Balancing Hoods Under Attack

Feb. 15, 2012
Just as other energy standards have chosen to embrace air balancing as a specified method to measure the performance of HVAC systems, the State of California has proposed a ban on air balancing hoods as we know them.

Just as other energy standards have chosen to embrace air balancing as a specified method to measure the performance of HVAC systems, the State of California has proposed a ban on air balancing hoods as we know them. With thousands of HVAC contractors and energy professionals heavily invested in commercial balancing hoods and enjoying success by satisfying consumers with comfort and energy savings, why would the rules change in the middle of the game?

With the International Energy Conservation Code, the new Energy Star Standards, DOE and ACCA Quality Installation all requiring air balancing in their new standards, why would California suddenly write proven air balancing hoods out of their standards and specify “fan powered flow capture hoods” as their only acceptable test instrument for measuring airflow at grilles and registers? Especially when it’s new to the market and literally 99% of field practitioners own and operate balancing hoods that meet all other industry standards already in place. The published cost of this new hood is close to $3000 and is nearly 25% more expensive than well performing hoods that are already on the market.

For over 50 years, air balancing has been the specified method for quantifying and verifying HVAC system performance. Suddenly a powered flow capture hood is offered and it’s immediately written into the 2013 Title 24 standards, excluding the hoods that have been used for decades. Many HVAC contractors and energy raters are infuriated by this decision as they have invested in proven technology and adapted system performance measurement as their primary test method for measuring, diagnosing and repairing HVAC systems.

The threat to us all is that in energy standards, as California goes, the rest of the country often blindly follows.

More than Hoods Only

Believing air balancing hoods to be a stand-alone test instrument, researchers unfamiliar with long standing air balancing techniques have sought to discredit balancing hoods by proposing garbage bag and laundry basket airflow measurement methods for the last decade. Plastic bags and duct pressurization fans attached to cardboard boxes have clearly lost the battle. Meanwhile, thousands of field practitioners have gained the skills and mastered the discipline to use the entire suite of air balancing instruments together to measure and verify live system performance.

As clearly defined in ASHRAE Standard 111, SMACNA and the other industry standards previously mentioned, measuring HVAC system performance requires the combination of airflow, static pressure, airflow traverses, temperature, fan speed, fan laws, air density correction, psychometrics, fluid dynamics, velocity and electrical measurements to agree before a sound decision of airflow performance can be made.

Air balance hoods are one in a collection of test instruments required to effectively measure airflow and system performance.

Functionally Impossible

A review of the proposed changes in the California energy standards reveals that return air flows are to be measured with this new powered flow hood. The reality is that the skirt opening of this device is only 16-in. x 16-in. and is incapable of measuring over 90% of California return grilles, because the type of grilles used in residential systems typically exceed these dimensions. Existing air balancing hoods will easily measure higher return grille airflows and fit on 90% of California return grilles.

In addition, the maximum capacity of the fan powered hood is only rated to measure up to 300 CFM. This further disqualifies it for use in California where few homes have more than two returns per system and the average return grille airflow exceeds 600 CFM. The over anxious move to integrate this hood was so speedy that authors of this proposed revision failed to realize it cannot function for the use it was intended in 90% of the homes it’s specified to test.

Additional Concerns

Additional concerns about the new hood include the effort required to achieve a reliable reading. According to the user’s manual, various rings must be placed in the fan powered discharge of the hood in order to overcome the backpressure of the narrow throat of the hood. Once the correct ring has been inserted into the hood, switch positions on the hood must be adjusted for each change in the ring size. Also, when the operator of the device needs to switch from return to supply airflow readings, another reconfiguration must be performed by removing the fan assembly from the hood housing, the controls must be reconfigured and then the hood must be reassembled.

Also, the fan-powered hood is battery powered and has a battery life of typically less than 45 minutes and requires a 3-hour recharging period. If the batteries should become weak and require recharging, the fan controller will need to be replaced with a corded model before balancing may resume. Add to this the fact that this hood is heavier than its competitors and it could require substantially more time and include much more chance for error than a traditional air-balancing hood.

The manufacturer of the new powered flow hood has been unable to adequately address the question of the powered hood’s ability to work with an HVAC system’s variable speed fan. As the fan powered hood works to achieve neutral pressure, the functions and programming of a variable speed fan may cause the system fan to hunt to compensate for the changing pressure imposed by the fan powered hood. Since more and more variable speed fans are required to overcome the pressure drop of today’s filters and coils this issue is of critical importance.

Hood Manufacturer Objections

A comment to this proposed change has been filed by TSI, a manufacturer of the balancing hood used most frequency for residential HVAC system testing and balancing in the United States.

Additional scientific evidence has been provided from further testing completed by TSI following the publication of a decade old report published by Lawrence Berkley National Laboratories. In its original form, this report has been the primary foundation for the rejection of balancing hoods by the building science community. The data shows satisfactory performance of the TSI models 8371 and 8373 balancing hoods that are excluded from the original LBNL report. To review the TSI report, click here.

If the California Energy Commission has decided to specify individual test instruments, would it not be appropriate to add this new powered hood to the list of existing industry proven air-balancing hoods that they have previously accepted instead of immediately disqualifying all others?

Sign the Petition

To sign a petition requesting the California Energy Commission to retain the existing balancing hoods that have previously accepted, go to [email protected] and sign the petition by email online. Let’s be heard, or be forced to use an inferior balancing hood.

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company and membership organization. If you're an HVAC contractor or technician interested in a free report on how to use a balancing hood to diagnose an HVAC system, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at for free information, articles and downloads.