When Will They Learn?

[WAYNE3298]

When will it become common knowledge that trying to control building static pressure and outside air quantity related to building pressure by airflow measurement and calculation doesn't work?
I have seen that tried several times and have never seen it work.
I am working on a job where the airflow of 6 residential AHU'S in an enclosure have a return fan that delivers air to them because the units do not have the static pressure to deliver design airflow on their own. The return fan speed is being controlled by a VFD with input from the AHU airflows. The result is the AHU fans are dragging the return fan along thus increasing the static pressure they need to deliver airflow. They of course are short.
Control of the return fan speed is going to be changed to maintain the static pressure at the AHU inlets. The way it should have been done in the beginning.

[bradluke0]

How are they controlling the outside air ?

[WAYNE3298]

At present they are not controlling outside air. The return fan speed thus negative static pressure available to draw in outside air varies with the speed of the ECM fan motors. They used the fan curve for the return fan and AHU ECM motor CFM readout.

[bradluke0]

You can figure out what negative static you need to draw in the required amount of outside air and make that your setpoint . Are the six ahu's ducted in parallel with each other ? Do the ahu's ramp up and down or do any of them shut off while the other ones stay running ? Interesting setup....

[WAYNE3298]

I talked them into changing the return fan control to maintain a negative static pressure (adjustable) at the outside air dampers. The AHU'S run continuously so no matter what they do the outside air will remain constant except when CO2 levels rise.
This is a very interesting setup because the unit is located in the basement but serves the floor above and there is a lot of crazy ductwork. Maintaining design outside air is not possible because the AHU ESP is 0,8 inches which does not include the filter design pressure drop of 0.5 inches. CO2 control will have to do most of the fresh air work.
I'm walking a tight rope on this one.

[Subcool215]

You are right. Controlling to static pressure is the best way. I'm a Service tech and air balancer now. But I was a controls tech in my early carreer. Working on mostly siemens and johnson controls controlling large building air handlers. And controling fans to static pressure was always the best way. Air movement is very turbulent. Its been 15 years since I was doing controls like that. But I bet at some point, somebody thought controlling to calculated cfm was a good idea because thats how vav's are controlled. But I think air flow inside an air handler is waytoo turbulent to get a reading you can actually control a slow reacting vfd with. Johnson controls were the first ones to even monitor ahu cfm before siemens. And i can remember the cfm value jumping up and down by at lease a few thousand cfm very quickly.
Theres more yhan one way to skin a cat. Maybe it will get perfected. Or maybe something is wrong with that particular control program. But controlling by static pressure always worked just fine to me.

[Cagey57]

Wayne,
It is common knowledge ... to individuals that have "been there, tried that and found it plain old doesn't work".
I have no idea where some of these CE's get the idea that building pressurization can be calculated without regard to any other factor.
I worked on a school that the CE refused to listen to "good sense" and, after 3 years, we (controls guy's) had to simply walk away because we lacked the "PE" on staff to override the original CE's design.

To quote Forest Gump "Stupid is as stupid does" ;-)

(edit) To answer the OP, they won't learn until someone smacks them over the head with the correct method.

[WAYNE3298]

Cagey the problem you had is more common than you might think. I was recently dropped from doing balance in a school district because I insisted in writing that the problems be addressed. The space occupants loved me but the powers that be didn't.
On one occasion I was told to stay away from the lady in charge of a building because she was mean and talking to her would only cause problems. I walked by her office and saw she was wearing a sweater on a day that was 100F outside. I went in and ask her if she was always cold and wanted me to do something about it. She said yes because I am always so cold I'm miserable. I raised her temperature set point and adjusted her airflows so no air blew on her. I went back two days later and ask if everything was OK. The lady was so happy I thought as ugly as I am she might kiss me. The lady wasn't mean she was miserable.
If I had been the balancer on your job you would have had all my support.

[Cagey57]

Cagey the problem you had is more common than you might think. I was recently dropped from doing balance in a school district because I insisted in writing that the problems be addressed. The space occupants loved me but the powers that be didn't.
On one occasion I was told to stay away from the lady in charge of a building because she was mean and talking to her would only cause problems. I walked by her office and saw she was wearing a sweater on a day that was 100F outside. I went in and ask her if she was always cold and wanted me to do something about it. She said yes because I am always so cold I'm miserable. I raised her temperature set point and adjusted her airflows so no air blew on her. I went back two days later and ask if everything was OK. The lady was so happy I thought as ugly as I am she might kiss me. The lady wasn't mean she was miserable.
If I had been the balancer on your job you would have had all my support.

Wayne,
The Balancer and I were on the same page. The CE was not about to relinquish the, as the spec said, "CFM Offset to maintain building Static pressure". The email string went on for pages and pages with so many different colors it was outrageous.
The end user reached out to us as soon as the warranty was up asking for a solution. Our normal Go To engineers wouldn't touch it because there was so much to undo before it could be fixed. Basic No Win situation for a 2nd CE. The school district didn't have the money ($$$$$.$$) to fix it correctly so it remained SNAFU !

Oh, forgot to mention it was a LEEDS building, Gold for sure possibly Platinum. Again, SNAFU !!!!

[WAYNE3298]

How is it that school districts don't have the money to fix something right but have the money to pay utility bills much higher than they should be?

[Cagey57]

How is it that school districts don't have the money to fix something right but have the money to pay utility bills much higher than they should be?

That is the conundrum of Government entities.
I think the quote is "If you don't have time to do it right when will you have time to do it over" ? or something like that ...

See Signature lines, kinda sums thing up IMO.