GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Best Method for Bringing in Makeup Air

Airfix | Posted in General Questions on

So we have a 600 CFM vent hood going into a “pretty good” house, new construction climate 6b.

I’ve read the pros and cons of using a vent hood and due to our cooking habits we feel the vent hood is a necessary evil although I’ve been pushing for something in the 400 cfm range my wife found one to meet her style requirements that is 600 cfm and doesn’t come with a smaller exhaust.

The hood can be bought with an interlinked make up air damper which we were going to procure.

My HVAC guy wants to run the make-up air directly into the return line of the furnace.

My thoughts were to run it directly into my mechanical room, have it get partially pre-warmed in the mechanical room, then run up to the kitchen (directly above) through holes drilled in the subfloor then out through some grills in the cabinet toe kick.

I’m concerned about running directly into the furnace return line because whenever the blower kicks in there is going to be a negative pressure pulling cold air from outside past the damper.  I know the seals on those motorized butterfly dampers are not great and it doesn’t seem like a smart idea to have them be pulled on all the time because I’m sure the seals will wear out quicker and they will surely leak more with the suction.

Using my method there is no suction on the damper until there is a net negative pressure in the home i.e. when the vent hood is turned on.  In that case the damper will be open which is what I want.

I think I might need to make it an active system so when the vent hood turns on there a damper opens and a fan kicks on to pull in the required air flow.

Does anybody see any problems with my plan to bring the air into the mechanical room?  How do people “normally” bring in makeup air to the house?

Steve

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Geir_Gaseidnes | | #1

    I am facing the very same challenge, and also in Zone 6. Given a tight envelope, and other machinery pushing and pulling air independent of cooking, I distrust an automated makeup air damper for the vent hood. I think a direct link between the damper and the vent hood makes most sense, open-on-demand. We are looking at Broan for the damper.

    https://www.broan.com/Accessories/Range-Hood-Accessories/Ducting-Dampers-Adapters-Caps/MD6T

    I do worry about cold toes while sautéeing, however.

  2. JC72 | | #2

    https://www.buildingscience.com/documents/insights/bsi-070-first-deal-with-the-manure

    Sounds like you're between the high and low CFM suggests in the link above.

    Normally people live in leaky homes so make up air isn't such a big deal or they just open a window.

  3. Airfix | | #3

    Geir,

    I'm not sure what an automated makeup air damper is. I was planning to use one like in your link that is electrically connected so that when the vent hood turns on, the damper opens. I think I'm going to have the make up air be active so that there is a fan that turns on in the makeup air line that flows 600 CFM into the house (Per John's link it should be a little more than 600 cfm). I'm not sure if this can be made variable to match the selected hood speed or not.

    Instead of a vent at the kick plate, maybe I could run it up inside the wall and have a vent above my cupboards. So cold head instead of cold toes?

    Steve

    1. Expert Member
      BILL WICHERS | | #4

      An automated makeup air damper basically opens an air pathway from outdoors into your house when you run your vent hood. Assuming you have your vent hood venting to outdoors (not a recirculating type), then the air it blows out that vent to the outdoors needs to be replaced inside the house by an equal amount of “makeup” (replacement) air from outdoors.

      In a typical leaky house, this makeup air comes through all the little cracks and gaps all over the house. In a tight house that is well air sealed, there sometimes isn’t enough leakage to makeup the air the vent hood exhausts, so the vent hood pulls a slight negative pressure inside the house. This can cause problems, especially for gas appliances that use natural draft exhaust where the negative pressure can cause backdrafting.

      The automated makeup air damper intentionally opens to the outdoors when needed for the vent hood to operate, but remains closed the rest of the time so that you don’t lose conditioned air.

      Bill

      1. qofmiwok | | #8

        I think he's questioning the prior answer, as I am too because that person said "I distrust an automated makeup air damper for the vent hood. I think a direct link between the damper and the vent hood makes most sense, open-on-demand. " I also don't understand the distinction between what he is calling an automated damper vs an "open-on-demand". Maybe he really means the method of opening, whether a pressure sensor, a current sensor, or a manual on/off switch?

  4. Expert Member
    Peter Engle | | #5

    We have a variable capacity range hood, up to 1100 cfm. We need it for the type of cooking we do. We also find that the kitchen heats up quite a bit when the range is going full-tilt. In those conditions, we go native: Just open the windows for makeup air. Anything less probably won't work anyhow. Opening the top sash a couple of inches is enough to comfortably mix the makeup air with the rest of the kitchen air. A well-designed hood captures most of the smoke on the first pass. These events are relatively short compared to the rest of the time that the place is closed up. I find that the ease of use (opening a window) is well worth the small energy penalty vs. a complicated and interlocked system with motors, fans, ductwork, etc.

  5. Airfix | | #6

    I'm resurrecting this thread as nobody commented on my HVAC guy's plan. I talked to him again yesterday and he is determined the best way of doing the makeup air is with the furnace blower.

    Like mentioned in the OP he wants a direct line from outside with a damper connected directly to the return air of the hvac system.

    When the vent hood starts the makeup damper will open the furnace blower will start and the zone damper for the kitchen zone will open and makeup air would be supplied to the whole zone.

    What are your thoughts on using the furnace air handler (it's variable speed) for a 600cfm makeup air?

    Steve

  6. Expert Member
    AKOS TOTH | | #7

    Steve,

    A 600CFM hood doesn't need 600CFM or makeup air. Depending on how tight the house is and number of baths, you probably only need around 200CFM.

    Your idea would work much better, there is no need to for all the interlocking/zones/blowers for the makeup air.

    Makeup air should be supplied to outside the kitchen area, this gives you the best capture efficiency plus keeps any cooking smells more contained.

    I would even go one step further and install and adjustable barometric relief damper instead of a motorized one, no need to let in makeup air unless the house is actually getting depressurized.

  7. Expert Member
    ARMANDO COBO | | #9

    Good Lord of Mercy... does anyone read codes anymore? Ain’t that a shame that an HVAC contractor doesn’t have a clue about codes?
    M1503.6Makeup air required.
    Where one or more gas, liquid or solid fuel-burning appliance that is neither direct-vent nor uses a mechanical draft venting system is located within a dwelling unit’s air barrier, each exhaust system capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be mechanically or passively provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with not fewer than one damper complying with Section M1503.6.2.
    Exception: Makeup air is not required for exhaust systems installed for the exclusive purpose of space cooling and intended to be operated only when windows or other air inlets are open.
    M1503.6.1Location.
    Kitchen exhaust makeup air shall be discharged into the same room in which the exhaust system is located or into rooms or duct systems that communicate through one or more permanent openings with the room in which such exhaust system is located. Such permanent openings shall have a net cross-sectional area not less than the required area of the makeup air supply openings.
    M1503.6.2Makeup air dampers.
    Where makeup air is required by Section M1503.6, makeup air dampers shall comply with this section. Each damper shall be a gravity damper or an electrically operated damper that automatically opens when the exhaust system operates. Dampers shall be located to allow access for inspection, service, repair and replacement without removing permanent construction or any other ducts not connected to the damper being inspected, serviced, repaired or replaced. Gravity or barometric dampers shall not be used in passive makeup air systems except where the dampers are rated to provide the design makeup airflow at a pressure differential of 0.01 in. w.c. (3 Pa) or less.

    1. Jon_R | | #10

      > exhaust makeup air shall be discharged into the same room in which the exhaust system is located or into rooms or duct systems ...

      It's worth noting that the discharge location can have a huge effect on needed hvac capacity and/or comfort.

    2. DebraLombardLEEDAP | | #11

      Yep, make up air required in IRC-2015, M1503.4 Makeup Air Required

      1. Expert Member
        MALCOLM TAYLOR | | #12

        The assumption being we are all under the same code.

      2. virtus | | #13

        This is one of the more confusing areas, but after reading through sections of my local building code, it almost feels deliberately vague in certain areas to give inspectors a great deal of latitude

        "M1503.6Makeup air required.
        Where one or more gas, liquid or solid fuel-burning appliance that is neither direct-vent nor uses a mechanical draft venting system is located within a dwelling unit’s air barrier, each exhaust system capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be mechanically or passively provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with not fewer than one damper complying with Section M1503.6.2."

        This can be interpreted as if you have an induction cooking appliance, direct vent gas fireplace and power vent furnace and hot water heater, there is no code requirement for make up air even if there is a practical one.

        My local building code states make up air is required to ensure that operation of exhaust system, exhaust equipment and combustion equipment is not adversely affected. Seems like an opportunity for the inspector to enforce his/her interpretation.

        1. Expert Member
          PETER G ENGLE PE | | #14

          Your understanding is correct: Using the specific language of the code, makeup air is not required if you have no combustion appliances or if they are all sealed combustion. However, your 600 cfm fan won't draw 600 cfm if your house is well sealed and there is no makeup air. The house will simply depressurize to the fan's static pressure, then no more air moves. Even with passive makeup air, you've got to depressurize the house before the makeup air does any good at all. Powered makeup air is probably the best way to go, and then you've got the issue of all of that unconditioned air going somewhere. If you expect your mechanical room to be hot, your approach could work. But if you're going to have HPWH in the mechanical room, chances it will already be cool and flooding the room with cold exterior air would not be a good idea.

          1. CarsonB | | #16

            "But if you're going to have HPWH in the mechanical room, chances it will already be cool and flooding the room with cold exterior air would not be a good idea." I have this exact scenario. Still, better to decrease HPWH efficiency for a little bit than introduce a dedicated resistance heater for the makeup air in my mind. I don't see blowing 400 cfm+ of below freezing air directly onto my feet during winter going too well.

    3. qofmiwok | | #15

      "Where one or more gas, liquid or solid fuel-burning appliance"
      I don't think this thread is exclusive to fuel burning appliances.
      I for example have an induction cooktop and still want good venting for the toxins created by cooking food.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |