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Musings of an Energy Nerd

Forced-Air Systems With Room-to-Room Temperature Imbalances

Are so-called ‘smart vents’ a solution?

This HVAC register includes a battery-operated motorized damper.The so-called "smart vent" is manufactured by a San Francisco company called Flair. The Flair system requires several wall-mounted thermostats that communicate with the registers via a smart phone app. Photo courtesy of Flair Smart Vents.

The majority of U.S. homes have a forced-air heating or cooling system—that is, a system that use ducts to deliver warm or cool air. These systems usually include a furnace, an air-source heat pump, a split-system air conditioner, or some combination of these appliances.

Some of these homes have a persistent problem: room-to-room temperature imbalances. Perhaps the house includes a bonus room over the garage that is cold in winter and hot in summer. Perhaps the house has a west-facing room that overheats when the afternoon sun strikes the oversized windows. Perhaps the house has a small addition over a crawlspace—a room that never seems to reach the thermostat setpoint. Perhaps the second floor of a two-story house is always too hot during the summer. The individual architectural quirks that contribute to these problems vary, but the basic complaint is the same: one or two rooms are chronically colder or hotter than the occupants want.

One solution attempted by some homeowners is to adjust the louvers on the registers or diffusers in various rooms of the house—closing some and opening others—in hopes of convincing the air from the furnace or air conditioner to favor the room that needs more conditioned air.

Some entrepreneurs have taken this idea a step further, and have marketed registers with motorized dampers controlled by wall-mounted thermostats or by an app on the homeowner’s smart phone. In theory, the motorized dampers are supposed to open and close as necessary to correct room-to-room temperature imbalances.

Do these solutions work? Kind of. But these “solutions” don’t get to the roots of the problem.

A look at available devices

Several manufacturers sell similar registers with motorized dampers:

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  1. GBA Editor
    Allison A. Bailes III, PhD | | #1

    Good article, Martin. I guess the smart vent folks still haven't figured it out, but since they're still around, somebody must be buying these things.

    1. AndyKosick | | #7

      I think the reason they're selling is that people can't find professionals capable of solving these problems for them.

  2. Expert Member
    ARMANDO COBO | | #2

    Sooo... we can't get homeowners to stop messing with the Tstat settings and messing the HVAC system setup, and now we have to worry about "smart" vents?
    I wish "Although zoning a forced-air system is difficult and rarely done well—and may not even be needed in a house with an excellent thermal envelope" gets reinforced more often. Maybe a separate article on the subject would be warranted. Martin, Allison, anyone?
    Useful article, thanks.

  3. Expert Member
    Peter Engle | | #3

    People keep buying these things because their HVAC systems and thermal envelopes are all screwed up and that can't be easily fixed once the house is finished. Grasping at straws is about all they can do.

    1. GBA Editor
      Martin Holladay | | #4

      I understand the motivation. Another factor is the tendency for so-called "smart vent" manufacturers to exaggerate the benefits of these devices.

      But (as I imagine you know) grasping at straws isn't the only option. Instead of spending $1,300 on a bunch of battery-operated registers, homeowners could invest a similar amount (or less!) in canned spray foam, caulk, and tape, and start sealing their home's air leaks.

      1. Expert Member
        Peter Engle | | #5

        Absolutely. And, they can still adjust the register dampers manually for free. But that's still not going to fix the problems with that uninsulated bonus room over the garage that only has a single flex duct supplying it....

      2. Expert Member
        MALCOLM TAYLOR | | #6

        Neither caulking, foam or tape can be activated by a smart phone. I'd imagine the choice is an easy one for most millennials.

  4. AndyKosick | | #8

    Thanks for this Martin, I've seen these advertised on my facebook feed and have meant to dig into them, but you did it for me. This seems like the right place to chime in with an overdue comment, as it's related, and I'd like to know what you think Martin. As I said above, the real problem is people can't find anyone to deal with these issues in the right way. Which leads me to this...

    If you're not following what Nate Adams and Ted Kidd are doing with they're HVAC 2.0 project, you should be. I have not always seen eye to eye with them, still don't, but I understand they're frustration. A great deal of thought made me realize a while ago that they have now got one thing very, very right that everyone at GBA should be paying attention too, and it's this >>>> If we're going to solve for existing housing (and fast for the sake of climate), we HAVE to do so through HVAC. To be clear, this does not mean only HVAC solutions, but HVAC has to be the way to the problem. I'll give three reasons for this:

    First, HVAC, as a trade, really is our biggest problem. You don't worry about a splinter in your hand when you're bleeding out from a gun shot wound. We need focus here, the next reason will make this even more clear. My path from carpenter to home performance has led me to the crux of HVAC. It's taken me too long to realize it.

    Second, there's not enough energy savings to justify the cost of improving existing homes, and when people have a comfort problems they think HVAC. Also, HVAC is the thing that's replaced or serviced most often in a home. Right now, HVAC is there first and sells them an oversized box that doesn't solve the problem and there's no money left to do anything else.

    Third, HVAC is essentially what electrifies a home, which is what solves for climate. I've had it wrong, we've had it wrong, there are many cases where HVAC is the right thing to do first, especially in the near term. It may often be in combination with envelope.

    What HVAC 2.0 is trying to do is help contractors understand that they sell comfort and not equipment, and that half the time it's an envelope problem. BPI kind of tried this but it didn't work. HVAC 2.0 is trying to build a business model that makes this approach look profitable, instead of like they're just giving away half their work. I think it might have the best chance of having a meaningful impact of anything I've seen.

    I've been overwhelmed and, I think, distracted by new home rating and code blower doors over the past couple years. The most important work is actually in existing homes. As I shift back to that work that I started out to do, my primary goal is to connect with local HVAC and help them become problem solvers for their costumers. Which just happens to mean envelope improvements, and heat pumps.

    1. GBA Editor
      Martin Holladay | | #10

      I agree with many of your points. You noted, "First, HVAC, as a trade, really is our biggest problem." I've been repeating something similar for years. For example, in an article titled "Still Fighting the Same Battles, 20 Years Later," I wrote, "In recent years, I’ve come to describe residential HVAC contractors as 'uniquely incompetent.' Even though I’m painting with a broad brush, there is ample evidence to back up my description. ... Most HVAC contractors can’t perform an accurate Manual J calculation, even though it is code-mandated. Most residential HVAC equipment is oversized. Many duct systems are unacceptably leaky. Many duct systems are located outside of a home’s conditioned space. Most ventilation systems are poorly designed, poorly installed, and rarely commissioned. In short, these contractors are doing a bad job. This is an industry in crisis."

      In this article, I wrote, "It's all about the envelope." You write that "half the time it's an envelope problem," and you imply that the other half of the time, it's an HVAC equipment problem. Rather than arguing about the relative contributions of envelope and equipment to comfort problems, we can probably agree that in many existing houses, both factors affect the homeowners' complaints.

      I know that Nate Adams often solves comfort problems by ripping out relatively new HVAC equipment and installing brand-new equipment. He does this more often than the typical home performance contractor. When I reviewed Nate Adams's book (in an article called "Energy Retrofit Advice From Ohio"), here's what I wrote: "Unlike most weatherization contractors, Adams isn’t afraid to tell customers that they should get rid of their HVAC equipment, even if it is only two or three years old. Getting rid of relatively new HVAC equipment is an expensive measure, but Adams thinks the cost is justified. He writes, 'Ripping out nearly new furnaces and air conditioners is frustrating for homeowners, and it’s frustrating for me to break the bad news. Sadly, we do it a lot.'"

      If Adams can find customers who can afford to pay $20,000 for envelope improvements, and who simultaneously are willing to pay $20,000 for HVAC equipment improvements, then all power to him. I'm sure that his customers are satisfied with the results. The real problem with the home performance industry is that good solutions to these issues cost so much that few homeowners can afford them.

  5. CarsonZone5B | | #9

    One possible application of something like this that comes to mind is solving the closed bedroom door problem of using the 1 or 2 minisplit in a house suggestion on GBA. Having a through wall smart fan that detects temperature gradients may be a cheaper solution than setting up additional zones on your minisplit system and perhaps smarter than setting up a fan to a separate thermostat.

    1. GBA Editor
      Martin Holladay | | #11

      I don't think that a through-the-wall smart fan -- a product that doesn't exist, since none of the products mentioned on this page include a fan -- would solve the problem you're describing. To learn why, see this article: "Using a Bath Fan to Equalize Room Temperatures."

      1. CarsonZone5B | | #12

        Thanks Martin, that article was really helpful. I guess the easy answer is to just install cadet heaters as insurance down the line from cold wife syndrome.

        1. Expert Member
          MALCOLM TAYLOR | | #13


          Another alternative is to just install an outlet at a convenient location, and add a wall mounted heater if you find it necessary. You could even remove these during the summer and swing seasons:

          1. CarsonZone5B | | #22

            A good point Malcolm, it's hard to tell if this will really be an issue at all.

  6. cold_feet | | #14

    Martin you said, "I know that Nate Adams often solves comfort problems by ripping out relatively new HVAC equipment and installing brand-new equipment."

    What can companies/contractors do with the relatively new equipment that's removed? Can it be donated, assuming it is in good working condition? Do the components get recycled/downcycled? I know that this is off topic from the article, but your comment reminded me that, as a homeowner, I've never learned or thought to ask until now.

    1. GBA Editor
      Martin Holladay | | #15

      Cold Feet,
      When Nate Adams is working on a job that involves removal of functioning HVAC equipment, I have no idea what he does with the equipment after removal.

      But people have been selling used furnaces on Craigs List (and through classified ads, in the old days) for years. Auto mechanics buy used furnaces to keep their auto shops warm.

    2. AndyKosick | | #16

      One of the underlying goals in the work of Nate Adams (and myself) is actually electrification. It also turns out that right sized, fully modulating heat pumps are one of the best ways to solve comfort problems by matching loads during moderate weather. I say this because the goal is for gas furnaces and inefficient AC not to go back into service.

      A contractor I work with locally has a roll-off for recycling metal right next the shop. The old equipment should be recycled. Even at low metal prices it's better than paying to dumpster it.

      I try to save and reuse a lot of things, the shear volume of construction waste is what turned my head towards green building years ago, but there are some things whose time is done.

      1. cold_feet | | #17

        Sorry Martin, I wasn't expecting you (or anyone) to know what one particular guy in the world does. I'm glad to hear that some will repurpose the usable equipment and others will choose to recycle the parts and/or metal. Thank you both for responding!

  7. synergytodd | | #18

    Closing grilles rarely if ever works. Using fan boosters never works especially when the compressor turns off. We recommend an energy audit using a blower door and infrared camera to identify leaks and breaks in the thermal envelope as well as any large leaks in the ductwork of the home. We then measure the static pressure of the HVAC system. More times than not, we find that the system has high static on the return side from having an undersized return, lack of returns or jumper ducts in bedrooms, a restricted return duct, a dirty coil, a restrictive filter or a combination of them all.

  8. user-6998645 | | #19

    Martin, in your response to Andy, you said "Most ventilation systems are poorly designed, poorly installed, and rarely commissioned. In short, these contractors are doing a bad job. This is an industry in crisis."".
    Is this really an industry in crisis I ask? I don't sense any crisis from them, I sense the crisis is with us who expect them to do a better job, but one that is not enforced by trade schools, designers, code officials and regulators, and ultimately by builders. The HVAC industry is happy to continue along as they have for years and they won't change unless pressed. Many in the HVAC industry are making great money doing what they've been doing, and changing their business means risk, potential losses, having to relearn and retool, etc. And for what as joe/jane public is not willing to pay for a better system in the first place.
    This article and its many comments are excellent and I love the passion that we put into our work, our concerns and ultimately getting things right. Sadly, it is going to take a national/international effort to change our residential and light commercial HVAC industry in my opinion.

    1. Expert Member
      ARMANDO COBO | | #20

      +1... I thought I was hearing myself think!
      I've been saying the same for years, and you can include the same "philosophy" with the entire building system. Having said that, only educated clients demand that too.

    2. GBA Editor
      Martin Holladay | | #21

      User 6998645 / Conrad,
      I think it's possible for an industry to be in crisis, even if industry participants are happily making a profit. But my definition of "crisis" might be different from yours.

      I fault HVAC contractors, but there's plenty of blame to share. We can also fault regulators, general contractors, architects, and homeowners. Here's what I wrote in a 2018 article called "Solutions to the Attic Duct Problem": "Energy experts have been advising builders for at least thirty years that it’s a bad idea to locate ducts in vented unconditioned attics, yet the practice persists. Why? Building scientist Joe Lstiburek blames the problem on architects and builders, and he’s partly right. But it’s also fair to blame regulators, most of whom are too timid to approve code changes that would make the practice illegal."

      1. user-6998645 | | #23

        Thanks Martin,
        My thoughts are not that far off from yours. Greatly appreciate all I've read here.

  9. ThickRoof | | #24

    Would these be useful in a situation where the occupant of a given room may desire a slightly different (1-2 deg) temperature than the rest of the home/zone?

    I'm going through an HVAC overhaul to properly size our system and ducting where the system is likely oversized and the ducting likely undersized (typical). Luckily we are building an addition so the system might end up properly sized saving us on equipment, but the ducts need to be replaced. I'm debating between zoning the upstairs/downstairs separately or just having a single zone with properly sized ducting. Alongside that debate I am considering smart vents to help adjust temperature slightly, particularly in bedrooms where the occupant might want it warmer/colder than the zone setpoint. This would be in lieu of something like a mini-split. Our compressor and air handler are both variable speed.

    1. graygreen | | #28

      Definitely zone the upstairs/downstairs separately if you are redoing duct work. You can always change your mind and use smart vents, but not the other way around.

      One thing to keep in mind is that a smart vent can only restrict flow. If you have a room that is too cold in the winter the band-aid approach that works better is to use a register with a booster fan. The Flair system isn't guaranteed to increase output in a room unless you have vents in enough locations (near the thermostat) so that the furnace runs longer before the thermostat hits its set point.

  10. graygreen | | #25

    This article seems to assume that a house is a single story. In a multi-story house with central heating/AC there is a fundamental issue of temperature stratification that is more difficult to solve than insulating a single room. There is always going to be some low-hanging fruit for improving the energy situation of a house- that will help with stratification- however it isn't necessarily going to be economical to completely solve stratification this way.

    It can't be solved by commissioning. In the winter more heat is needed on the lower levels and in the summer more cool is needed on the upper. In a central system this is only solvable by zoning each floor.

    Some duct work is very amenable to zoning, and some is not. To implement 2nd floor zoning in my house requires an extensive re-work of the ductwork. I have a ballpark estimate given to me of $5k- that's too much so I am not going to bother with getting a full quote. And I am lucky that I don't have a ceiling in my basement that needs to get torn down and put back up.

    On the other hand, I could get a smart vent system and spend around $2k.
    I have 2 neighbors with similar houses that have done both approaches, and both of them are happy with their outcome. Which approach would you take?

    The benefit of the traditional HVAC approach seems to be not relying on WiFi and batteries. With the smart vent approach there are additional benefits- I can buy their thermostat (Flair calls it a puck) and put it in any room in the house and zone that individual room so that a guest can determine the temperature of their own room.

    Any zoning system can interfere with the ventilation functionality. Flair vents are smart enough to open back up when just the fan is blowing. However, if there is heat/cool needed elsewhere for a long time (as modern variable speed systems are designed!) then the vents will be closed off for ventilation for a long time.

    I am looking into the possibility of trying to install a ducted mini split on the second floor with all new duct work- but I already know that is going to be very expensive. In the mean time I am improving the energy situation and will monitor to see how much it improves stratification.

    1. MartinHolladay | | #26

      Gray Green,
      You wrote, "This article seems to assume that a house is a single story. In a multi-story house with central heating/AC there is a fundamental issue of temperature stratification that is more difficult to solve than insulating a single room."

      In fact, my article addresses the issues you raise. See the final two paragraphs:

      "Although zoning a forced-air system is difficult and rarely done well—and may not even be needed in a house with an excellent thermal envelope—homeowners who insist on a zoned forced-air system may be able to get what they want. If you want a zoned forced-air system, you’ll need to hire a good mechanical engineer or a skilled HVAC contractor. As Allison Bailes pointed out in his article on the topic, you don’t want a single-speed furnace; nor do you want a bypass duct. You want multistage equipment and a designer who understands static pressure calculations.

      "One alternative to a zoned forced-air system is installing dual appliances—in other words, two furnaces or air-source heat pumps. (In many cases, this would mean one appliance per floor in a two-story house.) Another alternative is to switch from full-sized American HVAC appliances to Japanese minisplit units—either ductless minisplits or ducted minisplits."

      1. graygreen | | #27

        Sorry, I should have been more clear. There are statements like this being made in the beginning of the article: "Do these solutions work? Kind of. But these “solutions” don’t get to the roots of the problem."

        In then in the last 2 paragraphs we do have a mention of a problem that they do actually get to the root of the problem of- multi-floor zoning. But the smart vents are left out as a solution.

        It would be useful to compare different scenarios. The Flair system seems like it would be a lot better in terms of maintaining proper pressure than a bypass duct:

        Also left out is the use case of adjusting temperatures in just one room to a user's preference rather than for normalizing temperature across the house.

        And in comparisons, let's not forget about economics! There's an assumption that there's an equal choice between the "right solution" and smart vents.

        I appreciate the viewpoint coming from building science professionals, but its all a little bit armchair. It would be great to talk to people using this system and see what their experience is and what their options are. I have done this in my neighborhood and the smart vents are winning on economics for multi-floor zoning of a central system whose original ductwork is not readily amenable to multi-floor zoning.

        1. Izzza | | #29

          I like the idea of as precise micro-zones as possible. Currently in our condo we have a thermostat for the main areas plus a separate controller in the bedroom including the ensuite bathroom. It’s very nice to be able to control the temp in bedroom/bathroom only and it’s almost immediate that it can get to the set temp since it’s a small area. In our new build we originally had 3 air handlers so each floor could be zoned.

          We decided to combine the first and second floor to one unit and a separate smaller one for the basement. It would have been nice but we saved over $10k by making this change and also didn’t need the gigantic mech closet upstairs. I asked about smart vents early on but the engineer didn’t like them and said there are better ways to do it. The engineer was great, but he doesn’t know what everything costs because prices seem very unpredictable in this category. My builder, who was very good estimating almost everything else, totally underestimated the HVAC costs which could have really screwed us since we are cost-plus. HVAC seems like the Wild West, here in Ontario anyway.

          Economics should be part of the discussion, you’re right, because while many of us want the best of the best and efficiency nerds always want the “optimal” solution… everyone has a budget whether it’s $500k or $1M or $5M so sometimes we have to be strategic and choose the slightly suboptimal option for the most value payoff.

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