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  • A Bad Duct System Trumps High Efficiency HVAC Equipment

    May 3, 2015 | Blog | admin
  • by Jeff Sizemore

    In the past two years, I've helped my parents-in-law make dramatic improvements in their home. Two years ago, I tackled their building envelope, sealing the air leaks, isolating the mechanical room, and adding insulation. One year ago, I advised them on the purchase of their new high efficiency, sealed combustion furnace and variable speed air conditioner. This year, my task is air flow in the duct system.

    When I did the building envelope work, they saw a dramatic reduction in their energy bills. Replacing their old, inefficient heating and cooling system, however, didn't have as much an effect on their energy bills as I expected. In the past six months, I've been learning a lot about the mechanics of air-flow in duct systems and have started applying it to my favorite guinea pig: my in-laws house.

    So, last weekend I went over to their house and measured the static pressure. It was supposed to be no more than 0.5 inches of water column (w.c.), but I found that it was actually 1.05" w.c. Wow! When I took the 6" pleated filter out, that dropped it to 0.83", which is better but still too high. With the filter back in, I then measured the pressure in the supply plenum, which should have been about 0.1", and found 0.02" instead. That low pressure indicated a problem, too.

    At that point, I wasn't sure about the best way to proceed with diagnosing the problem and prescribing a solution, so I called my friends a local air conditioning company. A technician came over today, and I got to watch him go through his procedure for analyzing the problem. He verified my measurements across the blower and at the supply plenum and then measured at other points in the duct system.

    What Ken found is that the static pressures on the return side of the duct system seem to be the main problem. They're way too high (or low, rather, since they're negative), indicating that the blower is pulling as hard as it can but isn't getting enough air. This situation can lead to two problems in the high efficiency equipment here:

    • The variable speed, electronically commutated motor (ECM) used in high efficiency equipment is much more efficient than the standard motor IF the air-flow is correct. In my in-laws' house, the motor's having to fight for air and ends up working harder - and wasting energy - as a result.
    • To keep the evaporator coil from freezing up, high efficiency air conditioners can reduce the refrigerant cycling through the system if there's not enough air flow. That means not only do you get less air flowing through the system, but it doesn't get cooled as much. It also doesn't dehumidify as well.

    The solution to the static pressure and air-flow problem in this house entails modifying the return side of the duct system so that it moves more air. Here's what they're going to do:

    • Replace the return plenum with a larger one.
    • Replace the filter with a larger one.
    • Replace all the return grilles with new ones that allow more air flow.
    • Add another return vent in the dining room.

    Retrofitting an existing duct system can be a process of trial and error. After making the changes listed above, we'll look at the static pressures and air-flow again and see if they were sufficient to bring the operating conditions back into whack. (Hey, if something can be out of whack, that must mean it can also be in whack, right?)

    Before they get in there and do their work, however, I'm going to measure the actual air flow and see how far it is from what it should be. It's a 5 ton (60,000 Btu/hr) system, so the air-flow should be about 2000 cubic feet per minute total. I'll keep you posted on this project as it proceeds.