Obstacles to Air Balancing Residential HVAC Systems
Consider using a residential pre-balancing checklist
Air balancing is becoming more common across the country and being enforced on many new residential HVAC installations due to code and utility program changes. In some areas, air balancing requirements are stretched to meet minimum compliance while others are strictly followed.
Regardless of specifications, it’s tough to get a system where it can be air balanced. Being accountable for proper airflow delivery causes a tech to examine every portion of an installation differently. Airflow is no longer assumed since test instruments now measure it.
Do the systems your company services meet the tough standards needed to achieve proper air balance?
Let’s look at some of the most common obstacles to achieving a successful air balance on a residential HVAC system.
NO BALANCING DAMPERS
The No. 1 obstacle to balancing any forced-air system is dampers missing from branch ducts. A tech will have a hard time adjusting airflow to individual registers when these valuable accessories are absent.
On many older duct systems, balancing dampers came standard on every installation. As installs became low-cost driven, dampers were one of the first accessories to be removed to cut cost. Installations never recovered from this mentality with dampers viewed as obsolete.
When dampers are included, I’ve seen many occasions where access to the dampers is nonexistent. They are often installed so that they are out of reach to the balancer once drywall is installed.
Forward-thinking installers are forced to place dampers in the duct near the register. This should be a last resort, not a first option due to noise and added complications while balancing.
One trick many balancers have tried when dampers are missing is to use registers to balance airflow. This practice can have some unintended consequences as throw and spread are altered when vanes are closed. Noise is also an issue with this method.
To prevent this 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 once everything else is in place.
Our industry has a major problem with restrictive duct systems based on rule of thumb sizing. The side effects of this practice often take an extended period of time to show up in a system where airflow isn’t measured. They show up immediately when an air balance is performed.
If techs are still using the 0.10 rule of thumb to size duct systems, chances are very high the air balance will fail. The ducts won’t be large enough to move the right amount of airflow, and it appears as soon as a balancing hood is placed on a register.
Additional restrictions occur in the installation of the duct system. Properly sized duct systems will fail an air balance when poor installation conditions exist. When ducts are installed with kinks, sharp transitions, and poor fittings, the results are excessive pressure drops and poor airflow.
Be aware that both design and installation are crucial to a successful balance. One of these poorly carried out characteristics can cancel out the other done correctly.
Air must make it from the air-handling equipment to the register. Airflow lost through duct leakage is unavailable at the register and will result in an airflow shortage.
To control air, it first must be contained. This is a painful truth to anyone who tries to balance a duct system that has not been sealed. When a duct system leaks, there is no controlling air’s final destination. If conditions exist in a duct system that allow air to bypass its intended delivery point, it will take the path of least resistance all day long.
To prevent this from hurting the success of an air balance, make sure ducts are sealed correctly with proper materials. Any mechanical seam should be accounted for as a potential leakage site.
Most blowers are left with fan speed settings at factory default positions. Unfortunately, this may be incorrect for the airflow needs of the installation. Many balancing residential systems are created by companies that work outside the HVAC industry and do not have the training to set up a blower for the required amount of airflow.
In some air-handling equipment, the fan may be too weak to move the required amount of airflow. The majority of residential fans are rated to operate at a maximum of 0.50 inches of water column. The typical piece of equipment can’t move the necessary airflow when static pressure is over this nameplate rating.
There are some fans out there that can’t move the required airflow even when it registers below the 0.50 rating. The only way to discover these is to do the research. In many cases, weaker fans can barely handle the addition of a coil and filter. Add a duct system to this combination, and the result is a system that can’t be balanced.
To prevent this from being an issue, the installing contractor has the responsibility to assure the equipment has a fan capable of moving the right amount of airflow. It’s a tough discussion when a brand new piece of equipment can’t perform as intended.
RESTRICTIVE COILS AND FILTERS
Newer coils are an obstacle air balancers often deal with. Coils made in recent years have more fins per inch and thicker slabs than previous models.
This results in coils that are so restrictive to airflow that the actual total eternal static pressure of the system is often 200 percent of the equipment’s listed rating. When this happens, equipment airflow is often less than half of what is required due to excessive coil resistance.
Be sure the installing contractor refers to manufacturer coil pressure drop data before they randomly choose a coil. By using the coil’s wet/dry condition and desired fan airflow, they can determine if the pressure drop of the coil is within an acceptable range before installation occurs.
Incorrectly sized filters and lack of surface area are another challenge. Any balancer who has attempted to balance a furnace with a 5-ton blower with a single return drop and a 16-by-25-inch filter mounted on the cabinet knows the frustration of trying to overcome obstacles such as this.
Many filters are too small or the media used is too restrictive for the application. When it comes to filter sizing, it’s always safe to keep the filter face velocity below the manufacturer’s recommendations.
MAINTAINING PROPER BALANCE
To assure a successful air balance, the methods a tech uses to install systems will probably have to be adjusted. Begin by evaluating the top obstacles mentioned above and see if they are present. Only when these obstacles are corrected can systems be correctly balanced.
If balancing is on the agenda, it’s a good idea to have a residential pre-balancing checklist to keep a tech straight. This provides workers with all the essential details to account for on one document. If you would like a copy of a residential pre-balancing checklist, send me an email.
Publication date: 5/15/2017