Troubleshooting HVAC Air Flow Systems

It's been said that in real estate, there are three important things to consider when buying a property:

Location, location, location.

In the servicing of HVAC equipment, there are three important things to consider:

Air flow, air flow, air flow.

Often, when an inexperienced technician responds to a "not cooling enough" complaint, they focus on the refrigeration system (and adding refrigerant) when they should be considering the air flow in the system first. And, a first step in evaluating an air flow system is understanding the fundamentals of air volume and static pressure in a given situation. (See the illustration below)

HVACR Trouble Web Site HVAC Air Flow Fig1

Figure 1


The first thing to understand about the system shown here is what the total air flow is in the system and why. When you look at the return side of the illustration, you'll see that the total air flow is 1200 CFM. By applying a simple rule of design that manufacturers shoot for an air flow of 400 CFM per ton, we can understand that this is a 3-ton system.

When you check the supply side of the system, you'll see the same volume of air being delivered there, and divided in half to be distributed to the two sections of the main trunk.

And, following the main trunk, you can also see that the individual branches are delivering air into the individual rooms in the building.

In addition to the air flow shown in CFM, you can also see the static air pressure measurements shown in water column inches.

Overall, this illustration shows a system that is operating normally and in balance since the same amount of air flow that is entering the return is leaving the supply, each room is getting the required air flow according to system design, and the static pressure difference between the return and the supply at the point just beyond the indoor coil is only .05 "W.C. ......

......Entering pressure at the fan inlet is -.02 "W.C., and pressure at the fan outlet is .02 "W.C., which adds up to a total of .04 "W.C., and adding .01 "W.C. after the air has gone through the indoor coil, brings us to the total of .05 "W.C. static pressure.

The point to understand here is that if we had a customer who was complaining that the last room on the right (indicated at the top right of our illustration) is "always too warm", you can take steps to troubleshoot this situation.

The first thing to do is check the static pressure between the return and the supply. If it's not within the maximum .05 "W.C. recommended by the manufacturer, then we would know that we have to solve that situation first. If it was within the specifications, though, we could then move on to finding a way to balance the air flow to the affected room.

With an air flow hood or anemometer, measuring that room would show less than the required 200 CFM, and measuring all the other supply registers would show that there was excessive air being delivered at one or more register location. With the lack of balance evaluated, adjusting the registers or system dampers would solve the customer's complaint.

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