Air Balancing, Air Balance Reports

After a new heating, ventilating, air conditioning system has been installed in a building, it is likely that the amount of air moving through all parts of the system will need some adjusting. This is air balancing. My experience is that this part of the job is frequently a problem.

Why does the system require adjusting? In reasonably simple systems, you should design the system so that air flows through all parts of the system can be calculated and known. The system gets built the way you designed it, and when it's turned on, it flows just as you designed it. That's the cheapest way to build a system, and you should do it if you can. But on so many jobs, it cannot be so simple. There may be too many branches, with air flow divided to too many outlets, say a total of 5000 cubic feet per minute (cfm) air flow divided to 10 outlets at 150 cfm, 10 outlets at 300 cfm and 1 outlet at 500 cfm, and there are 2 bathroom exhaust fans at 250 cfm each, and 5 return inlets at 500 cfm each. Are you so confident that you can size the inlets, outlets and duct so accurately that without any adjustments, when the system is turned on, it will flow correctly to every part of the system? Probably not. And consider that the duct may not be placed exactly as you planned, due to some last minute interferences or changes by others. For any complex duct system, you want to design your best, of course, but you also want to be able to make adjustments. That means requiring dampers in the duct, so that an "Air Balancer" can make adjustments to the air flows when the system is first started up. The dampers are reasonably cheap, usually they're a single piece of sheet metal on a rod through the center of a duct, that can be slightly rotated and locked in place, slowing the air flow to one duct.

Seems reasonable so far.

Problem: After the contractor has installed all the duct, according to your plan, the general feeling is that the construction is done. There is a general resentment that we now have to wait for someone to make adjustments.

Problem: If you allow the air balancing to be done by the contractor, what prevents the contractor from doing a quick and inaccurate job, so the contractor spends the absolute minimum on the balancing? For this reason, most Owners hire an Air Balancer independent of the Contractor and the Construction contract. This is good in principle, but it is a burden on the Owner who doesn't need this extra complication in the job, and the Contractor isn't always happy to have an independent person come onto the job and be in the middle of the construction, but not be under the Contractor's control. Especially if the Air Balancer finds fault with the Contractor's work, or if the Air Balancer and Contractor don't coordinate their schedules well, and the Air Balancer delays the job.

Problem: What prevents the Air Balancer from doing a quick and inaccurate job, or worse, a slow and inaccurate job?

Problem: What if the Air Balancer balances many of the inlets and outlets to within 7% of their design value, but you wanted everything to be within 5% of design value? Do you fire the Air Balancer or tell him/her to go back and correct it, or do you just say "Oh I give up. It's not worth the trouble, let's just be done with the job?" Remember, everyone wants to be done with the job. Why are you, Mr./Ms. Engineer, being such a hard-nose and requiring more air balancing, slowing down the job?

Problem: There's trouble with the Air Balancing so you go to the specifications to see what should be done. Arghhh! The Air Balancing specifications are a mess, who wrote these, didn't anybody review them? They're no help at all!

Problem: The Air Balancing specifications call for all sorts of work that you don't want. Vibration testing, Sound measurements. I can't believe we're paying for this! What for?