Does Your Building Blow? Long As It Doesn’t Suck.
On your second try, you put a little extra juice into it and that door finally lets loose and a whoosh. Yup that building sucks… It’s building static pressure is negative and is drawing outside air in.
On the flip side how about when you can’t keep the front doors shut and they stay cracked open and air whistling through? This would mean the building is over pressurized and not exactly a good thing either.
Building Static Pressure is the Way to Measure How a Building Breathes
Yes, a building actually breathes based on whether it is positively or negatively pressurized. It is always recommended that a building be slightly pressurized to keep unwanted outside air and contaminates from entering the building envelope. Building static pressure and how to maintain it is actually a pretty simple thing to understand if you break it down into a couple of simple parts.
A Manometer or Differential Pressure Sensor is needed for measurements
If you are looking to try and take a manual measurement you are going to need a Manometer and alot of tubing. Manometers are really just a device that measures the pressure differential between two points.
Now, I am a real techie type of guy and love electronics but I will choose a Dwyer Magnehelic over a digital handheld manometer that costs 3 times as much any day! If you do decide to use a digital one be sure to get one over $100 or you may find out you just spent $99 for crappy readings on a tool that may not last long.
If you are looking to incorporate measurement of the building’s static pressure in to the building automation system or DDC controls system, then you are going to need a differential pressure sensor. There are many manufacturers out there that offer these types of sensors. Here are a few that have worked well for me in the past:
- ACI/DP – Low Differential Pressure Transmitter
- Modus M30/40 and T30/40 Series Differential Pressure Transmitters
- Omega PX274/277 Differential Pressure Transmitter with Field Selectable Ranges
Poly Tubing and any other Tubing for the Distance
Regardless of which device you choose you are gonna need some poly tubing and lots of it! You can’t just get 3 or 4 feet and stick it on each of the High and Low ports of your manometer or pressure transmitter. You’re gonna need to go the distance!
Make sure your high port or inside the building measurement point isn’t too close to the opening of a door or large window. Try to keep it a good 20 to 30 feet from a large opening or on the other side of a wall or two to help separate the room with the opening.
Your Low port should always be placed to the outside of the building envelope and for a permanent placement to an automation system you are going to want an Outdoor Pressure Pickup Port Enclosure that will negate wind and rain. This is very important.
A few inches might as well be a mile.
When measuring your Buildings Static Pressure, typically you will be wanting to maintain 0.05″ of static pressure. Yeah really… POSITIVE 5/100ths of an inch! You only need it to be ever so slightly positive to help keep the air flowing out and not in.
Imagine if you push too much… the next month’s utility bill is going to have someone yelling at someone else about air conditioning the whole city and I’m here to make sure that person ain’t you.
Understanding the Ins and Outs of Your Building
There are many ways air finds its way into and out of a building. I don’t want to make this post go on longer than it needs to so I am going to give you a link to an easy to read PDF that was written by CanAm Building Envelope Specialists. It does throw a few terms and strategies that might be a little too ‘Engineer-ery’, but feel free to ask more explanations in the comments below.
Does Your Building Suck? - by Tony Woods, President
Believe it or not, your building probably does indeed suck. Highrise, low-rise, office or residential, it makes no difference. The truth is that just about every building does it. If you have ever noticed that parts of your building are draftier than others, that people on the 10th floor can smell second-hand cigarette smoke from the 5th floor, that there’s a whistling sound in the elevator lobby, that the outside trim on the upper floors is beginning to deteriorate or that static electricity is a real problem on cold windy days, it’s almost certainly because your building sucks.
This PDF document does discuss alot further the causes of air infiltration, as well as some remedies that can be used to correct the problem.
How to Control All This Pressure
Fear not, the Engineer is always right and planned ahead… WHOA! I just ducked out of the way of a flying shoe. Weird…
Because most buildings have some sort of pre-planned ventilation you probably won’t be the one that needs to come up with an entire design, but you will need to understand what to control and how it affects the buildings static pressure.
If you are pulling IN outside air from somewhere then you need to compensate for it by exhausting it. This can be done in any of the following forms:
- Naturally through the buildings openings, doorways and windows (no control over)
- Relief dampers on the building usually located on the roof or top of a building
- Relief dampers located at or in the HVAC units bringing in the outside air
- Exhaust fans both VFD speed controlled or single speed
Any one of these or in most cases, a combination of a couple, will allow the air brought in from the outside to be exhausted back out. It is up to you to figure out which option allows you to affect the building pressure the most and still have a smooth range to modulate it with.
On and off like my last girlfriend won’t cut it.
Now you could have a large exhaust fan up top that pulls 2,000 to 20,000 cfm but if it isn’t speed controlled or has some modulated dampers on the inlet side; it really isn’t gonna help you. You can’t be turning such a large piece of equipment on and off and expect to keep something as small as .05″wc in check.
Now if you can get a variable frequency drive on that baby and make a slow and easy PID loop to go with it… then you got the makings of something that can truly maintain that slightly positive static pressure we are looking for.
Be on the look out for anything you as a controls person can modulate by speeding up/slowing down or opening/closing to be able to manipulate the building static pressure to hover right over set point. Depending on the situation you might see quick changes in the pressure while others you will need to make small adjustments and wait to see if there any changes.
Don’t Choke the Fresh Air or the Occupants as a Solution
The one thing you can’t do as a permanent fix to the problem is close down the outside air dampers or slow down any make up air fans. This will seem to fix your pressurization problem, obvioulsy if you bring in less outside air you won’t need to exhaust as much.
But what you’ve really done is make the overall building problem worse by not allowing enough fresh air that is needed to maintain a healthy building environment. This could have much more expensive and long term problems and issues, so be sure to maintain your designed ventilation and air changes per hour and look to controlling one of the above parts of your building automation system.
What building static pressure problems have you come across in your experience? What were the problems and what was the final solution?