ASHRAE Standard 90.1 for Spiral Flat Oval Duct Systems
Many of the jurisdictions we work in are just now getting building code updates that bring them up to the 2010 or 2013 ASHRAE Standard 90.1 energy code. It’s not surprising that the new energy codes have not yet started affecting how engineers design duct systems. Here’s a spoiler alert – if you still design duct systems around rectangular ductwork, you are not going to meet the new energy codes without taking some extraordinary measures.
What changed? The new ASHRAE energy codes are the culmination of years of study and debate to address something we’ve known for years – duct systems leak too much and it’s been costing billions of dollars. One 2005 study conservatively estimated duct leakage cost as $2.9B per year. The new codes have eliminated some faulty practices that have existed for decades. First of all, there is no more “Class B” and “Class C” duct sealing. All ducts – even low-pressure ducts – must be sealed to Class A.
That means all joints and all seams except spiral lock-seams must be sealed. Even though all ducts are not required to be leak tested, they must still meet the same leakage requirements of the duct that must be tested. The maximum permitted duct leakage is “Class 4” – 4 CFM per 100 ft2 of duct surface at 1” WG. You need look no further than the SMACNA HVAC Air Duct Leakage Test Manual to see it clearly shown that the expected leakage of a properly sealed rectangular Class A sealed duct system is Class 6. For round and flat oval duct systems it is Class 3. Can rectangular duct be sealed enough to meet this new standard? Probably, but not without buckets of sealant and practices far beyond what most engineers and contractors are used to. This applies to the low-pressure duct as well.
When you get into the economics of this change it makes a lot of sense. Choosing the right duct options to meet these new standards can have little or no net impact on cost. Trying to “fix” the wrong products to meet the new standard will cost a lot. In the end, an estimated cost to the owner of $1.75/CFM/year for duct system leakage – regardless of where in the system it was happening – makes the payback for better systems worthwhile.
ASHRAE Advanced Energy Design Guides –– “Round duct is preferred over rectangular duct. However, space (height) restrictions may require flat oval ductwork to achieve the low-turbulence qualities of round ductwork.” That is the wording in your ASHRAE Advanced Energy Design Guides. It is a practical and common-sense approach to sustainable construction and meet the net energy usage goals of ASHRAE and industry standards.