# Thread: Variation in room pressures in day and night time in a tall building.

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## Variation in room pressures in day and night time in a tall building.

Hi,
I have just joined this site today, if my post is not appropriate for this forum, please suggest/move as needed.

I am a manufacturing guy and working on improving the process. We have a facility of height about 30mtrs. This has several floors say 10 floors and there are about 4 AHUs catering to group of floors, for example top 2 floors are catered by one AHU and so on. We have automatic monitoring of pressures for individual rooms.

These rooms/floors are all interconnected through a central well inside the tall building. So in fact air can move up/ down. We are seeing that top floor pressures rises when the ground floor pressure drops and vice versa. As of now we hypothesize that this is possibly caused by day and night times weather changes, air velocity and temperature changes. Though such hypothesis is not confirmed fully, because there is little differences in day to day behavior of above observation.

Our goal is to make these floors at relatively stable pressure, current typical range of room pressures is ~0.2 in/wc to 1.0 in/wc. Kindly suggest, what might be the reason for such behavior and how to overcome this.

Thank you in anticipation.

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I did some quick searching for values of pressure based on elevation. It appears that going form 0 to 100 feet would lead to an approximate drop in pressure of about .108”HG which converts to about 1.5”WC.

Atmospheric pressure changes with elevation because the amount of air weighing down on you changes.

You should expect to see about a 1.5” WC difference from the bottom of your building to the top simply due to elevation. I’d imagine that it would be more useful to reference each room with the pressure of the environment outside at that elevation.

I’d imagine you could theoretically use exhaust or make up air at each floor to pressurize or depressurize to adjust for the pressure based upon elevation but that would take some serious engineering.

Here’s one funny thing I realized thinking about this. I typically think that a gas flows from high to low pressure but I don’t think that would be the case in this situation because of gravity. Our difference in pressure is only due to gravity and therefore would not be overcome without any help. I don’t think you have to worry about significant uncontrolled airflow because of this difference in pressure. A 1.5”WC difference in pressure between two rooms side by side would be a different story.

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Thanks Shane.
Your comments are a new insight for us on this matter. Considering the natural tendency of such behavior, we can still accept that top floors are at somewhat lower pressure relative to down floors, but our aim is to make them stable at those pressures then anytime of the day.

For example (currently), during the day, down floors are at 1 in/wc then top floors are noted at 0.2 in/wc whereas evening/night times, down floor reduces to 0.7 in/wc and top floors rises to 0.5 in/wc. I would like to understand how to make them stable at a particular pressure such as down floor at 0.8 mbar and top floors at 0.4 mbar. Currently there is sinusoidal trend during day and night.

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Where are the pressures taken in the building and in reference to what? These pressure sensing devices usually have two ports on them “low” and “high”. Where is low connected and where is high connected?

I can’t quite explain why your pressures vary as they do at this point. What trend do you see in temperatures between the bottom and top floor?

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5. What kind of problems are you having?

Are occupants complaining?

Breezes at elevator shafts, doors not opening/ closing?

Or are you just trying to balance the building, just for the sake of having it balanced?

If you have a central shaft interconnecting all floors, you will be fighting the affects of temperature on the structure. Those temperature differences from bottom to top will cause air to move through the entire building.

And, like Shane said, what is your reference point for the other side of your meter? Is it outside the building?

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## Variation in room pressures in day and night time in a tall building.

Do all the doors get closed (or opened) at night when you see this change occur? Janitorial staff?

Is it a sudden change or a graduel change? Over what timeframe does it require to complete the change?

Where are the sensors located? You said each room but where in each room, and do you monitor the common area also? Where is the reference pressure taken from?

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A building pressure of 1.5" WC would blow your windows out. You mentioned monitoring the room pressures but are you attempting to control the pressures? Your building pressure goal with reference to the outside should be around 0.05 to 0.08 inches WC. A good control tech and TAB tech can resolve your problems.

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Originally Posted by thatguy
Do all the doors get closed (or opened) at night when you see this change occur? Janitorial staff?

Is it a sudden change or a graduel change? Over what timeframe does it require to complete the change?

Where are the sensors located? You said each room but where in each room, and do you monitor the common area also? Where is the reference pressure taken from?
Should consider both doors and windows.

Do you have exhaust fans set on time clocks?

Do you have economizers that are using outdoor air for cooling at night?

Are the roof top units operating on VFD? You could control the fan speed to maintain pressures based off their readings from the sensors...

I’m just spitballing here since we don’t really have much info to go by.

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Let’s see if this makes any sense.
Pressure and temperature are directly proportional, one rises, and so does the other. (That’s physics.)
During the day, the top of the building heats up faster than the lower ones, so the top floors will have higher pressure. At night the opposite happens. The bottom floors are warmer than the top floors due to thermal transfer from the earth irradiating it’s heat back, so you will have a higher pressure than the top floors that cool faster.

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10. Originally Posted by WAYNE3298
A building pressure of 1.5" WC would blow your windows out..
THIS ^^^^^^^

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Stack effect for a 100 foot tall building ranges from about 0.01 inches WC at -20F to 0.25 WC inches at 68F. I haven't calculated it for higher temperatures but it is easily done if needed.
These numbers are the pressure differences from the ground to the top of the building.

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Originally Posted by mahenkj2
For example (currently), during the day, down floors are at 1 in/wc then top floors are noted at 0.2 in/wc whereas evening/night times, down floor reduces to 0.7 in/wc and top floors rises to 0.5 in/wc. I would like to understand how to make them stable at a particular pressure such as down floor at 0.8 mbar and top floors at 0.4 mbar. Currently there is sinusoidal trend during day and night.
Are you using 2 different pressure units on purpose? Even if the pressures are all mbar they are still high. Your building pressure monitors may need to be calibrated. Wind at night might also be causing pressure changes but that can be negated with a proper setup.

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Originally Posted by WAYNE3298
Stack effect for a 100 foot tall building ranges from about 0.01 inches WC at -20F to 0.25 WC inches at 68F. I haven't calculated it for higher temperatures but it is easily done if needed.
These numbers are the pressure differences from the ground to the top of the building.
Very interesting. Could you post a link to this data please? I got my numbers from barometric data but am no expert.

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15. Originally Posted by Shane Bird
Very interesting. Could you post a link to this data please? I got my numbers from barometric data but am no expert.

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Wayne is a retired Mechanical engineer. I'm betting those numbers are stored in his Grey matter! Lol!

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Shane I did the calcs on my MATHCADD program so there is no link. That program can do array calculations which is how this was done. Once the array is set up all I have to do is change the parameters and it will recalculate to include the information added. If you wish I'll PM you the basic formula.

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Hello everyone,
Amused with the response on this site and depth of knowledge. I would like to share a detailed note on all the questions being asked and so that everyone can understand the issue in detail, so please be patient. I am just preparing it.
Thanks.

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I cant attach an image, can someone help me here please.

19. If you are using the TapaTalk App, it's pretty easy.

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Yes, I tested Advanced, but I can see Image can only be attached as URL link. How to attach the image stored on my PC. I have prepared a sheet showing structure description and trends of pressures.

Surprisingly, I even could not find link to help topics on such questions on this site.

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