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Community and Q&A

Makeup air

Dave B | Posted in Energy Efficiency and Durability on

Hi 

So I plan on putting in a Broan kitchen vent model PM250SSP (250cfm), there is an option to upgrade to a 390cfm unit, but what I’ve read 250 should be enough for most everyday cooking? I seem to get conflicting information regarding if the unit needs makeup air. The house is being built to be tight, so I am not sure if it requires a unit, if so would a 6″ Broan makeup unit be enough?, the kitchen vent is 7″ of course its also power by a fan.  If it is required would the attached plan work with having it come in under the toe kick of the kitchen cupboards? 

Thanks for any advice.
Dave
Zone 6

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Replies

  1. Tom Wheeler | | #1

    399 cfm and below does not require make up air.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Dave,
    You may want to read these two articles:

    "Makeup Air for Range Hoods"

    "All About Makeup Air"

    1. Dave B | | #3

      Thanks Martin, I have read these and other articles, it seems the 250cfm vent hood should not need make-up air, even if the house is tight(1.5ACH or less), and not small ( ours is 2200sq' per floors) . Is this correct thinking??

      After reading these again I am confused on the dryer, I thought all dryers venting outside would need make up air? (In a tight house)
      I was planning on a heat pump dryer but finding it difficult to get any choices up here in Ontario
      Except for small units.
      Do dryer manufacturer rate cfm discharge?
      If I was to go to a regular dryer, venting outside I assume if it's under 250cfm again no make up air required?
      Thanks again

      1. User avatar GBA Editor
        Martin Holladay | | #6

        Dave,
        Q. "It seems the 250 cfm vent hood should not need make-up air, even if the house is tight (1.5ACH or less), and not small (ours is 2200sq' per floors) . Is this correct thinking?"

        A. It's hard to generalize. If a house is extremely airtight -- under 0.6 ach50 -- it's certainly possible for a 250 cfm exhaust fan to be starved of makeup air. In all cases, this problem can be alleviated by opening a window when the fan is in use. Whether or not this approach is acceptable depends on the occupants' opinions.

        Q. "I am confused on the dryer. I thought all dryers venting outside would need make up air in a tight house."

        A. That's not true. Most clothes dryers work fine, even in a tight house. Again, this discussion depends on your definition of "tight." Once a house gets tighter than the Passivhaus target of 0.6 ach50, it's possible that a clothes dryer might benefit from a little makeup air.

        Q. "Do dryer manufacturer rate cfm discharge?"

        A. You can always call up the dryer manufacturer and ask. Most clothes dryers are rated at 100 to 225 cfm.

  3. John Clark | | #4

    The size of the blower motor for the most part depends on the TOTAL heat output of the range. A good rule of thumb is 100 cfm : 10k BTU. So for example if you have a 4 burner gas range with a total heat output (all 4 burners on high) of 50k BTU's then you'd need a fan to push about 500 cfm's. For electric it's 100 cfm : 10 inches of width of the range. Keep in mind you won't ALWAYS have to operate the fan at max speed.

    Other factors: The number of bends in your exhaust piping. Lenght of duct and # of bends = slightly more CFM. Down sizing of the exhaust duct = more CFM. Larger hood size relative to the range = less CFM. Ideally the hood should be 6-inches wider ((3-inch overhang on each side) than the width of the range .

    From what I'm seeing with production builders over the past few years is that they're FINALLY adding range hoods which exhaust to the outdoors, but in their typical cheap [email protected] way.
    What I mean by that is:

    #1 The range hood is only as wide as the range itself (keeps cost of cabinets down)
    #2 Use a low CFM blower because they install a medium BTU gas or electric burners BUT
    A: Install the range against an interior wall of all places and consequently use rectangular duct inside the stud wall which lowers efficiency.
    B: Because they put such as premium on adding as much living area into the smallest space they make all sorts of compromises with regards to plumbing, electrical, HVAC runs AND range hood duct runs. When it comes to running the exhaust duct for the range hood I typically see them snake it an extremely long distance in order to reach an exterior wall and I almost guarantee you they'll use duct which is undersized. They have to because there's no room since they wanted save money on the height of the open web floor trusses for the second floor and the room layout of the second floor prevents them from using straight pipe to the roof. Sometimes the metal exhaust duct ends are just taped (poorly) together and god only knows how much kitchen exhaust is leaking into the air space between the first and second floors as it makes its way to the exterior.

    I know I sound annoyed, because in my opinion it is annoying. A typical home buyer doesn't know to think about these things so they're none-the-wiser. The homebuyer gets his/her range hood but most likely not as efficient as it should be.

  4. Dave B | | #5

    I will be doing an induction stove, the kitchen vents out in a short length with one 90. The vent it's self calls for a 7" exhaust duct.

    1. John Clark | | #7

      Sounds like an ideal situation for a low speed hood with sufficient capture area.

  5. Akos | | #8

    Dave,

    A 1.5ACH 4400sqft of house leaks 880cfm at 0.2" water pressure. Looking at the blower curve of hood you selected (ignoring duct losess), you would get a bit over 250cfm out of it and would depressurize the house to around 20 pa.

    If you don't have any combustion appliances, that is not something I would worry about. Cracking a window open while cooking would boost the flow slightly (10%) if needed.

    P.S. Generally the range hoods with axial blowers (a standard fan) tend to be louder especially at higher flows. Something with a radial blower is usually significantly quieter and loose less flow as the inlet filter gets clogged.

  6. W Ramsay | | #9

    It really depends on what styles of cooking you do and your tolerance for odors of meals past. Pan frying or stir frying need much better performance than boiling water for Raman. Some good info here: https://www.gardenweb.com/discussions/5161173/hood-faq

    @Akos is technically correct but real world performance can be much worse or much better. Changes in pressure on various sides of the building can have a much greater effect than how sealed the house is. If the hood is being vented in to a high pressure side of the house (typically windward) the performance of many consumer hoods can approach zero. Lawrence Livermore did a bunch of tests on this about 10 years ago and the results were surprisingly bad.

  7. Dave B | | #10

    So my wife thinks we should be going with the next size up 390 cfm, she is worried the smaller unit won't be enough for certain meals.
    It's a broan PM390SSP
    I assume either crack a window or make up unit then for this size.?
    I am hoping to get the house 1.5 ach50 or better

    Thanks

    1. Akos | | #12

      Dave,

      That is a much fan but needs a proper hood around it. It also does more pressure, even with a tight house, you will still get pretty good flow. With your 1.5ACH50 you would still get around 375cfm out it. It would depressurize the house more, if you don't have any combustion appliances, I don't think it would be an issue.

  8. W Ramsay | | #11

    Can you describe 'certain meals'.

    The Broan you mentioned has a small aperture and zero containment area. It won't work very well with even 1200 CFM. More in the link I posted above. I'd look at Zephyr, Modernaire and others. If you want it to work very well at all then make sure it's at least 6" wider than your range and 27" deep.

  9. Dave B | | #13

    Certain meals are fish frying, or others with high oil content.

    The house has no combustion appliances, all electric.
    The unit will be fitted in a custom built hood which we can make any size, although if you saying the unit itself is to small in dimensions Maybe I will try to find something else.

  10. Akos | | #14

    Unless you have a wide stove, the intake dimensions of the blower won't matter much. Where a larger inlet help is with grease filter, larger filter will be less restrictive as the grease builds up. The commercial style slatted grease filters are better for this as they can't get clogged.

    For me the biggest issue, which is hard to see until after the install is noise. It is fine for the hood to make some noise on high, but on low, you really don't want to hear it. This is where lot of the hoods fail.

  11. W Ramsay | | #15

    Not necessarily. Hot effluent from cooking rises up and expands out a bit as it does so. It takes an enormous amount of airflow, like multiple thousands of CFM's, to overcome that and pull the column in to a smaller aperture. This is why the opening of hoods (aperture) should extend out at least 3-6" beyond the range and one reason why a large open containment area is necessary (the other is that cooking effluent is quite bursty and far more so than a blower can keep up with).

    Anything that does not naturally rise up in to the aperture and containment area will spread out throughout the house.

    Good hood performance is much more about the hood design than the CFM's, despite what the marking dept's of consumer hood manufacturers say.

  12. Dave B | | #16

    Does anyone have a recommendation to kitchen vents that are in the 300-400 cfm that are not crazy noise.
    Akos, I have not heard this version running so I am unsure of the noise level. I would prefer the grate style screen also

    W RAMSEY, Any suggestions on good design for a hood. The stove will be an induction unit standard size.

    Thanks for your replies

    1. Akos | | #17

      I've bough some of smaller budget units from this place:
      https://www.kitchenhoods.ca
      Don't know how good their "professional" style ones are. Might be good to browse for ideas.

      Seems to be a decent insert, bit high on the CFM, but don't have to run it full:

      https://www.kitchenhoods.ca/shop/range-hoods-on-sale/built-in-range-hoods-for-wood-hoods-on-sale/30-36-inserts/powerful-built-in-range-hood-insert-liner-34-inches.html

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