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Community and Q&A

Central fan integrated supply + Range Hood/Make-up air

Michael D | Posted in Mechanicals on

Building a “pretty-good” house in Zone 5
* 2″ exterior foam with taped and staggered seams
* r-15 mineral wool in interior
* attic will be conditioned with 4″ closed cell foam and mineral wool

I can’t know before the blower test how much leakage there will be but assuming the house will do well on the test.

The current plan is to use “central-fan-integrated supply ventilation system” with HVAC unit with a variable speed blower in the attic and basement each with a ductwork with motorized damper to bring in fresh air.

The S.O. is insisting on a big commercial style range requiring a large high CFM hood (I would change this if I could but some fights are not worth having). I have looked at Martin’s 2010 article on the large range hoods required by these monsters:
“Make up air for a residential hood” https://www.greenbuildingadvisor.com/blogs/dept/musings/makeup-air-range-hoods?page=2.
This makes me a bit concerned about backdrafting. In the article (2010) the manufacturers seemed blissfully ignorant about the make-up air issue, however today I see many of the manufacturers recommending motorized dampers and external make-up air sources in addition to the hood which is a step forward.

In my case since I will already have a fresh air source with a motorized damper and I was thinking if I have the hood open the damper for the HVAC system to make-up air to be drawn in through the existing ductwork. This would a)save a little bit of money/time and b) have the make-up air distributed through-out the house which would be better on days with extreme temperatures as the cold/hot air would not be concentrated in one area.

My question to the GB crowd is does this seem reasonable? and will the approach pull in enough make-up air through the ductwork when the HVAC motor is not on or could it still possibly cause backdrafting??

Thanks in advance

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Michael,
    The short answer to your question is No, this won't work.

    The reason has to do with air flow rates.

    Typical air flow rates for a ventilation system (for example, your central-fan-integrated supply ventilation system) are in the range of 50 to 100 cfm.

    If you have (as you put it) a monster range hood fan, you are probably talking about 600 to 1,200 cfm. So you need an entirely different approach. My article (Makeup Air for Range Hoods) explains what you need to do.

  2. Michael D | | #2

    Thanks for the quick answer that is exactly why I asked - back to a different approach.

  3. Michael D | | #3

    Martin,

    I am now a bit confused or perhaps I was not clear enough (or I am mis-defining the term central-fan-integrated system) as I went back to your article and one suggestion was to use the Broan dampers (who coincidentally make the hood I am getting) and to read their installation guide.

    I read their installation guide and they suggest exactly what I proposed which is to put a fresh air source with damper on the return of the forced air system (pic attached). The other addition from Broan is a pressure sensitive switch. That is also what I thought you mean by central-fan-integrate system which is a fresh air source with a damper connected to the return air plenum. This would mean that the hood will pull enough air through the system itself as long as your sizing is correct (and Broan offers a tool to help design the correct size http://www.broan.com/tools-landing-page/make-up-air-damper-tool)

  4. GBA Editor
    Martin Holladay | | #4

    Michael,
    Every tight house needs a mechanical ventilation system. What type of ventilation system are you planning to install?

    A mechanical ventilation system is different from a makeup air system. The phrase "central-fan-integrated supply ventilation system" usually refers to a ventilation system, not a makeup air system. This type of ventilation system includes a motorized damper controlled by an AirCycler or FanCycler control. It is different from a makeup air system from Broan.

    To learn more about central-fan-integrated supply ventilation systems, see Designing a Good Ventilation System.

    You are correct that if you install a makeup air system from Broan, and if you follow the Broan instructions, you will have addressed the makeup air problem.

    However, your home may still need a ventilation system.

  5. Michael D | | #5

    I think we are now in violent agreement :0)

    The central fan integrated system (from your article) requires:
    1) A duct that introduces outdoor air to the furnace’s return-air plenum;
    2) A motorized damper in the fresh air duct;
    3) An AirCycler control to monitor the run-time of the furnace blower and to control the motorized damper.

    Broan's make up air design requires:
    1) A duct that introduces outdoor air to the furnace’s return-air plenum;
    2) A motorized damper in the fresh air duct;
    3) An transformer to open the duct when the hood is venting

    What I am proposing is to simply use the same fresh air duct and damper for the make-up air hood so here are the cases:
    Hood off, fresh air cycler signal off ==> furnace blower off, damper closed
    Hood off, fresh air cycler signal on ==> furnace blower on, damper open
    Hood on, fresh air cycler signal off==> furnace blower off, damper open
    Hood on, fresh air cycler signal on==> furnace blower on, damper open

  6. GBA Editor
    Martin Holladay | | #6

    Michael,
    For ventilation, you want 50 cfm to 100 cfm.

    For a makeup air unit, you may want 1,200 cfm.

    So how are you going to commission your proposed system? If you adjust your system to supply 60 cfm, you'll have a good ventilation system -- but no makeup air.

    If you adjust your system to supply 1,200 cfm, you'll have a very bad ventilation system -- and an energy penalty worthy of the Guiness Book of World Records.

  7. Michael D | | #7

    Then perhaps its two supply ducts with two dampers and they are off different sizes but both still attached to the return of the central air system.

    In which case the question is what causes most of the flow rate - the blower or the size of the duct (physics would say a combination of both). But if it is mostly the blower and the fresh air duct is sized big enough then when the hood is on it will pull 1200 cfm of air from the ductwork and when the furnace is on at low speed is will pull the lower CFM rate.

    My hvac person already proposed a 6" duct for ventilation and Broan typically wants 8" for make up air so then the answer probably is two different fresh air supplies and dampers - the savings is not worth any risk.

  8. GBA Editor
    Martin Holladay | | #8

    Michael,
    Your proposal will probably work if you include two outdoor air ducts and two motorized dampers, separately controlled. These systems are commissioned with manual dampers that are adjusted while the flow rate is measured; once the desired flow rate is achieved, the manual dampers are locked in place.

    It's possible, however, that commissioning will be tricky. There are four cases:
    1. Makeup air system off, ventilation system off.
    2. Makeup air system off, ventilation system on.
    3. Makeup air system on, ventilation system on.
    4. Makeup air system on, ventilation system off.

    The contractor who commissions the system will need to test air flows in all four modes. All I can say is, good luck.

  9. Michael D | | #9

    So now we are getting to the crux of the question - your "All I can say is good luck" does not instill me with much confidence.

    Would it then be smarter to use an HRV or ERV for balanced ventilation and then use the fresh air source with damper for make up air only for the hood so there are less complications with balancing the system or would we still need to take into account hood operation with the HRV/ERV. Either way the "hood on" case is rare in the overall scheme of things since it might be on for 10-20 minutes every few evenings.

  10. GBA Editor
    Martin Holladay | | #10

    GBA readers,
    Has anyone tried what Michael proposes? How did things work out?

  11. John Semmelhack | | #11

    First (for Michael) - what appliances are you worried about backdrafting? If you have a natural draft fireplace or woodstove, and the house is tight, then DO NOT put in a high-power range hood.

    The concept that Michael is suggesting will only help to reduce the depressurization...it won't eliminate it....and if the range hood fan really is 1,200cfm, AND the house is quite tight, AND the make-up air duct is only 8", then there will STILL be significant depressurization when the range hood is on.

    Let's say the house tests at ~350cfm50 on the blower door test, which yields an equivalent hole area approximately 8" round. With a 1,200cfm range hood and no make-up air duct, we would expect the house to depressurize to in the ballpark of 350Pa. An additional 8" hole with no resistance would reduce that depressurization to about 100Pa...however, in practice an 8" make-up air duct with a screened exterior hood and ductwork might only be equal to about a 6"-7" unrestricted hole, so I'd expect the actual depressurization to be more like 125Pa to 175Pa.

    If you're going to put in a commercial-size range and commercial-size range hood, you should install a commercial style (powered) make-up air system.

  12. John Semmelhack | | #12

    A side note to this conversation - the actual minimum airflow required to "entrain" and remove all of the heat, moisture, combustion gases and odors coming off a particular range has as much to do with the shape and installation of the hood as it has to do with the airflow. The most recent ASHRAE Journal has a great article on the topic - http://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201511/index.php?startid=10#/28

  13. Michael D | | #13

    John,

    You are right to ask about my appliances - since I am building new I will use direct vent high efficiency furnaces and a direct vent water heater so I guess those are not a worry for backdrafting. Our only fireplace will be sealed as well. So I guess for backdrafting issues that leaves the dryer.

    Even so, should I not still be worried about makeup air for the hood - and you mention commercial make-up air system - what suggestions would you have? My builder is already discussing external blowers because he does not like the opposite issue (air infiltration from the hood when it is off.)

    At this point maybe it will be easier to switch to a smaller range and a new wife :oP

  14. Michael D | | #14

    Thanks Chris - your response really helps - this range is dual fuel and I can not see us ever using more than 3 burners (Thanksgiving) at once so with specs that is worst case 58K Btus (3 biggest burners on high) and 580 CFM so half as bad as 1200 CFM. I am getting closer to thinking the ERV for ventilation and fresh air intake with damper for hood make-up air will work. If we cook more than that design threshold we can open a window.

  15. John Clark | | #15

    The installation and technical guides on the Broan website give you the necessary details and it doesn't appear that you actually need a fan pushing air into the housing envelope but just an opening to relieve the pressure differential.

    Tip with regard to range hoods. The cfm requirement assumes all of the gas burners on the pro-style range are lit and at full power. Also don't forget that exhaust duct sizing, # of bends, and length all have a negative impact upon the amount of air that's actually moving through the system.

    Range hood should be a min of 6 inches wider than the range/cooktop to insure sufficient capture area.

    Compromise: Pro-style Induction range. This range type commands a lower CFM requirement because the range itself doesn't emit any heat unlike gas so you're only having to exhaust the emissions from the food you're cooking.

    Induction: 100 cfm per 10" width of the cooktop = Total cfm Ex: 36" wide induction range would need a fan pushing 360 cfm.

    Gas: Total BTU output of the range / 100 = Total cfm Ex. Typical pro-style gas range (25k btu burners x2, 15k btu burner x1 , 10k btu burner x1). Total btu 75,000 requires a fan pushing 750 cfm,

  16. John Clark | | #16

    Michael,

    No problem. I was in the process of buying a range hood a few months ago and these guidelines appeared, from my own research, to be fairly standard.

    One thing I forgot to add is that you'll need to up size the fan if the range/cooktop is on an island.

    http://www.prolinerangehoods.com/1-801-973-3959/support/electric-vs-gas-cooktops-and-how-to-size-your-range-hood/

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