Air balancing is the procedure whereby dampers in a ducted air system are adjusted to provide comfort in a space. In a commercial application, when a building is designed, the mechanical engineer will specify how much air he wants delivered to a specific space. The amount of air will be specified in CFM (cubic feet per minute), L/S (liters per second). The calculation made by the engineer is based upon the recommended number of air changes per hour and the temperature conditions. Several factors will be built into the design that considers the heat loss and heat gain of a specific area, internal heat loads from lighting, staff and machinery, to duct sizing and available air from the central unit. Every air handling unit that is or has been made had to go through extensive independent testing to verify the volume of air the unit can deliver under various conditions. If an office space or a home had requirements, for example, of 1,200 CFM and a system was installed that could only deliver 900 CFM, it should be understandable that the system will not provide the required comfort levels (at least under maximum and minimum load conditions. The variable in all this is the human factor. If a human feels comfortable in his or her office at 300 Cubic feet of air movement per minute, yet the engineer says his figures suggest 400 CFM, there will be a complaint by the occupant. This is why there are two forms of air balancing… “Technical Air Balancing” where the air flow to a space is measured with instrumentation and the balancing dampers are set to maintain a specific air flow rate as designed by the engineer. The second form of air balancing is “Comfort Balancing” where the rate of air flow is adjusted (quite honestly by hit or miss) to the individual taste or the person occupying the space. When I say “hit or miss” it is because it will sometimes take a few visits with a trial period in between to reach the comfort goal of the occupant. Other considerationsfor comfort levels in a space are the noise of the air entering the space, the location of entry air into the space and the temperature of the air. Anyone that has worked in an office with more than a handful of people has heard the complaints that I have dubbed “The three Bears Syndrome”… someone is always too hot, someone always too cold and someone is always just right. I have found throughout the years that the same person, under the exact same conditions will be comfortable one day and feeling like Hell hath frozen over the next day. Air balancing is virtually mandatory as the report will identify any glaring deficiencies for correction.
When an air balance is performed the technician will start at the air handling unit and record any identifying information such as model and serial numbers than measure the voltage and current draw. The current draw on the fan motor is important as it lets everyone know if there is room for adjustment, should it be required. The technician will also measure and record the pressure of air in the system (for both the supply and return) called static pressure. Once this is complete the technician will measure the air flow at each individual outlet on the system. I have seen some air balancers that simply record the information and others that make adjustments along the way or after the original test is complete. This is also where the science of air balancing breaks down and the explanation is simple. If a system is delivering 1,000 CFM at the unit and the first outlet is set to the specification (say 150 CFM), the next diffuser that is adjusted (whether for more or less air) will change the air flow at the first outlet that was adjusted. This same scenario will occur at each outlet adjusted further down the line. Typically the outlets closer to the fan provide the best air flow and the outlets farthest from the fan produce the least air flow and also experience losses in temperature. Multiple tests of the system would bring the system closer each time, but involve so much labor that the price would be prohibitive particularly when a comfort balance is what will make the occupants most comfortable.
In a residential application there are registers in the floor or ceiling that normally have an adjustment to open or close for the air flow and if the resister was lifted out of the floor you may also see a round damper in the pipe that can be opened or closed to adjust air flow. In a residential application, most owners would close off all registers in the basement to force more cooling to the upper levels during the A/C season. This will improve the situation somewhat, but remember the fan will now attempt to propel the same volume of air through fewer outlets causing more pressure and more noise. If sufficient registers are closed, the increase in pressure can actually result in less air flow and a freeze up of the A/C system. High school physics taught us that hot air rises and cold air falls. If the basement of a home is like an icebox, close the doors to the basement and ensure that there is return air from the basement (preferably at a low level in the basement) and run the fan on your furnace 24/7 to circulate the cold basement air upstairs. In my own home I have raised the A/C coil of my system 16 “ off the top of the furnace, allowing me to install three basement runs “upstream” or before the cooling coil. This means the air entering my basement will be heated in the winter and recirculated air at no lower temperature than the return air during the cooling season (usually around 72 to 75 F).
In commercial applications the balancing dampers for an outlet can be installed in the ceiling diffuser in multiple styles and price ranges or where the branch duct for the diffuser attaches to the main trunk duct.
Return air inlets are every bit as important for location and size as the supply outlets. In commercial office spaces I have found greater comfort levels for our Customers when the return air in an exterior zone office is above and along the full width of windows. This serves to remove the chill on the glass in winter and the solar heat gain in the summer, before these temperatures make the occupant uncomfortable. The supply outlet in the ceiling is best located near the centre of room where the air flow is diffused equally on all 4 sides of the diffuser and not blasting directly on someone’s neck. Your HVAC contractor can advise on this matter and provide the materials and labor to ease the discomfort.