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Makeup air in all electric home?

Rob_Shearer | Posted in Mechanicals on

I have read a lot of blogs and questions that, quite reasonably, recommend makeup air for kitchen exhaust fans in tight homes. The use of these fans can cause backdrafting in combustion appliances. But what if the home does not have any combustion appliances? We typically design very tight homes, around 1 ACH(50) with no combustion appliances. We use heat pump water heaters, Mitsubishi mini splits, and electric condensing dryers. We also use Induction ranges and cooktops. No fireplace. We tell our clients that if they want a fireplace, they can have one…outside. I am still somewhat concerned about all of the nasty stuff created by the process of cooking. I know that a lot of people use recirculating hoods and combine that with an extra bit of HRV exhaust in the kitchen area, and that is exactly what we have been doing. But I wonder if we could install a reasonable kitchen hood without makeup air in a home that has no potential for backdraft? I am still hesitant about putting a hole that big in our envelope with nothing but a thin damper between the outside and the inside, but I also want safe healthy air for our clients to breathe. Thoughts?

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  1. bobhol | | #1

    I am in the same position ...I am heating with mini splits and am building a tight house dryer vent is a Heartland which is the best I can find for air tightness but I need something similar for my kitchen vent fan/microwave hood is about a 250 cfm fan and the Heartland needs more force to be used than the fan can put out and I dont want a drafty vent into the kitchen.

  2. GBA Editor
    Martin Holladay | | #2

    A powerful range hood fan will not cause any backdrafting problems if there are no combustion appliances that can backdraft. However, if the range hood fan is starved for makeup air, the fan motor will struggle, and the fan will be less effective at removing smoke and odors.

    Whether or not the fan needs a makeup air source depends on the tightness of the home's envelope.
    If the homeowner is satisfied with the performance of the fan, you're all set.

    If the fan draws poorly, crack open a window.

    If the fan draws poorly and the homeowner doesn't want to crack a window, the fan needs a source of makeup air.

  3. Expert Member
    Dana Dorsett | | #3

    In any house with an HRV, running the exhaust ventilation will unbalance the HRV while the exhaust vent is running, but the HRV system is a fairly low impedance air intake for "reasonable" sized kitchen & bath exhaust. There may still be capacity issues if running 2 bath fans, the clothes dryer & kitchen exhaust all at the same time though.

  4. exeric | | #4

    Bob, there is no easy answer to your question. There generally is no "good" ready made solution to proper airflow and airtightness with a passive damper, but you can definitely make one that works for you without too much difficulty. What I do, and what I would do if I were you, is buy the best inline retrofit passive dampers I can find for the diameter of duct you will be using.

    I don't know if you are familiar with the Kill-a-Watt devices but they are a small, cheap device (20 bucks) that plug into the outlet feeding the fan and then the fan plugs into it. It indicates the watts used by the fan digitally. Usually a fan that is stalling because there is lower air flow than it wants will consume more watts just like any stalling motor would. Similarly, if the flow is completely unrestricted it will always read the same power and it will always be at its lowest level. This is also no good because you will not know if the reason the fan power is always low because the damper is leaking.

    The proper way to visualize it is as a power curve turning up from a straight horizontal line on a graph. You want to move away from the horizontal line so you can have an idea just how leaky or restricting the damper valve is. Where the power use starts to turn up will indicate where the damper is beginning to open. (You may have to open a door to get a good indication, and if you do it may mean you also need a makeup air damper.) It shouldn't be too much higher than the fans rated power use but it should be a little higher. That means the damper is doing its job.

    If the damper is too tight and stalling the fan you will get quite a bit more than the rates power use of the fan. Most replacement in line dampers have more than one spring. The ones I recently got have three. Just begin cutting the spring "legs" one at a time so that the power used by the fan reduces a little closer to it rated power useage. This isn't a reversible operation so it will take a little care and finesse to get a suitable back pressure for each fan/damper system you are using this system to commission.

  5. wjrobinson | | #5

    Eric, are you serious? Ever hear of the KISS rule?

  6. exeric | | #6

    I guess I should add that this applies to medium to smaller fans more than big honk'in fans that move 1000cfm or more. Those are pretty uncritical I think but I still think one should get the best in line damper you can get to insure its well sealed when closed.

  7. exeric | | #7

    No, I'm not kidding. The Panasonic whisper green fans can consume up to 2 or 3 times more power when the airflow is too restricted than when the damper is opening properly. Check the Panasonic operation manual for the charts of power use. In the ventilate mode that means 2 or 3 times more power use 24/7 than you may be expecting. It adds up.

    Just because you hadn't thought of this does not mean its irrelevant. One shouldn't question a new idea just because its new to YOU. You also didn't know about the rating system of drywall surfaces that has been in effect for years. What else isn't KISS enough for you?

    EDIT: Just went to the Panasonic website and they apparently removed the info in the operations manual about power use when was backpressure was greater than wg = 0.1. If memory serves they used to also rate power usage at wg = 0.3. Trust me, those fans use a lot more power to get the rated CFM at the higher back pressure and the easiest way to tell if that's the situation is with a Kill-a-Watt. Much easier than using a flow meter, especially since the Panasonics are made to always operate at rated CFM within certain backpressure limits. They just use a lot more power when wg goes above 0.1.

    Apparently Panasonic realized in their marketing dept that they were sharing too much information and removed the info at higher water gauge ratings.

  8. wjrobinson | | #8

    Eric, you're right, complicated is better. Levels of drywall finish... etc........ you go. Here we don't do all the lower levels of drywall finish so no levels, get it? Fans have been installed poorly in the past, true. Now we are taking great care to install the ductwork properly. Maybe someday we'll start testing the amperage for proper flow but for now.... it gets checked for operation.

    I am going to play with your theory soon and see what my watt meter and fan tell me.

  9. wjrobinson | | #9

    I just blocked the flow of a fan.

    Blocked flow 44.9 watts draw
    Not blocked 44.5 watts draw

    1% increase in work for the fan when this fan was blocked. How many fan testers does it take to screw in a light bulb, LED or otherwise? I don't know, will have to run a test.

  10. exeric | | #10

    AJ, perhaps I should be more specific. Fans are not all the same. Some fans, like the Panasonic are made to compensate for more restriction to keep a level CFM. The Panasonics DC ones are made like that. Other fans, and I was wrong in my first post implying that electricity in ALL fans operating under restriction will be higher, do not. Most just decrease output then under that restriction unlike the whisper greens I later referenced.

    Think about it AJ - one of the two, flow rate in CFM or input power has to change under increased restriction. It isn't rocket science. Did you use a Panasonic whisper green with that test? If you did then I capitulate. If not then you have to do the test with a compensating fan like the Panasonic.

  11. Rob_Shearer | | #11

    Thanks all, this was helpful!

  12. Hobieda | | #12

    Thermador makes a separate under-cabinet recirc unit, with no electrical hookup - it's basically a box, with two activated carbon filters, and a 6 inch duct inlet connection, that's intended to be used with their 600 cfm downdraft kitchen fan. If your code allows it, you could hook that to any 600 cfm fan with a 6 inch duct. If you have mechanical ventilation as well, the actual removal of the carbon filtered odors after that might take a little longer, but it would work.

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