Infiltration values for room with drop ceiling

[IanVG]

I am tasked with designing a two AHU replacement system for one floor of a five story university building built in the 1960's (includes mostly offices, small classrooms, computer labs, and a research laboratory (exhaust required)). I was told to consider infiltration to determine how positive my building should be. Air is currently returned to the AHU via the hallways. There is no return air moving in the plenum space. All rooms have ~8 drop ceiling (2x2 ceiling grid system). The construction below the ceiling grid is different from above. There are large windows on each face of the building and the building is a rectangular shape. The windows are known to be leaky (allows water and spiders in). I have been reading through some ASHRAE handbooks, articles and journals but feel no closer to an answer. I feel like a lot of words have been spilled over how to perform tests to determine how leaky a building is - that is not useful for me, as I will in no way have the money or the will of the owner to perform that test.

I am looking to estimate how positive (OA CFM - Exhaust CFM) I need my entire floor to be to prevent infiltration of outside air into the building. I also have a lab space that includes these leaky windows, so maybe estimating the amount of OA coming in through the window would be nice.

I am getting too far into the weeds, and I believe I am looking for simplified plug-n-play values here. E.g. cfm/sf, turnover, etc. Does anybody have recommendations on how to proceed? No one in my office has done a calculation like this, and I honestly don't know where to begin. Does anyone have any recommendations? I am attaching a floor plan of the fourth floor (where I plan to do my work).

 

[HVAC-Novice]

For load I assume 0.04 cfm/ft² for tight buildings, and up to 0.25 for leaky buildings. 0.08 for improved buildings, 0.12 for average. This is highly subjective and unless you were involved in air sealing, or it was measured, you have to err on the higher side.
this also would be above average infiltration. So you can get away with less.

BTW, you can't use egress paths, like corridors, as plenum. Based on you description, this seems to be the case here.

 

[IanVG]

@HVAC-Novice thank you! And yes, I realize that based on code the corridors cannot be used as the return air path. Unfortunately, I do not have the funds to renovate the space as such. I am constrained by the existing conditions. And just to clarify, by cfm/ft², the square footage refers to total exterior wall area, correct? I want to confirm that this is not a floor based approximation. and I should probably run with a leakier estimate for the exterior walls that are above plenum vs the exterior walls that are below plenum. E.g. does 0.20 cfm/ft² for exterior walls above plenum and 0.15 cfm/² exterior walls below plenum seem approximately right? If i need to use one value, I will probably run with 0.20 cfm/ft², as we are aware that the windows at least pose a serious leakage problem.

 

[IanVG]

And also just to write my thinking process out loud, if I know that:

North AHU:[ul]
[li]Total Exhaust (CFM) 50[/li]
[li]Total OA (CFM) 393[/li]
[li]Total Supply (CFM) 3092[/li]
[/ul]

South AHU:[ul]
[li]Total Exhaust (CFM) 1056[/li]
[li]Total OA (CFM) 1250[/li]
[li]Total Supply (CFM) 4357[/li]
[/ul]

(with the Total OA CFM being the flexible number). Then I need to ensure that the Total OA CFM is at least greater than the Total Exhaust CFM for each AHU by the amount specified by the infiltration number. E.g. if the CFM I calculate from 0.20 CFM/ft² is, let's say, 1000 CFM for the entire floor, (so 500 CFM for each half of the building), then the current North AHU Total OA CFM value would need to be bumped up to 50+500 CFM = 550 CFM and the South AHU Total OA CFM value would need to be bumped up to 1056+500 CFM = 1556 CFM for the building to remain relatively positively pressurized compared to the outside. Is this logic correct?

 

[HVAC-Novice]

I would consider all enclosure surfaces that contribute to infiltration. Exterior walls and roofs. I only would add floor area if the floor is exposed to outside. If it is a crawlspace, you likely have little wind effect etc.
you also could make OA intake to be 105% of exhaust and most of the time pressure will be positive. If you have some infiltration some hours of the year - no big deal.
Note that all the measurements are imperfect and field measurements aren't great. So a lot depends on how well your flow stations work. Just slightly positive is enough. You also don't want to create problems with doors and the force required to open or close them.

Infiltration for load purposes is different since we assume the worst in each room. In reality, on one side of the building air infiltrates, and exfiltrates on the other side (think wind, or chimney effect causing infiltration and exfiltration).

 

[nuuvox000]

Blower door tests are pretty common; I'm sure there's a company near you that will do it.

Remember, if the owner doesn't want to spend the money to do it the right way, just make sure you send an email to them stating that without the actual test values from a blower door test, you are only making an educated guess and they may have issues down the road. Then, when they get upset, point them back to the email.

"I was told to consider infiltration to determine how positive my building should be"
Why? What is the issue that the person told you to do this is worried about?