Controversy and confusion reign as contractors in the state of California work to decipher new regulations limiting the use of bypass ducts in residential zoned heating and cooling systems.
The code change is part of the state’s Title 24 regulations implemented July 1, 2014, and while the rule is not a complete ban on the use of bypass ducts, it greatly limits their usage in newly constructed residential buildings.
According to the Title 24 rules, any newly constructed low-rise residential building including zonally controlled central forced-air cooling systems must be capable of simultaneously delivering, in every zonal control mode, an airflow from the dwelling through the air-handler fan and to the dwelling, of greater than or equal to 350 cfm per ton of nominal cooling capacity. The air-handling unit fan efficacy must also operate at a value of less than or equal to 0.58 watts per cfm as confirmed by field verification and diagnostic testing in accordance with the applicable procedures specified by the state’s code.
An exception exists for multispeed compressor systems, variable-speed compressor systems, or single-speed compressor systems that utilize the performance compliance approach if those systems demonstrate compliance for airflow (cfm per ton) and fan efficacy (watt per cfm) by operating the system at maximum compressor capacity and system fan speed with all zones calling for conditioning.
Zonally controlled systems are eligible for energy compliance credits in heating under Title 24, but the requirements are stringent in order to provide the energy benefits. A zoning system using duct dampers, and/or bypass ducts, may be allowed under a performance approach, but may not qualify for the zonally controlled credits. This would not mean the zoning system itself would be noncompliant.
Systems may be eligible through prescriptive or performance ap-
proaches. When homes utilizing the prescriptive approach have automatic zonal control, they are prohibited from using bypass ducts that divert supply air directly back to the return air stream. Using the performance approach, there is an energy penalty for systems choosing to utilize bypass ducts for zonal control unless there are at least 350 cfm per ton flowing in the most restrictive zonal control mode.
The California Energy Commission (CEC) has approved select compliance software packages to help properly design and submit zoning systems with bypass dampers. Verification must reveal an improved system airflow rate and/or reduced fan power draw, including more efficient motors and ducts that have less resistance to airflow, and determination of fan efficacy utilizing simultaneous measurement of system watt draw and airflow rate. Permit applications for designs utilizing bypass dampers should be accompanied by the appropriate certificate of compliance.
For more information, visit http://bit.ly/CaliforniaTitle24.
The CEC believes such regulation is necessary because it deems bypass ducts — short ducts placed between the supply plenum and return air plenum in a zoned system — inefficient. CEC representatives sponsored a “2013 Building Energy Efficiency Standards Staff Workshop on Residential Zoned Air Conditioners” case study in July 2011 examining HVAC efficiency at 80 Sacramento-area homes built in 2007 or later.
The study, facilitated by Bruce Wilcox, John Proctor, and Rick Chitwood, examined 16 multiple or zoned split systems. Wilcox, Proctor, and Chitwood determined the targeted cooling systems featured high cooling duct pressures, low-capacity efficiency, and high cooling fan wattage. They also concluded that any added comfort provided by zoned systems is accompanied by increased energy cost.
“The purpose of all this is to make sure that each unit performs efficiently,” said Proctor. “It’s not to beat up contractors or make life difficult for them. It’s to make sure that when you put in a unit with a particular efficiency, it actually performs up to that stated efficiency. Running air in a circle and losing efficiency doesn’t make any sense.”
Two Sides of the Discussion
The validity and accuracy of the CEC’s assumptions have been called into question by some, including Mike Holscher, senior project engineer at Indianapolis-based Jackson Systems LLC, a manufacturer of zoning products and controls.
“What happened was John Proctor said that zoning with bypass ducts deteriorates equipment. We disagreed and wanted to take a look at his tests and assumptions that were made,” said Holscher. “We looked at an actual house the CEC used for its case study and identified multiple red flags. We found the opposite results came back from what was actually proposed and that a lot of misconceptions remain out there about bypass ducts.”
Dick Foster, president, ZONEFIRST, a zoned systems manufacturer, said, despite a popular misconception, the Title 24 regulation is not an all-out ban on bypass ducts.
“If you are going to put in a zone single-speed damper system and a bypass duct, then you simply have to declare it,” said Foster. “If you do put in the bypass duct, then California uses a couple of different ways to comply. One is a prescriptive method, and one is a performance method.
“The homes the CEC used for its test cases were older with poorly designed duct systems from the beginning. They should have never been used to make these determinations,” continued Foster. “The CEC also tested homes without zoning that had performance issues due to poor design or installation. It’s important to note that a new home designed and installed with zoning and a bypass according to ACCA Manuals J and ZR, and CEC guidelines, showed no issues with efficiency drop. The problem is not zoning, it’s why aren’t all systems designed and installed according to industry-accepted guidelines?”
Holscher said Jackson Systems’ Barometric Zone Damper (BZD) circumvents the need for a bypass duct altogether, acknowledging that many are considering the BZD as a bypass duct alternative in the wake of California’s Title 24 restrictions.
“Jackson Systems’ BZD is spring loaded and opens when the duct static reaches 0.3 inch,” said Russ Donnici, president, Mechanical Air Service Inc., San Jose, California. “They have worked well; however, we’ve had to add some weights to increase the bleed point to 0.5 inch in some cases. Typically, when we are called in to check a nonperforming system installed by another company, and the system has a bypass damper, it is almost always not set properly. It’s almost like service techs don’t own a static pressure meter.”
“It’s our belief that zoning actually reduces energy consumption, so it’s really a win-win for consumers,” said Tom Jackson, CEO, Jackson Systems. “We always talk about comfort first, so if the second story of a two-story house is 10°F hotter than the main floor, you are going to be mad as hell. Comfort first, but, the great thing is, not only does zoning provide comfort, but energy is saved as well. The government shouldn’t have the control here.”
Implementation of the Regulation
The stark contrast in opinions regarding the efficiency of bypass ducts is secondary to the actual implementation of the regulation and its effects. However, those effects and ramifications have been minimal, according to Foster.
“It went into effect in July but has not affected business to this point,” said Foster. “Contractors haven’t gotten much feedback just yet, as most just set the dampers to their minimum positions. This means you never fully shut off a system 100 percent. The CEC allows minimum position on zone dampers, and, over time in particular homes, it creates customer callback. I’ve seen it where it can overheat or overcool a zone.”
So, how do contractors become fully educated on proper zoning, both under the new rules in California and in general? ACCA’s Manuals J, D, and ZR may be a good place to start. Manual J covers residential load calculation, Manual D covers residential duct systems, and Manual ZR covers residential zoning systems.
“For contractors, if you really want the system to work efficiently, you need to make sure you always have 350 cfm per ton flowing through the system,” said Foster. “If you have a two-zone system, then you are either going to have to allow some air to go through a zone that isn’t asking for cooling, use larger ducts for both zones, or use less restrictive ducts.”
Now that the rule is official in California, is there a chance similar regulation could spread across the country?
“Some people say that as California goes, so the country goes,” said Jackson. “There are several states, like North Carolina, that basically require zoning on two-story homes. Other states did a much better job partnering with contractor associations and didn’t come out with some crazy rule like California, which did not test zoning properly. Other states are trying to do the right thing.”
“There is a tendency for stuff to spread across the country, obviously, but not everything,” said Proctor. Citing a Carrier-ASHRAE study as evidence, Proctor said: “With respect to the bypass, the numbers are really very clear. Use of the bypass is extremely detrimental to the efficiency of the unit. It doesn’t make sense to cripple the efficiency of an air conditioner. People are so used to bypasses that they think they sometimes have to use them when they really don’t.”
Publication date: 12/22/2014