How would you react if you were unexpectedly told your new installation needed to be air balanced? Would you welcome the testing, or would a slight panic fall over you? Could you meet the challenging standards required to achieve a proper air balance?
It's a tough discussion when a new installation doesn't perform as expected. Having to prove proper airflow delivery forces you to examine every portion of an installation differently. Any system issues will be revealed and require correction.
In the past, air balancing was a specialized service that High-Performance HVAC professionals offered customers to ensure they received what they paid for. It was proof of a job well done. Air balancing is now more common in new residential installations because of code and utility program requirements.
Unless you prepare, you may get caught with an installation that can't be balanced. Let's look at some air balancing challenges that are easy to avoid and could prevent successful air balancing of your residential HVAC system installations.
Incorrect Fan Airflow
A successful air balance starts with proper airflow at the air handling equipment. Many HERS raters and air balancers balancing residential systems today are from outside the HVAC industry and are not trained to set a blower for the correct airflow. They need your help.
Most residential fans are left with the speed settings at factory default positions. Unfortunately, this may not be correct for the installation's airflow requirements. The old rule of thumb of high speed for cooling operation and low speed for heating operation doesn't apply anymore.
While not as common as in years past, some air handling equipment still have fans that may be too weak to move the required airflow. The only way to discover such equipment is to research the manufacturer's specifications.
To prevent incorrect fan airflow from being an issue, you must set up the equipment to move the right amount of airflow necessary to meet system specifications.
Improper Coil and Filter Selection
If everything is right with the air handling equipment's fan setup, the air must next overcome the coil and air filter resistance. It likely won't be a problem if these components are selected correctly. However, if you ignore specifications, these accessories can kill airflow before it can make it through the duct system. Newer coils have more resistance than previous models. You may run into serious problems if you do not account for this.
Always refer to the manufacturer coil pressure drop data when choosing a coil. Don't randomly choose one just because it fits the indoor equipment dimensions. Instead, use the coil manufacturer's wet/dry conditions and desired fan airflow to determine if the coil pressure drop falls within an acceptable range before installation.
Incorrectly sized air filters and a lack of surface area are other barriers to proper airflow. Most air filters are too small, or the media used is too restrictive for the airflow moving through it. A quick check of the filter manufacturer's data can help guide you to the right filter.
Look for a filter that allows required fan airflow through it at a face velocity of 500 feet per minute (fpm) or less. These steps ensure the filter won't restrict airflow. However, if you're in doubt, go bigger because you can't oversize an air filter. Also, be sure to leave a clean filter for the balancer in case there's debris in the return duct system.
Poor Duct Design and Rules of Thumb
Once air passes through the coil and filter, it must circulate through the duct system. If you're still using rules of thumb for duct sizing, chances are you'll fail an air balance. Your ducts won't be large enough to move the proper airflow, and you'll know it as soon as the balancing process begins. Rules of thumb can't hide from a balancing hood.
Additional restrictions can still happen. Even properly designed duct systems will fail an air balance if poorly installed. Ducts with kinks, sharp transitions, and poor fittings result in excessive pressure drops and reduced airflow. None of this sloppiness is accounted for in the design. Be aware that both design and installation are crucial to a successful air balance. One of these characteristics, poorly executed, can cancel out the other.
The solution to poor duct design and installation is tougher than any other air balancing barrier. It involves multiple steps that may bleed over to numerous roles within a company. Breaking it down to a two-step approach helps. Start with your duct designs and get them right. Then, focus on how those designs get installed by your installation crews. It sounds easy on paper, but it takes a lot of work.
Duct Leakage
Even on perfectly designed and installed duct systems, air must make it from the air handling equipment to the supply registers and return grilles. Duct leakage is a guaranteed way to fail an air balance because of airflow deficiency.
Remember, you can't control air unless you contain it! This is a painful truth to anyone who has tried to balance an unsealed duct system. We know air takes the path of least resistance, but forget to apply this principle in the field. When a duct system leaks, air follows this rule, escapes through any unsealed mechanical connection, and never makes it to the intended delivery point.
To prevent duct leakage from hurting your air balance success, correctly seal the ducts with proper materials, like mastic. Account for any mechanical seams or penetrations as potential leaks.
No Balancing Dampers
The most obvious barrier to balancing any HVAC system is a lack of balancing dampers in branch ducts. You'll have a hard time adjusting airflow to individual registers when dampers are missing.
On the other hand, I have seen many installations where dampers were installed, but were inaccessible. Keep this in mind and place dampers where they are easy to get to. If there is drywall preventing access, install access panels, or install the dampers in the duct, near the register. However, this technique should be a last resort, not a first option.
One trick many balancers use when dampers are missing is to use the supply register dampers to balance airflow. This practice can have unintended consequences, such as poor airflow delivery and noise issues.
To prevent dampers from being an issue, everyone from the general contractor to the installation crew must be on the same page when air balancing is part of the project. It can't be an afterthought at the end of the job.
Supply Register Choices
The supply registers are the final barrier air must pass through before it's measured with a balancing hood. If any are missing, the system can't be balanced.
These accessories are easy to forget if we consider them as only vent covers. However, they directly influence room comfort, air distribution, and noise levels. It may sound obvious, but the register in a room must be able to move the airflow required for that space.
To prevent this issue, look at the manufacturer's engineering specifications for a particular register or diffuser you're considering. Don't assume a 4x10 stamp-faced register always delivers 100 cubic feet per minute (cfm) of airflow. Also consider other factors, such as "throw" and "spread," because they determine how well a register delivers air into a room.
Steps to Ensure a Successful Air Balance
Preparing and paying attention to details are keys to a successful air balance. You'll probably have to adjust some things in your design, selection, and installation processes to ensure that.
Evaluate the challenges mentioned above and see if they exist in your installations. Only when corrected can you balance your systems. Having a single document with all the essential air
balancing details is also a good idea. Email me if you would like a copy of NCI's Residential Pre-Balancing Checklist.
Knowing your systems work as intended relieves a lot of the anxiety that goes along with someone else's air balancing your systems. The best way to learn about the air balancing process is to do it yourself. Use this process to gain confidence in your system's performance and adjust as needed. You never know what you'll find.