question would remain as to whether they must be included in the fan power calculations of other systems in the building or if they are to be modeled equally in the proposed and baseline cases since.
d) All Fans in the Building – All fans in the building, regardless of function, are included in the calculations since the fact that they move air means they potentially provide cooling effect. This would mean any fan (switched, tied to BMS, ventilation only, etc.) would be included in the calculations and no exceptions would be allowed for any fans to be treated as miscellaneous exhaust modeled equally in the proposed and baseline cases. This option is difficult because baseline systems do not necessarily serve the same areas as proposed systems and it may not be clear which baseline system the fan power should be associated with.
There may also be additional options not presented that better define how the section should be interpreted.
Any help would be great since it appears review of this matter is inconsistent.
Ante Vulin
Sustainability ManagerYR&G
80 thumbs up
October 17, 2012 - 6:41 pm
Matt, I agree that this is a confusing subject. I usually treat exhaust fans as identical between the baseline and the proposed. But looking into this again, the contents of Section 6.5.3 of 90.1-2007 and the definition of "fan system" in the 90.1-2007 User's manual seem to make it clear that exhaust fans are included in the fan power allowance if they are greater than 1 HP and if they exhaust conditioned air. The Advanced Energy Modeling for LEED guidance also states that the total fan power for the baseline system design reflects the sum of "power modeled for supply, exhaust, return, and relief fans." I would be interested to hear more about the specific review comments you or others have received on the subject.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
October 18, 2012 - 11:10 am
We agree with option b). We would typically include exhaust fans as part of the system if they are controlled centrally. If they are on a separate control like a wall switch then they are independent and should be modeled identically above the G3.1.2.9 calculations. Whether the exhaust fans serve all or only a portion of the area is irrelevant.
Matt Edwards
ME Engineers Inc.27 thumbs up
October 18, 2012 - 11:25 am
Thanks to all for weighing in on this. I tend to also agree with Option B since it encompasses fans that need to run in order for a complete HVAC system to function. Toilet exhaust fans often fall into this category in commercial buildings.
As a side note, I did also find in the ASHRAE 90.1-2007 User's Manual that "all fans that operate at peak design conditions" are to be included in the calculations, but that doesn't really address switched fans that are technically designed to be on but don't respond at all to outside temperature "design conditions". It makes sense to me that fans provided for alternate purposes (as evidenced by switched control or other independent control) are miscellaneous exhaust and should be modeled equally in the proposed and baseline cases.