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Residential island range hood ducting and makeup air questions

Phaelon56Phaelon56 Member Posts: 7
Hello to all. I am a new member here, and this is my first post. I wasn't certain of which forum category this should be posted in - if it should be moved and I should repost the question, please advise and I will do so.

My wife and I just purchased a house in the Raleigh North Carolina area. It is a two-story 2700 ft.² Cape Cod style house built in 1986 with reasonably tight control over outside air infiltration, but it is certainly far from airtight. I am posting today to get some advice about how to proceed with installing a new ducted range hood over the island that is the center of the kitchen work area. The kitchen is wide open into the family room with total space in this combined room of roughly 4800 ft.³. At the far end there is a 10 foot wide x 8 foot tall opening into the living room, which has a wood-burning fireplace. We don't expect to use that fireplace very often when we are cooking but that is possible. We don't do heavy frying, but do lots of stove top cooking. I am a serious home cook. We do nearly all of our cooking at home, and don't dine out frequently.

The present configuration has a 30 inch slide in electric range in the center island, with a downdraft assembly, which I'm guessing is ducted out through the crawlspace under the house. I plan to remove this and install a dual fuel 30" or 36" Kucht range with gas cooktop. It is not a commercial range, but is built to a fairly rugged standard, and probably has maximum output of 60,000 BTU, not including the heat from the oven (if and when we are using it.) I've been doing lots of research on the required size of the range hood for this amount of cubic space, as well as the type of cooking that we plan to do.

My intention is to use an island range hood - probably a Z-Line - and run ducting through the ceiling joists to exit out the back of the house above the deck (the joists run perpendicular to the exterior wall - I should have an unimpeded run.) I'm guessing that my optimal duct material can be assembled in interlocking sections, then sealed properly with foil tape, and slid in from the outside to the point where I add and seal the elbow that comes down into the stack coming up from the hood. I have also considered a single run of spiral pipe but installing that myself would be challenging. have found it exceedingly difficult to find comprehensive or accurate information about performance and fire safety for this type of of configuration. I've ruled out any kind of flex duct, and will only use smooth surface rigid ducting. Having once been a commercial coffee roaster, and having previously installed positive pressure high temperature stainless steel ducting for my 12 kg batch size roaster, I believe in the importance of properly sealed ducting that can periodically be cleaned from the outlet end.

Here are my current questions (note - I have already done a fair amount of reading in this forum about residential range hood installs, and did not find the answers to these questions):

- Should I use double wall duct that has an air space between the outer shell in the inner liner? I know that this would mitigate my concern about heat buildup, but with 10 inch joists, it would mandate that I use 8 inch O.D. duct with 6" I.D for the inner liner. If this is the most prudent approach, is it worth the extra money to get the type of duct in which there is mineral wool insulation between the inner liner and outer shell? Do I need to install a thimble where the duct goes through the exterior wall?

- Is the amount of heat that will be passing through the ducting at a high enough level that I even need to use double wall? My coffee roaster ducting was capable of 500° sustained temperature, with a 2000° maximum for short durations. I'm guessing that the air being pulled through the hood will be no more than a few hundred degrees by the time it hits the first elbow to make it straight run toward the outside. By the way, the horizontal distance from the elbow above the range should to the outside of the exterior wall where the duct will end, is about 10 feet.

- I have seriously considered a range hood that has a 900 CFM capacity when the fan is on high. I had initially thought that 600 CFM would be more than adequate, but when I look at the total cubic footage in this large room, I'm inclined toward the larger capacity, even if I never have to put the fan on high.

- Our house is far from airtight; the clothes dryer is in a closet on the second floor, and the electric water heater is in a small enclosure attached to the outside of the house with its own entry and venting. Heating is provided by heat pump with an outdoor condenser system. That being said, I am concerned about the possible need for makeup air. Would it be prudent to add some sort of makeup air blower that is bringing air back into the room? I would have it coupled electrically so that it would automatically open a damper and be activated when the hood fan was running. Also - if I provide make up air, then where should it enter the kitchen?

- Note: I fully anticipate that I will require professional trade help to ensure that my needs are properly calculated, and also anticipate hiring an electrician at a minimum - possibly an HVAC specialist. I'm posting here because I want to fully understand the requirements before I consult with a professional in person.

I've been known to overbuild things in the past, and don't want overkill on this project, but we love to cook and plan to be in this house for 30 years or longer. I want to do it the right way!

I am not asking for nor interested in discussing any specific pricing here, as that is against forum rules (as I understand them,) but I am budgeting roughly $1600 for equipment and materials, will be doing as much of the installation myself as is practical, and will rely on paying the skilled trades person for the balance of work. Any and all input and feedback will be greatly appreciated!


  • Jamie HallJamie Hall Member Posts: 9,417
    Years ago -- much longer than I care to list -- I did building inspections of restaurants and the like. I don't think I even encountered a need for double wall ducting for the range hoods, so I wouldn't worry about that. Provision for cleaning the ductwork, however, is essential. In restaurant type applications, I also required a grease filter -- nothing fancy, but a stainless steel mesh affair -- at the entrance to the ducting itself, where it could be gotten at and cleaned regularly.

    Almost all of those installations had the exhaust fan on the outside of the building.

    I also required built in fire protection for commercial units -- but I doubt that you will be required to have that.

    As to air volume and makeup air, since you don't have any fuel burning appliances other than that fireplace and the range, you have less of a problem than you might. I'd be much inclined to try running without any. If the need arose, you could easily add an exterior vent, seems to me, in the future.

    In any event -- and this is very important -- check with your local fire marshal or building inspector on all this. They have the last word, and opinions and codes do differ.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • STEVEusaPASTEVEusaPA Member Posts: 2,447
    edited May 2018
    I think you definitely need make up air, unless your fireplace is sealed combustion. The make up air doesn't have to come into the kitchen. Think of when they do an energy audit and blower door test (on your front door).
    As far as piping, you should get instructions with the hood. I wouldn't think you need double wall, but smooth wall is obviously better. Make sure you have a way to access and clean it.
    I'm no expert on this, but maybe an air-to-air (fresh air) exchanger would do the trick. Or something like this:
    Ultra-Aire 70H whole house ventilating dehumidifier (piped for exterior make up air).
    After it's all installed, make sure the installer checks your house with the fireplace flue open, range hood on full blast, any exhaust fans running and the clothes dryer running to make sure you don't back draft the chimney.
    @captainco would be the expert on this, hopefully he'll weigh in.

    You're not going to get anywhere near 500° in the range hood with all burners on high.
  • Phaelon56Phaelon56 Member Posts: 7
    edited May 2018
    Thanks to all! The hood will already have a stainless steel mesh baffle filter to catch the initial grease (if it does not then I will get one that does!) The suggestion about the whole house ventilating dehumidifier is a good one. I have looked at EVR and HVR also, but not sure which one would be best in this climate. I returned to the network tech world last year (Sales Engineer) after a 4 year stint doing commercial insurance inspections. The commercial range hoods I inspected all had single wall ducting, but vented directly through a back wall or through the ceiling to the roof (single story buildings.) I always had to check the inspection dates for cleaning and for the Ansul fire suppression systems (which I definitely don't need.) Towards the end of that inspection career, the underwriters started mandating that I pull a filter to photograph the inside of the hood, and also get a photo of the fusible link. A few of those were very scary looking!
  • ratioratio Member Posts: 1,775
    Ask your local building dept on the requirements for a residential kitchen exhaust fan, that'll be your starting point. Avoid talking about grease or how often you will be cooking there, you don't want the building dept to start thinking you need a commercial grease hood.

    I'd get a pilot hole up somewhere & take a look down the joist space ASAP. It's quite possible that Sparky ran a piece of wire through the joists where you are planning on running. Sometimes the wires can be pushed out of the way, but not always.

  • Larry WeingartenLarry Weingarten Member Posts: 1,389
    Hello, Lawrence Berkeley National Labs has done quite a bit of research into how residential range hoods work or don't. Here is a link to one of their studies: . I agree with the others that single wall duct is all you need. I'm wondering if plastic could work? A benefit being no corrosion possibility, particularly in the long horizontal section. Also, a simple way to get makeup air may be just to open a window in the kitchen. B)

    Yours, Larry
  • IronmanIronman Member Posts: 4,731
    The IMC requires makeup air for a hood capable of exhausting 400 cfm or more.

    Single wall 26ga. pipe with the joints double wrapped with foil tape (UL 181) is all that you need. No screws.

    You're not venting an appliance: you're exhausting a cook top and there's more than ample air to dilute the heat from it to a safe level.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • LeonardLeonard Member Posts: 511
    edited May 2018
    My comments are all for commercial restaurants , not residential.

    Landlord for a commercial restaurant here in NH. Here city wants welded ducts on commercial, don't know residential, I guess grease drips out seams. And insurance company wants clean out ports for each straight section.

    As far as exhuast ducts they used a fire insulating blanket ~ 1-2 inch thick around welded steel ducts when they ran near wood, like rafters, suspended ceiling . Guess issue is if grease accumulates and there's a grease fire at stove then duct might catch fire and get pretty hot in there.

    Being an engineer I don't think much of plastic ducts for fire safety reasons. Here at my house we've got a ~ 4 x10 steel duct that's maybe 10ft long. I've given some thought to adding co2 noselles in it, just in case. I 've got a 20# CO2 tank from a previous tenant.

    In the commercial restaurant as far as make up air I've seen air being sucked DOWN hot water heater chimney due to vacuum drawn by exhaust hood fans. Tenants shut off makeup air to save money. Was so bad CO was accumulating in boiler room next to dinning room( Chinese restaurant, they don't care). You might get same effect in main furnace or fireplace depending on fan CFM and house leakage, I got no experience with those numbers . Restaurant had several high CFM exhaust fans for large hoods Maybe 5k CFM each.
  • heatheadheathead Member Posts: 80
    In looking at range hoods for a 36 pro range I have done it with blower unit built into the hood. 600 cfm Let just say it was really loud, couldn’t hear people very well when on. You end up not using it if it’s loud. When adding room on my house I Had to change the outside of termination on the house. Had new termination point installed with blower built into it called a remote blower. That takes the motor and the noise up to the roof or side of the house at termation point instead of near the stove. You can have a conversation with a remote blower on, couldn’t when the motor was built into hood.

    Code in my area requires a blower door test or remote air intake on anything over 600 cfm.. Air from outside goes to return on furnace. Put filter on the inside of that intake before furnace in addition to furnace filter.

    I told neighbor they had to do remote blower because then you actually use the fan. They didn’t like extra cost at first but now are great full because the fan quiet and they use it. 100 cfm per 30,000 btu I believe is optimal. Unless indoor grill option then go bigger. Lot of the kitchen designs have rule of thumb 6 inch’s wider than stove. Never follow that rule because of looks and cabinet space.
  • Phaelon56Phaelon56 Member Posts: 7
    Thanks to all (again!) Definitely no commercial cooking here, and most of the time I'm just cooking for my wife and I, but I like to do things like sear steaks in a cast iron skillet, then finish them in the oven. Done the right way, it generates a lot of smoke when you first sear the outside of the meat. This is a project I will do once, and plan to do it right. Based on everyone's comments, it's obvious that I don't need double wall, but I'm inclined to use stainless steel rather than galvanized, even though it will cost 3x or 4x the price. Stainless eliminates any future concerns about rust or corrosion, and I think it will probably clean more easily. The more I read about the noise levels, the more I think that it might be prudent to put in a remote blower that is at the outlet end. There are already three hanging solid rod pendant style lights in the center of the kitchen above the island. There's definitely power up there, but the Romex must run toward the inside wall in the center of the house and over to the switch, I'm still thinking there's an unimpeded run out between the rafters to the outside. As for makeup bear, I'm seriously looking into the previous suggestion that I consider a whole house dehumidifier system. I'm guessing something like that would probably be $1,000 installed or perhaps less but I will do some research. This will be our first summer here in North Carolina, and I'm told that it's incredibly hot and humid for about 3 months. I'm not pulling a permit to do this work, and don't have to, and I'm not inclined to get a building inspector involved especially because there don't seem to be really clear guidelines about residential range hoods. I just want to make sure that it's done in a professional manner and with safety as the utmost consideration.
  • rick in Alaskarick in Alaska Member Posts: 784
    The simplest air intake would be a "Fresh 80" air intake. They are used up here on houses where the people do not want to put in an HRV system due to cost, or because of retrofit concerns. The Fresh 80 is just an air flapper that is installed in the wall, that opens with any negative pressure in the house. This way, whenever the fan comes on, the damper opens and allows air in to the house to balance out the loss. You can also close them off if you need to.
    If you go that route, make sure the vent is in an area that when the air comes in in the winter, that the cold air does not "fall" down on an area where you do not want it, such as your couch.
  • pecmsgpecmsg Member Posts: 437
    Start with FanTech site. Several exhaust styles to decide. They also have makeup air systems.
    The make up does not have to be in the hood but can be done like that.
  • STEVEusaPASTEVEusaPA Member Posts: 2,447
    I still think being in NC, if you're pulling in fresh air, you're going to pulling in a lot of humidity most of the year, which is why I mentioned the ideas in my first post.
  • ratioratio Member Posts: 1,775
    If you decide to run the fresh air/makeup air into the return duct of the furnace, a few tips:
    • There is a set of terminals on the furnace control board marked "EAC", this is energized (120VAC often, occasionally 24VAC) whenever the blower motor is energized. Use it to power a spring-closed-power-open damper to keep the outside air closed when the furnace blower is not running. Be sure and measure the airflow to insure you're bringing in the correct amount.
    • Interlock the exhaust fan with the furnace blower (use a relay powered by the exhaust fan feed to make R to G, either in the furnace or at the stat).
    • Around here, commercial kitchens are required to have a temp sensor in the hood to bring the fan on automagically if an appliance is turned on without the exhaust fan already running. This is a nice feature, but might not be feasible.
  • Larry WeingartenLarry Weingarten Member Posts: 1,389
    @ratio: "Automagically" ...What a great word!! It sorta sums up my understanding of electronics. :p

    Yours, Larry
  • LeonardLeonard Member Posts: 511
    edited May 2018
    In restaurant hoods they like to pipe make up air to a line of vents just in front of wall mounted exhaust hood. That way you don't flush as much $$ conditioned air out of building ( they are moving lot of air , ~ 10k CFM). But might be a cosmetic issue in a house,

    Depending on how much cooking smoke misses the hood you might want to place makeup air vents little further away in ceiling to flush air thru kitchen.
  • JUGHNEJUGHNE Member Posts: 5,155
    If your existing downdraft system is (probably) 6" to daylight, then you could possibly use that for fresh air inlet.
    Just some type of grill above the floor. Maybe a toekick grill in the island. There is a butterfly damper that can be installed at the hood to swing either way, in or out.
  • Phaelon56Phaelon56 Member Posts: 7
    Thanks again to ALL for the great ideas! It's still a moving target - we don't close until June 6. The only modes we plan are all in the kitchen. Counter upgrade is a definite, as is the hood. I will have to remove the three pendant style lights currently mounted over the island - but if it works aesthetically. I will move the outer two closer to the ends of the island (with the hood in between. ) I am leaning towards a six burner duel fuel range with max burner output of 96,000 BTU's plus an electric oven with three oven baking elements that total 3850 watts (there is also an 850 broiler element, but it won't be used when over is in baking mode. Base on the potential that we *might* use all six burners simultaneously a few times each year, and also have the oven on at that time... I am leaning towards 900 CFM. I don't need the "automagic" feature ;-) as our range will always be attended when doing cooking, and I will turn the hood on only when needed. I will probably also leave it on the lower or middle speed setting. Have given thought to some having an external blower at the outside end of the duct, but am concerned that I'd have to remove the blower annually to clean the grease out of the duct. As for running the intake air in through the existing down draft hood ducting - sounds to me like a great idea! I am *assuming* that it's a real ducted downdraft. Even if it is just a recirculating filter... much easier to bring the make air in that way, then have a register on the back side of the island (crawl space is about 3 feet high and has easy access.) Still looking into whole house dehumidifiers, but not clear on how it could/would automatically kick on to bring in makeup air during the winter months when humidity is low.
  • JUGHNEJUGHNE Member Posts: 5,155
    One job I was on the owner was considering a hood over the island cooktop.
    In order for the hood to be effective in that case it would have been just at eye level to the bottom.
    They realized that it would block the view to the rest of the great room and went with the down flow vented cooktop. FWIW

    Have you considered the height requirement for effective draw?
  • Phaelon56Phaelon56 Member Posts: 7
    JUGHNE said:

    Have you considered the height requirement for effective draw?

    Yes I have thought about this. It's one big open room - 14 feet from the cabinet/sink wall to the exterior wall, and 30 feet from the far end of kitchen area to the wall at end of open dining area. The island runs parallel to the long wall. The hood will be 26" to 30" above the range top (28" is optimal - I think.) I already Photoshopped a hood into a picture of the existing kitchen. It's the type with a rectangular arced piece of glass on the bottom. Looks really good and does not block the view.

  • Phaelon56Phaelon56 Member Posts: 7
    I am now seriously considering a whole house dehumidifier system, which was recommended as a good way to get make-up air to replace what is exhausted out the range hood. I'm okay with the cost (have researched it) but not clear on a few things.
    1) Can it be triggered by activating the range hood\
    2) Instead, can it be triggered simply by a reduction in air pressure in the house?

    I'm thinking that if it works solely on sensing humidity, then in the winter it might not kick on when I use the good but the air in the house is relatively dry.
  • ZmanZman Member Posts: 4,572
    Triggering when the hood activates is pretty simple. Just put one of these any where on the power wire to the hood.

    A setup with outdoor and space pressure transducers is usually a custom commercial control.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Phaelon56Phaelon56 Member Posts: 7
    I guess we can bring this thread to a close. Got back into the house and got accurate measurements today, and also spoke at length with a local HVAC contractor. They will handle the hood install. I'm going with a 42" hood that handles 760 CFM maximum. The duct can vent through the roof with one 45 angle that will allow it to run straight up through the edge of the 2nd floor laundry closet. The local contractor stated that my overall AC capacity (2 tons for 1st floor and 2.5 tons for 2nd floor) is a tad undersized for this climate and the size of the house, but that means it will cycle to ON for longer when it's hot, and remove more humidity from the air. With all factors considered, his opinion is that we will be fine with a motorized damper to bring in makeup air. In other good news, the existing range does have a downdraft duct that vents out under the porch. The 6" damper is already in place - just needs to be replaced by a motorized one that is triggered to open when the hood is running. I love it when a job turns out to be less complicated than I had feared. How often does that happen? (rhetorical question!)
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