Wind effect on building Pressurization 2

[dosworld]

We have a running debate at work about building pressurization and the effect wind has. We all agree that wind can have an affect, but ASHRAE etc all pretty much say to control to ambient pressure and minimize wind effect rather than control to maximum stagnation pressure on the building. What is the best way to control building pressure with respect to outdoor conditions, ambient or stagnation pressure?

 

[ChasBean1]

What you all agree upon is right, but don’t try to have controls for wind pressure. Do neither (ambient nor stagnation pressure control). Simply supply a little more outdoor air than the building exhausts on a consistent basis. If you try control the building based on ambient conditions or stagnation pressure, you’re designing in problems that will result in control difficulties and use of excessive outdoor air.

 

[imok2]

Here's one concept

Define pressure zones and control the supply and return airflow rates into and out of each zone (D cfm). Monitor “bleed” airflow rates between critical spaces and reset the differential as required. In addition, monitor local exhaust flow rates if changes in flow rates during operation will significantly impact pressurization.
• Single air handler serving a single floor: control supply and return airflow rates at the air handler.
• Single air handler serving multiple floors: control supply and return airflow rates at each floor to compensate for stack effect.
• Single air handler serving multiple pressure zones on one or more floors: control supply and return airflow rates at each pressure zone.

 

[Zesti]

There can be two different effects at work:

- if the rooms inside the building that need to be kept at a certain pressure are adjacent to the outside wall of the building then windpressure on the wall can influence the pressure inside the room. For this reason critical rooms should not be adjacent to the outside wall or the outside wall should be made specifically airtight.

- if the outside air intakes and exhausts are not well placed this can have an effect (worse than the previous). If the OA intake is built into the wall then windpressure will influence the pressure inside all ducts. Same for exhaust openings. Both should therefore be placed in the roof, using a 'penthouse louvre' type.
Do not place there to close to the roof edge as wind will make a large vortex near the roof edge.

 

[dosworld]

Thanks all for your replies.

Chasbean, I have thought about not actively controlling the pressurization, but that seems to be a non-starter with my boss. We have fairly stringent requiremnts for building pressurization and are currently controlling to ambient with just normal differential pressure sensors. We use 4 sensors (one on each side of the building) and average them for our outside reference point. We sheild the sensors from the wind and seem to have some success with this.

However, there is an argument that seems somewhat valid that wind pressure would negate our averaged ambient pressure and effectively overcome our pressurization in the building. If this is so, and we have to actively pressurize, is it feasible to account for wind pressure (I don't think it is since in changes to rapidly)? How can this be accouted for.

Thanks again.

 

[Zesti]

So if I understand correctly, what you want is a positive pressure inside the building.

By definition, the average of 4 measurements will be lower than the maximum value out of those 4 measurements.

Therefore, if the set pressure difference is not large enough, there might not be a positive pressure between inside and outside at the side with the highest outside pressure.

Airflow around buildings can be strange indeed, and so can the locally occuring pressures.

In short, to me, saying "We have fairly stringent requirements for building pressurization" is not in tune with averaging 4 measurements.

(Wind pressure on outside walls can be a lot larger than any room pressurization: 100's of Pa vs. 10's of Pa...)

 

[marcoh]

From what I can gather the internal building pressure should be naturally pretty neutral. The infiltration on the positive side of the building due to wind pressure is offset by exfiltration on the lee side. There is an article on the Trane website about building pressurisation that explains it pretty well.

 

[show]

You might be interested in this device:

http://www.airmonitor.com/pdfs/industrial_public/brochures/BRO_SOAP.pdf

Good Luck

 

[Zesti]

Indeed, putting the measuring device outside the positive and negative pressure zones seems like a very sensible thing to do (figure 1)...

 

[ChasBean1]

I’ve seen returns modulated before to maintain a positive building pressure and in most cases you’ll find return air to be dramatically reduced in favor of a pressure that can be hard to achieve. For example, anything over +0.01” w.c. is getting up there in standard building construction. So the sensor needs to be a very low range. If there’s a loading dock door open or something, your returns will just shut off and you’ll be conditioning a bunch more outdoor air.

This is why I’m a more of a proponent of a cfm offset; slightly more OA than EA. This is simple and consistent whereas pressure control is a little of a house of cards and can dramatically increase air conditioning costs.

I agree with your conclusion in your last paragraph regarding trying to have HVAC react to wind gusts. It’s not practical. You’d need total pressure ports at the building faces. Gusts can be seconds and can shift directions in seconds – this would make the HVAC go nuts trying to keep up. At any given moment, it would probably be reacting to conditions two gusts ago in another direction, making things worse.

 

[dosworld]

Show, thank you for the article. It is interesting.

Zesti, I understand your comment regarding our "stringent requirements". The requirements are there, but the argument is if we have been doing it correctly. I agree that averaging the sensors is not taking into acount the hightest pressures. I think we should maybe keep our four sensors and use the highest as the reference. this however, is still only the highest ambient pressure (sort of) since it is sheilded from the wind and subject to high/low pressure zones due to the building. The sensor in the article seems interesting, but it still does not really answer if a control system can actually adjust to the wind anyway.

 

[dosworld]

Chasbean1, thanks for the reply again.

I agree chasing the wind is problematic. For years we have just measured and controled from the average of four sensors on the roof. this has been ok at best and difficult. We do not have return fans, but there has been a push to convince some to just set the OA values for ventilation plus some for pressure/infiltration and set the exhausts and relief to keep a slight positive, but to not really actively measure it. I don't think we would be any worse off than we are since we can't fight the wind anyway.

 

[AbbyNormal]

The building envelope will have a bigger impact than any mechanical system.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[GMcD]

What AbbyNormal said, and only if you are doing a Hospital or other clean room/lab operations should you need to get so hung up on building pressure control. Now, let's throw openable windows into the mix- then what? Try measuring and controlling building pressurization in that condition....

One of the fundamental things that MUST be incorporated into the Architects' documents is building envelope commissioning scopes- rain penetration, air leakage, solar performance, thermal bridging detailing checks, etc. How can the HVAC Designer/Engineer be expected to control building air pressure if the curtain wall or window-wall system leaks like a sieve. Try getting a blower door test done on a commercial building and see what you can find, very interesting information can be brought to light.