Flow Hood Testing: Proving a Home’s HVAC Performance

Most HVAC installations for a new home begin with the Air Conditioning Contractors of America (ACCA) Manual J load calculation.

If the values of the home’s building enclosure, room volumes, the location’s climate data, and orientation to the sun are properly entered into the program, the HVAC contractor can reliably determine the required air delivery, in terms of cubic feet per minute (CFM), for each conditioned room in the home.

Some HVAC contractors rely on “rules of thumb” to determine system and duct sizes and register locations, hoping to meet Manual J requirements, while others use the ACCA’s Manual S to properly size and specify the right equipment, and Manual D to properly design the conditioned air delivery system.

More sophisticated contractors will run the information through ACCA’s Manual T to specify the best registers and their placement to ensure that each room has an even temperature and that the home’s room-to-room temperature is consistent.

However, even if all these manuals are followed to create an excellent HVAC system, it still ultimately falls to the installer to follow the outputs correctly and install the ductwork with care. Therein lies the rub.

What To Watch For

The fact is, the duct layout created by Manual D fails to account for and show the home’s roof truss design, the location of the truss webs and bracing, roof pitch, plumbing and fire sprinkler locations, and other obstacles that hinder an “ideal” installation.

As such, in reality, the best way to run the ducts from the supply plenum to the register locations is often a very difficult task.

Unfortunately, our experience has shown that many installers lack the proper training to understand and follow basic flex duct installation rules as outlined by the Air Diffusion Council (ADC) guidebook, Flexible Duct Performance & Installation Standards. This guide is critical to ensure the system will deliver the right amount (CFM) of air called for by Manual J.

Poorly installed flex ducting is one of the most common culprits behind rooms that run too hot or cold in new homes. When ducts are installed with hard bends around framing or each other, are not properly stretched, or become pinched as they pass through truss webs, they will not deliver the intended amount of air.

RELATED: 

• Why Compact Duct Layouts for HVAC Make Sense
• Tips for Placing HVAC Ducts in Conditioned Space
• Avoid Unnecessary Expense and Homeowner Discomfort: Get HVAC Ducts Right

Take the Test

Most home builders today are familiar with the duct blaster test that ensures that an HVAC system (specifically its air delivery system) meets the maximum air leakage allowed by the latest energy and building codes. Builders often assume that if the system passes that test, it should operate just fine ... neglecting to consider that the CFM requirements called out by Manual J also must be met.

So how does a builder know for sure if they jive? Enter the flow hood test.

A flow hood test (see photo) measures the actual amount of air coming out of each register in each room of the house. Though the test has existed for decades, it is not commonly used in residential construction (but should be).

The results of the flow hood test should come close to matching the Manual J calculations. If a duct needs to be adjusted, it is beneficial to include dampers where the duct comes off the plenum or inside the register boot. Relying instead on an adjustable diffuser often leads to constant adjustment (intentional or accidental) by the occupants.

For solving the mystery of hot or cold rooms in a new or existing home, the flow hood test is the gold standard.

However, to get ahead of comfort issues, start with a properly designed and installed HVAC system and educate installers and project managers on how to evaluate a properly installed flex duct system according to the ADC guidelines. Let’s put an end to unnecessary comfort complaints!