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  1. #1
    Join Date
    Apr 2005
    Location
    Cincinnati
    Posts
    11
    Hi All - this is my first time posting to this particular site, many pardons in advance if I'm repeating a recent question. I just scanned through some recent threads and couldn't find anything applicable.

    We're building a 2200 square foot ranch in southern Ohio. It has a 12/3 vaulted ceiling on the main floor, and 9 ft ceilings in the walkout basement (also 2200 square feet).

    We're building a large kitchen in this house that includes a 600 CFM Vent-A-Hood range ventilator. It's vented to the outside through an 8" duct. As VAH vents do not use baffles or filters they are not plagued with the static pressure problems of other hoods, so they truly move about 600 CFM.

    I'd imagined that in this relatively modest-sized house the HVAC guys would recommend some dedicated make-up air system, with dampers and duct heaters and all that mess. But they just presented a plan that just upsized the size of the furnace fan (not sure to what) and upsizing the AC compressor from 2.5T to 3.5T. It's not a high-efficiency furnace, so there is an duct run that brings fresh air in.

    Does this sound reasonable to you? Their tech assured me that a separate make-up system like I described would be overkill - what do you all think? We don't want this house to drop in temperature 10deg everytime we cook during the winter.

  2. #2
    Join Date
    Apr 2003
    Location
    Eugene, Oregon
    Posts
    1,209


    I'd imagined that in this relatively modest-sized house the HVAC guys would recommend some dedicated make-up air system, with dampers and duct heaters and all that mess. But they just presented a plan that just upsized the size of the furnace fan (not sure to what) and upsizing the AC compressor from 2.5T to 3.5T. It's not a high-efficiency furnace, so there is an duct run that brings fresh air in.

    Does this sound reasonable to you?

    No!
    Increasing furnace size to accomodate make up air? Yikes!First check with your local building codes on make up air, they may have some requirements you have to go buy. Without the lay-out of your kitchen it may be a bit hard to get a concrete sollution. Maybe call a mechanical engineer in your area for ideas. Is their an architect involved?
    How about a vent in the kitchen with a reversed damper to open on negative pressure and close on positive pressure?
    Proud supporter of Springfield Millers and Oregon Ducks.

  3. #3
    Join Date
    Apr 2003
    Location
    Eugene, Oregon
    Posts
    1,209
    The vent I mention would be to the outside not the hvac system.
    Proud supporter of Springfield Millers and Oregon Ducks.

  4. #4
    Join Date
    Apr 2005
    Location
    Cincinnati
    Posts
    11
    millerman - thanks for the reply.

    I've read about some people using a passive system as you suggest, but it's always in more moderate climates than ours. In Ohio we sometimes get 100deg summers, and often times get 0 deg winters; won't this make for a cold/hot house when we're using the fan?

    Someone on another board recommended an EVH system like this one: http://www.ultimateair.com/store/recoupaerators.html . It provides about 200CFM of make-up.

    Lastly - if I approach the HVAC guys and argue this, I need to understand why. It sounds reasonable enough; the furnace is getting outside air, if we're in a negative situation, increase the blower output until we're neutral again. Is it bad because the system will cycle too much during normal times? I need some ammunition here.

  5. #5
    Join Date
    Apr 2003
    Location
    Eugene, Oregon
    Posts
    1,209
    ERV's are a great product for turning over fresh air. This system operates with your hvac system to exhaust stale air and bring in fresh air and recover the enrgy from the exhausted air. You can design a passive type vent with a motorized damper that opens when the exhaust hood is in operation, similar to a combustion air system used in commercial equipment. It can be quite simple to do. You would save a couple g's also.
    Proud supporter of Springfield Millers and Oregon Ducks.

  6. #6
    Join Date
    Jun 2004
    Location
    Florida
    Posts
    556
    upsizing a ton for 600 CFM o/a seems excessive to me. I think what he might have done is add 600 CFM of O/A to his load calc. Maybe he is just trying to sell a larger unit. Is that all he is doing? Perhaps he is going to introduce o/a through the duct system. Don't be confused with make-up air and ventilation air. If so I think 600 o/a on a 3.5 ton is too much.

    maybe I'm missing something. I thought that ERVs require as much exhaust air as they bring in O/A. They condition the o/a somewhat by transferring the heat from the exhaust or vise versa. This would not help his problem unless he ran the exhaust from his hood through it. I doubt he would want to do that since that air would be grease laden and hot. If he just exhausted inside air the house would still be on negative pressure and I don't see any benefit. Wouldn't he then be getting around 1200 cfm of O/A




  7. #7
    Join Date
    Apr 2005
    Location
    Cincinnati
    Posts
    11
    dhanna - thanks for the reply.

    I'm by no means an HVAC expert, so forgive me if I butcher any jargon.

    He told me that since we're using a standard 80% efficiency furnace, that the outside air will automatically be ducted into the furnace. It doesn't seem like there's any way that that would draw 600 CFM from the outside, though. I thought most of the return air came from return ducts inside the house(?).

    As for trying to sell a bigger unit, it might just be that he's taking the easy way out. I've made it clear that I was willing to pay for an active make-up system, so it seems like that might cost more than a compressor upgrade; it would make more sense to do the footwork and charge me the more expensive option. Maybe he's got no experience with this and he's too proud to say it. I'm not sure.

  8. #8
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,509
    i think this may help you out
    - EZ Vent Heat Recovery Ventilators
    this is the only site i could find on easy vent or easy air
    pulls in outside air and exhausts inside air
    connects to the supply and return of your ductwork and with the mextrix heat exchanger it warms or cools the outside air before it enters the duct work
    have put a lot in mainly in tight homes where the house cant breath enough
    also have one in the office and they work great.
    to increase the size of your unit doesnt seem to make any sense.

  9. #9
    Join Date
    Apr 2005
    Location
    Cincinnati
    Posts
    11
    Thanks for the info, tinknocker.

    One thing - I can't find specs on this device, but aren't HRV systems pretty much exchanging inside and outside air 1:1? If I need to make-up for a negative pressure situation, I would think this wouldn't get me anywhere.

    Although I guess my main concern with negative pressure is the risk of pulling radon from the basement, etc., so this kind of system might negate the bad effects of this situation.

  10. #10
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,509
    rstagg
    you are correct it will not make up for negative pressure
    just give the house a way to breath and exchange stail air for fresh air
    i am assuming this is a good size hood vent you are using
    putting out side air into you return ducts isnt going to make much differance either
    before you get to far into this check with the manufactor of the hood. find out if they have the same one with make up air vent capabilities built in to it
    where one fan pulls and one blows in all in the same hood
    this will resolve any consern of negative pressure
    or put a grille in the ceiling of the kitchen near the stove with a motorized damper and a 8 or 10 in pipe through the roof so the hood can draw the air it needs with that. might being in cold or hot air is the main draw back.

  11. #11
    Join Date
    Apr 2002
    Posts
    11,808
    Originally posted by rstagg
    Hi All - this is my first time posting to this particular site, many pardons in advance if I'm repeating a recent question. I just scanned through some recent threads and couldn't find anything applicable.

    We're building a 2200 square foot ranch in southern Ohio. It has a 12/3 vaulted ceiling on the main floor, and 9 ft ceilings in the walkout basement (also 2200 square feet).

    We're building a large kitchen in this house that includes a 600 CFM Vent-A-Hood range ventilator. It's vented to the outside through an 8" duct. As VAH vents do not use baffles or filters they are not plagued with the static pressure problems of other hoods, so they truly move about 600 CFM.

    I'd imagined that in this relatively modest-sized house the HVAC guys would recommend some dedicated make-up air system, with dampers and duct heaters and all that mess. But they just presented a plan that just upsized the size of the furnace fan (not sure to what) and upsizing the AC compressor from 2.5T to 3.5T. It's not a high-efficiency furnace, so there is an duct run that brings fresh air in.

    Does this sound reasonable to you? Their tech assured me that a separate make-up system like I described would be overkill - what do you all think? We don't want this house to drop in temperature 10deg everytime we cook during the winter.
    600 CFM is an enormous amount of air to exhaust from a home.

    The best advice you have been given is a dedicated make up air system.

    You could get a small dedicated fan and an electric duct heater. The duct heater could have an SCR control and set to heat up air to a set temperature.

    Perhaps 0F is a cold day in Ohio, so a 12 kW duct heater could temper the air up from 0 to 63F. Would have to see your house plans before I could tell you where to dump that make up air.

    The worst thing you could do would be to over size a furnace, especially a non-condensing furnace (not a high efficiency) and bring in 600 CFM of winter air. You will rot out your heat exchanger quickly.

    In summer the house will warm up and get more humid.

    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  12. #12
    Join Date
    Jul 2004
    Location
    midwest
    Posts
    2,868
    My first question is why do you need so much exhaust? Like Carnak said 600 cfm is a lot of air and no matter how you try to solve this problem your going to get a draft through the kitchen.

  13. #13
    Join Date
    Apr 2005
    Location
    Cincinnati
    Posts
    11
    Carnak - thanks for the advice. It's what I was thinking, just wanted some backup from pros like you. Funny that these guys won't figure out the *more expensive* option for me. 'Guess they don't like a challenge.

    Trane - this is actually a modest amount of pull for the range we're installing. It's a commercial-style range with 6 18k BTU burners. Of course, we won't always (or ever) have 6 burners running full-out, but 4 or 5 running at various levels won't be unusual. We're both avid, trained foodies, my wife a former professional. BTW - it's not unheard of for people installing 60-inch ranges with grills and griddles to install a 1200 CFM hood. Granted, I think most of these people have more money than kitchen skills, and like a pretty trophy, but that's not us.

    One of the most bothersome things to me is these guys keep talking down to me as if I couldn't possibly understand the technical side of what they're talking about. As a professional engineer, it's really frustrating. I certainly don't have even close to all of your domain knowledge in this area, but you can have a descriptive conversation without losing me. 'Nice to know there's a place like this where people won't treat you like a child.


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