Stairwell Pressurization

[Big Unit]

Have A 39 story building with two stairwells. Each stairwell has one constant volume fan and one freq. drive controlled fan. We will be holding a set pressure with the freq. drive to adjust for doors opening and closing. Where should we put the static pressure sensors? I'm thinking one in each stairwell around the 14th floor. Fans are on the roof. Thanks.

[Chris_Worthington]

"if" it is critical then why not 2 or more and average with some fail safe logic, should one sensor crap out?

[kdocsr05]

Are you looking for pressure control during smoke purge?

[Big Unit]

Are you looking for pressure control during smoke purge?

Yes. I should clarify that this is for life safety and tied into the fire alarm system.

[kdocsr05]

Assuming we are talking about a life safety stairwell pressurization system with the modulating fan an element of the building smoke purge system, I offer the following.

With positive pressure relationships in the stairwells and potentially negative pressure relationships established on the fire floor by the ‘purge fans’ used to control smoke, forces required to open a door to the stairwell becomes significant. The concern is that someone might not be able to open the door against the pressure differential induced force, and thus would be unable to enter the stairwell. Our local codes restrict this force, which requires that a pressure control system be provided for the stairwell. Inspectors often want this stairwell pressure control system to be demonstrated as a condition of acceptance, selecting the fire floor and doors to test at random. The problem is the required amount of force to open a door has no definition and is left to the inspectors discretion.

I do not think you will find an application standard here. The verification process may involve setting up the ‘pressure sandwich’ on every floor and then testing all doors, then repositioning the sensor as adjustments become necessary which can be time-consuming.

Smoke purge and stairwell pressurization are evolving post 911 and it has become almost impossible to keep up with all the changes. We are put in a situation were you learn as you go with little or no direction. Then during commissioning all the big shots show up and everything changes.

I have been involved with commissioning smoke purge and pressurization at the NYC Port Authority Bus Terminal for ten weeks, worked there today on a pre-inspection and my head is spinning. We had a different designer today than last week, who has a new interpretation of the local fire code.

[kostasgreece]

Just to add my two cents.

Kd has detailed described the issues that may arise...

I am not based in US so i dont have to deal with local codes, but i have recently involved on a high security US building and the presurization control is a nightmare...

Get ready for a serious period of not ending living hell....

In order to build and maintain a pressure stack you would need two things...
1. A good design that takes into consideration building pressure stack and builts it in normal operation lets say "by default".
2. A well commissioned and programed "reaction" mechanism that shall pull back pressure stack in design values when this ones goes off.

Not having a clear view of the whole design i would recommend to test in an aproprieate manner what happens on normal mode when everything is settled, in othe words crosscheck the design and find possible flaws...
You may experience that a potential door opening shall collapse your stack and then you need to chase once again and re-built stack etc...

After that you can spot with a critical eye where are points of best monitoring space pressure..., with light hart I could say one sensor at low rise, one at mid and one at high rise... observation will lead you on definition of low mid high (by the way you need to have installed sensors on the beginning and start recording..)

Second (again without knowing the mechanical design), the two fans may create you a slight problem maintaining the pressure even with a good design..

Just to have an idea I had space DPs in ALL critical spaces, with modulating dampers and VFD fans... and still the damn pressure stack some times collapse and then you need a serious amount of time to rebuilt it..

Anyway extensive tests (trial and error sequence) only lead to the desirable result..

I hope that all the above worth two cents...

[Cagey57]

Biggest obstacle you have is getting someone to SAY where and how much differential you have to maintain.

If I had to guess I'd bet you have to maintain your differential relative to the basic building envelope but no one told you where to reference it right.

Do you reference the 19th floor ?, the 39th floor ?, the 1st floor ?


The last one I was involved in we had to maintain .01" relative to the basic building envelope, Referenced center of the building, during "Normal Operation". In "Fire Mode" we had to keep it (stairway) neutral to floors above and below (both positive to floor involved) floor involved, which was in full evacuation mode (negative to everything else) but stairway could never be negative to floor involved. It proved to be quite tricky to keep ALL of the presure relationships correct but we were only working a 10 story with 3 parking levels below.

You have your work cut out for you. I hope someone in a position of authority gives you a definitive target to aim for

[jslimjeff]

There is also the configuration of the VFD that must be looked at for life saftey operations. Some VFD has an Override function that must be used when doing stairwell pressurization. This override mode wil disable the keypad for external control, eliminate response to certain faults and warnings within the drive and acknoledge and disregard certain safties that are wired to the drive. I have been told by many contractors that use our drives for this very situation that the setpoint and sensors used are dependent on the local code. Some use high/low select, averaging and mutiple sensors. The internals of our VFD can do the stated above. BUt as this is life saftey please check with your local requirements.

[TURBOCHILL]

Here is the most useful link regarding stairwell pressurization .
https://www.cedengineering.com/userf...%20Systems.pdf

[theoldman]

Make sure that the registered professional fire protection engineer makes all critical design decisions and that it is all documented. That's what they get paid for and they are the responsible party. The last place you want to be is in front of a judge explaining why you made a design decision when you aren't qualified to do so.

[mwitt]

I would agree with pushing the PE to determine the pressure limits both high and low. Having worked for an engineering firm, this is fully their responsibility.

That said, we did once work out the answer. ADA limits force to 5 lbf for an interior door, but exterior doors are higher (close to 10 lbf). For fire doors they say it is up to the AHJ. But for arguments sake, say we have a force limit of 5lbf applied at 32" (near edge) of a 3'x8' door .... we get an allowed pressure of 0.07"wc. Which makes sense given the typical building pressure set-points around 0.05".

However, the door closure is going to add to this force. So if it is putting 4 lbf on the door, then the air pressure can only add 1 lbf or 0.014"wc. So the door closure really matters here.



One other point i want to add is that you will have to deal with stack effect if the stairwell isn't conditioned like the building. I often see these stairwells on the exterior and without much temperature conditioning. The temperature difference between the stairwell and the inside and outside of the building will create a pressure differential. The pressure differential is dependent upon the temperature difference, total height, and opening area between the spaces.

This can really play with buildings and pressure control. For instance lets play this out:

For the stairwells, they are very tight and sealed. No open penetrations, everything fire caulked. Pressure sensors are at the top, mid and bottom and the VFD controls to the minimum pressure sensor. It is summer, and the stairwell is hot compared to the building.

1. So with all doors closed, you have good even pressure (top to bottom) and solid control (likely a constant VFD speed if trended).

2. Someone opens a door at the bottom. Not a problem yet. With no other opening, the pressure drops at first, but then the VFD responds and a differential is returned.

3. Someone opens a door at the top with bottom open too. Now we have a stack effect. The temperature differential causes the stairwell air to want to rise up creating a higher pressure at the top and a lower pressure at the bottom. The VFD has to respond even more to maintain the minimum DP at the bottom. However, the new DP at the bottom plus the stack effect created a DP at the top that exceeds the allowed high limit.

Since you cannot change the opening size (i.e. door size), the DP limits, or the building height, the only was to control stack effect is to maintain a minimal temperature difference.



I merely mention this so that if it becomes a problem and you start to see this, you can point to something outside of your control. Often we are left to try and do the impossible without a leg to stand on.

[WAYNE3298]

Stairwell allowable pressure is determined by the authority having jurisdiction. Pressure is determined by the force required to start the door to open and that force is tested with a scale. The fan can be ramped up on pressure below set point but an open door negates pressure control capability. I have run many of these tests and they take forever because there is nothing you can do except put a guard on each door to make sure someone doesn't open it. No tests can be run when someone opens a door so every time that happens you have to wait for the system to recover before door pull force is measured.

[kontrolphreak]

What is everyone using to measure the force required to open the doors during the test? In the past I have used a belt tension gauge which works well, but just curios what others are using.

kontrol out

[orion242]

Last one I worked on TAB used nothing more than a scale with a hook on it. Nothing too fancy. Tie it to the door handle and pull.

Something like this.

https://www.amazon.com/TOOGOO-Handho...rds=rope+scale

[WAYNE3298]

A fish scale is the most popular one I have seen. I hated doing the tests because they always turned into 3 or 4 hours for a 30 minute test. Some people literally got fighting mad when you closed the stairwell and opened the door not caring that killed the test. They didn't realize nobody got paid until that test was accepted.

[orion242]

Exactly, they are a complete $hit show. Better part of a day pissed away in ones I have dealt with.

[WAYNE3298]

I always thought the best test would be a 85 or 90 pound 90 year old lady to just try to open the door.

[orion242]

The 90yr old lady may pass before fire marshall bill finally decides to show up for the test scheduled months prior...

[kontrolphreak]

So our test were the push-bar style door openers, for that the belt gauge is great. All the fire doors we tested were push to open from the floor to the stairwell, not sure if pulling the door open from the stairwell would be approved.

kontrol out

[orion242]

Pretty sure we had the same setup. Could be wrong but I think that is part of the building code.

Panic bars on the stairwell doors and interior doors open into the stairwell, exterior open to the outside. If it was reverse as the cattle rush the door, would be impossible to open with everyone shoving.

Sure its up to the ADJ. Pretty sure they didn't have anything special and tested by pulling the door open, but its been a few since I had to deal with this.