Par. G3.1.2.8 states: "If return or relief fans are specified in the proposed design, the baseline building design shall also be modeled with fans serving the same functions and sized for the baseline system supply fan air quantity less the minimum outdoor air, or 90% of the supply fan air quantity, whichever is larger."
Par.G3.1.2.9 provides an equation to calculate the electric power of fans. In tab. G3.1.2.9 shall I use as Ls the air flows obtained according to par. G3.1.2.8, both for the supply fan and for the return fan? I mean, shall I calculate electric power separately for supply fan and return fan, using for both equation of par. G3.1.2.9? Or does the equation provide the total power (supply fan + return fan)?
Could I find an example of calculation of electric power of return fans, please?
With Regards
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Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5907 thumbs up
November 19, 2012 - 12:23 pm
No you do not calculate power separately for the the supply and return fans. Only use the supply cfm to calculate the total power allowance which is distributed across the various fans. Yes, the G3.1.2.9 equation provides the total power.
In the Baseline the total fan power is all that matters since the supply and return fans always operate at the same time. You can distribute the fan power across the various fans as you see fit.
Francesco Passerini
engineer90 thumbs up
November 19, 2012 - 12:28 pm
Thank you, Marcus. And for "return or relief fans" what does "sized for the baseline system supply fan air quantity less the minimum outdoor air, or 90% of the supply fan air quantity, whichever is larger" mean?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5907 thumbs up
November 19, 2012 - 1:40 pm
There are two issues in that sentence. The first part is if the proposed has return/relief fans then they get included in the baseline. The second part refers to the sizing of the supply cfm minus OA or 90% whichever is larger.
Francesco Passerini
engineer90 thumbs up
November 20, 2012 - 3:31 am
My doubt regards the second issue. If "In the Baseline the total fan power is all that matters since the supply and return fans always operate at the same time. You can distribute the fan power across the various fans as you see fit" how shall I consider in my model the second statement (sizing of the return fans)?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5907 thumbs up
November 20, 2012 - 10:00 am
The second part deals with the sizing of the return air flow. The statement you quoted has to do with the baseline fan power. I am not clear about your doubt or confusion. If this distinction does not help perhaps you could clarify your question for me.
Francesco Passerini
engineer90 thumbs up
November 20, 2012 - 11:05 am
I think that the following statements are in contrast:
1. You can distribute the fan power across the various fans as you see fit
2. sized for the baseline system supply fan air quantity less the minimum outdoor air, or 90% of the supply fan air quantity, whichever is larger
I mean, the second statement seems a rule for sizing the return fans, therefore to distribute the fan power across the various fans I shall respect it, right?
Probably I would need to see an example.
Regards
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5907 thumbs up
November 20, 2012 - 12:31 pm
Here is an example provided by Tyler Thumma in my office:
Let’s say you have a Baseline VAV system (Type 5-8) which has a supply airflow of 1,000 cfm calculated by the simulation using a 20 degree F supply-air-to-room-air temperature difference per G3.1.2.8, and an outside airflow of 300 cfm. According to G3.1.2.8, the return fan airflow would be either the supply airflow minus the outdoor airflow (700 cfm) or 90% of the supply airflow (900 cfm), whichever is larger, so 900 cfm. The G3.1.2.9 fan power allowance is calculated to be 1,155 Watts using the supply airflow and assuming no pressure drop adjustments. This 1,155 Watts must be distributed between the supply and return fans, but how you distribute it is up to your discretion. One option would be to divide it proportionally to the airflows, so 608 Watts for the supply fan and 547 Watts for the return fan.
Francesco Passerini
engineer90 thumbs up
November 20, 2012 - 12:58 pm
Thank you for the example, Marcus! (and Tyler Thumma!)
Only a last observation: so it seems that the statement "sized for the baseline system supply fan air quantity less the minimum outdoor air, or 90% of the supply fan air quantity, whichever is larger" is used only for an option ("One option would be to divide it proportionally to the airflows, so 608 Watts for the supply fan and 547 Watts for the return fan"), doesn't it?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5907 thumbs up
November 20, 2012 - 2:05 pm
I am not certain if your conclusion is correct.
The statement from 90.1 is not optional for sizing the return air flow in the baseline. The impact of calculating the baseline return air flow does not matter in many cases unless you are applying one of the baseline fan power adjustments related to the return air flow. Then it will impact the baseline fan power.
What is optional is how you allocate the calculated fan power among the fans in the baseline system. However exactly how you allocate the fan power does not matter in most cases since all the fans operate at the same time. Again the only real impact will be related to the baseline fan power adjustments related to the return air flow.
Perhaps the confusion here is related to "designing" the baseline system. It is important to remember that the baseline system does not actually have to function in the real world. We are only concerned with the energy use.