HIGH-PERFORMANCE HVAC
CREATING TIGHTER DUCT SYSTEMS: An efficient strategy
W
hile preparing for a training program on the compelling history of energy efficiency improvements in Canadian homes over the last 30 years, I was reminded of a Natural Resources Canada (NRCan) initiative from back in 2004 to optimize the energy used specifically for the distribution of air in new and existing homes. It was recognized at the time that the electrical consumption of furnace fan motors was by no means as significant as the space heating or cooling energy consumption in homes. None the less, I was asked to facilitate an industry forum that established some long-term objectives to optimize electrical use of fans in HVAC systems. There were several initiatives proposed from the forum. The first was to reduce the overall capacity of heating and cooling systems by ensuring more accurate sizing of equipment. The outcome was the re-write of the CAN/CSA F280-12 Standard (Determining the Required Capacity of Residential Space Heating and Cooling Appliances). This was completed in 2012 and systems designed to the new standard are 30 to 40 per cent smaller on average than under the old standard. The second initiative was to improve fan motor efficiency. This has been accomplished as of 2019 with a very specific Fan Efficiency Rating (FER) regulation in North America that ensures either electronically commutated or constant torque fan motors are used in residential air handling equipment. The third initiative was to reduce overall air delivery needs by generally promoting energy efficiency in homes but also by reducing duct leakage. As far back as the early 1990s, duct leakage studies, primarily in the U.S., showed that as much as 20 per cent of the total system airflow was lost through duct work leaks before reaching supply outlet grilles, and as much as 50 per cent of return airflow was drawn through duct leaks rather than from the return grilles. It is important to acknowledge that the focus of U.S. studies was on duct systems installed in unconditioned spaces such as attics and crawlspaces whereby duct leakage resulted in a significant direct energy loss. By contrast, a 2002 study I completed for a large gas utility in southern Ontario on 60 homes (30 new construction and 30 older homes), found that supply duct leakage on typical sheet
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A powered flow hood is used to accurately measure airflow from each supply register in a home. metal duct systems was never less than 20 per cent and averaged 25 to 35 per cent. It shouldn’t be a surprise, given that it is most common in Canada to use joist or stud-lined cavities for much of the return duct system. Return duct leakage was so high, over 50 per cent, it hardly made sense to even try and measure or seal it. Unlike the U.S., most of the duct work in Canadian homes is located in conditioned space and thus there is an assumption that the duct leakage isn’t a direct energy loss to outside. However, in the 2002 study, in half of the houses studied, as much as five per cent of the duct leakage was found to be to the outside. In some cases, this was because there were ducts in unconditioned spaces such as ceilings over garages or in boxed in cavities adjacent to outside walls. In other cases, it was simply that the stud-lined cavities used for return air pathways, such as for high wall returns in upper floor bedrooms, were not thoroughly air sealed at the top plate of the wall to the attic. Even without direct duct leakage to outside, there are energy penalties connected to duct leakage in Canadian homes. The additional fan motor power to deliver the additional 25 per cent of air needed to compensate for duct leakage was estimated by
Gord is a professional engineer who has spent 35 years helping builders and HVAC contractors implement innovative technologies into high-performance homes. He has particular expertise in IAQ and airflow management in houses, and can be contacted at gordc@buildingknowledge.ca.
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2022-07-06 11:21 AM
