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Are Duct Booster Fans Crazy? Or will they solve insufficient cooling air reaching 2nd floor?

user-1121196 | Posted in Mechanicals on

2nd floor of my Kansas City 24yo home (bought 3 years ago) is consistently 6-12 degrees (F) warmer in summer than first floor. The problem I noticed from Day 1 is simply that the 2nd floor does not get enough air from the HVAC system. With the house fan on full blast, you can barely feel any air coming up through the floor registers in the bedrooms, while the registers downstairs are blasting out cold air.

Supposedly, a duct booster fan isn’t a good idea. However, after everything else I’ve tried that is supposed to address the problem, I think a duct booster fan is by far the least expensive and most effective solution that would directly address the problem. In fact, I’m coming to the point where I think everything else is stupid.

So if there’s a reason that I may be delusional, please convince me otherwise. But this is my throwdown challenging the “conventional wisdom” of building science on this one.

Here’s the situation as it stands:

A. All the ducts are inaccessible. Don’t tell me it’s a duct/supply sizing problem. Of course it is. I’m not going to spend $$$$ tearing apart the house to let an incompetent contractor try to fix what the original inept contractors did, if a <$200 solution will fix my problem. B. Supply trunk line to 3 bedrooms on the 2nd floor is smaller than to the 1st floor, and has a much longer run, with more bends, and crosses over the unconditioned garage. So a long, hard push for cold air to get to the hottest rooms; instead, the air follows the path of least resistance to rush out of the easy, short, huge trunk path of the 1st floor. C. Professional air sealing performed, R49 attic insulation (though I’m convinced insulation is a very minor factor for cooling concerns), solar attic ventilation fan. New low-e double glazed tinted argon windows. AC recently repaired (kink in coolant line, leveled, cleaned, recharged). Added a return vent in finished portion of basement. Old PSC blower, but still seems to function well. My potential solutions: 1. Install a duct booster fan (with pressure switch or linked to main blower) on the vertical rise of the trunk line that supplies the upstairs rooms — should pull more air upstairs and give it the extra push it needs to overcome the distance, rise to the second floor, bends, and undersized ducts and still throw the air into the rooms to mix well. 2. Install manual dampers in the trunk lines and adjust seasonally — best done in conjunction with the duct booster, to restrict 1st floor airflow that steals the lion’s share right now. Duct booster may correct the air pressure problem of cutting off the majority of the vents via the damper. 3. Use AeroSeal to seal duct leaks internally throughout the system and try to reduce loss of cold air and increase flow (at likely >$1,000 for a 10 year life, not sure this is worth it, or if it even works as marketed).
4. Install EcoBee or similar thermostat with multiple sensors to average out set temperature vs. actual with upstairs — problem is, without correcting supply, this will just freeze out the 1st floor while it tries futilely to bring down the 2nd floor temp (already keep 1st floor thermostat at a cold 72/70 at night just to try to keep the upstairs from getting too hot).
5. Replace old PSC blower with Evergreen ECM — still must overcome the draw of the 1st floor trunk line, and if it runs in lower speeds, will likely only exacerbate the problem of not pushing sufficient air upstairs, UNLESS combined with 1-4 above
6. Install a heavy curtain across stairwell to slow cold air spillage downstairs and hot air rising (the powerful chimney effect going on here); works for my in-laws in their split level, holding heat in at the lower level, not sure how well it will work to keep cold air in, particularly if we’re just not getting enough cold air up there in the first place
7. Complete redo of HVAC system (too expensive):
a. Access, resize ducts per Manual D, seal and insulate (but still have the same problem of long runs, bends, and vertical height to overcome);
b. Replace single stage AC with higher efficiency dual stage (same air flow problems at low speeds, however);
c. Add automatic zones (expensive, difficult to access ducts, still may have pressure problems, but perhaps an automatic system would be able to detect and correct, signaling the blower to blow harder for the upstairs zones when those zones are active)
d. OR Add a second heat pump unit dedicated for the upstairs — would have to figure out how to get an air handler access to the trunk lines and separate it from the garage
e. OR Add a ductless minisplit system for the upstairs — expensive, poor room-to-room balance of air, unlikely to extract sufficient moisture from air

I think 1 and 2 are my best bet by far, followed up by 4 and 6 as cheap options, and possibly 3 and 5. If I thought I could easily dedicate a new heat pump to take over the upstairs supply lines per 7d above, I’d probably do that, but new air handler in the garage = bad idea, and probably against code, so I’d need to enclose and finish a new mechanical room to code and probably reroute/do significant duct work.

So, why SHOULDN’T I install a duct booster and manual dampers?

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  1. Expert Member
    Dana Dorsett | | #1

    At the very least, test the duct leakage to know where it stands. If there's a disconnected duct losing most of it's air to a floor joist cavity or something, no amount of aeroseal or duct booster or flow balancing vane tweaking is going to fix the problem.

    If the ducts are reasonably tight (or can be made reasonably tight, manual adjustment vanes are probably the least problematic first stab at balancing the temperatures.

    Heating load and cooling load balancing on the same duct system never works perfectly on multi-story homes, but can usually be seasonally tweaked to suit. If the temperature balance was fine during the heating season the hot-upstairs/cold downstairs symptom during the cooling season is almost a given.

  2. chrisjri | | #2

    I had to put a curtain/ shade over an open staircase to keep my a/c from going down to the basement. I'm in the business so a roman shade on horizontal tracks did the trick.

    If the vents are in the floor of the first floor I would cover some up, or partly cover all with large books, to test the manual vent idea.

  3. GBA Editor
    Martin Holladay | | #3

    In the winter, you want your forced air system to deliver most of its air to the first floor.

    In the summer, you want your forced air system to deliver most of its air to the second floor.

    This is a common problem. Here is a link to an article about the problem: Keeping Cool in a Two-Story House.

    By far the simplest solution to the problem is to close all of the registers on your first floor. (Don't worry -- some air will leak through, even if they are closed.) Open them up again in the winter.

    Another approach -- a little more work to install, but easier to implement seasonally -- is to install a balancing damper in one of your main ducts -- the one serving your first floor -- so that you can choke off the air flow to the first floor during the summer.

    Either of these approaches is preferable to a duct booster fan.

  4. user-1121196 | | #4

    I realize the problem is common and there's no perfect solution; physics is against me. Just looking to minimize temperature differences for comfort as much as possible.

    Dana, I hadn't considered that there might be a large duct breach or disconnect robbing airflow. Could I do a preliminary check by comparing air flow at each register and looking for one with significant difference, or is the only way to go with a full test (which would presumably make assumptions about what the problem may be based on amount of overall leakage). If no breach, is AeroSeal worth it?

    Also, yes, winter balance is much better, though we do have a colder dormer room and still sometimes use space heaters to set back the temperature pretty low at night and just keep our bedrooms slightly warmer.

    Chris, thanks, it's a cheap enough options that I think I'll try it and see if it helps. Ikea room separating curtains and rod!

    Dana and Martin, looks like the manual dampers are a solid starting point. Should they be able to adjust to varying levels of restriction, or just completely close the unused zone seasonally?

    Will dampers or register closing not introduce a static pressure problem? I've always been fuzzy about that and how much to be concerned about it. For the record, I've closed all the vents on the first floor and in the guest bedroom upstairs for extended periods before and haven't seen much difference (though I think it was just a little better). I'm hoping dampers near the plenum will work much better.

    Just what IS the big problem with duct booster fans, btw? Competing fans throwing pressure and air flow out of wack, too hard to get them balanced? Restriction in the airway counteracts the fan's effect too much to be worth it? Forces too much air through any leakage points to be worth it? Uses too much energy/creates too much noise to be worth it? (this is more about comfort than energy efficiency, and I'm not concerned about noise due to the prospective location and other room fans it would be replacing). I can see those negatives, but had really expected the positive increase to airflow to overcome them by a significant enough margin to be worth it. Am I wrong? Or just do the damper and leakage tests first, then maybe duct boost if that's not enough?


  5. GBA Editor
    Martin Holladay | | #5

    The main problems with duct booster fans is that (a) they use electricity to solve a duct design problem, and (b) they delay permanent and more appropriate solutions to duct design problems.

    No one knows how to design and install good duct systems -- but lots of people are selling Bandaids.

  6. Expert Member
    Dana Dorsett | | #6

    Whether or not operable balancing vanes choke off flow too much is a function of your duct design and the coil/compressor sizing. It's true that if you throttle it back too much you'll have coil icing issues during the cooling season, but you can burn that bridge if/when you come to it. :-)

    Booster fans are only a useful band-aid for truly horrific duct designs where the duct is ridiculously undersized for it's length & flow and can't be replaced. For mere seasonal temperature balancing between heating & cooling mode, operable vanes/dampers are more likely to give satisfactory results.

    The aerosol sealers are "worth it" only if you have substantial duct leakage that can't be treated any other way. Before pressure-testing the duct leakage it's worth going around and sealing all the register boots to the subfloor or drywall, and sealing any exposed seams joints on hard-piped duct you have access to with duct mastic. Tape the seams of the air handler with foil tape too. THEN have somebody with a duct blaster come in and tell you how bad the rest of the leakage is.

    How leaky is TOO leaky?

    In California under Title 24 when commissioning new ducts if they leak more than 6% of the air handler's rated volume at 25 pascals pressure they fail, and retrofit duct sealing is required. That's a good target, but don't sweat it too much even if it's as high as 10% of air-handler rating. If it's as high a 20% (or more) it's time to look at aerosol sealants more seriously if the leakage is all behind drywall, not fixable by other methods.

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