Exhaust Vents - I Can't Be The Only One....

amck2March 17, 2014

who is confused. I'm changing out an OTR MW for a venting hood and I have been trying to educate myself on the proper size, best reviewed brands, etc.

My first question is about insert hoods, which I understand is the type I'd buy if I wanted the metal to be concealed by a wood covering - like a simple rectangular mantel. Should I look for manufacturers that specialize in these, or are they offered by most of the brands -Kobe, Zephyr, Braun - that I often see recommended here?

My second question is about make up air (MUA) which I didn't know I'd have to consider. I live in New England and have a gas fireplace in my LR, which is open to the kitchen area. Apparently, this is a concern.

I tried reading posts on the subject on the Appliance forum and I still don't understand how much of an impact it should have in my remodel. My 30" induction range will be on an outside wall where it will vent. When the contractor installs the fan/hood/exhaust system (I'm still not sure what term is generally used for this appliance) should he be able to take care of the MUA issue then?

I hate to be so dense about this. I hope someone can explain simply for me & others who are exchanging a microwave for a vent what has to be done re MUA. Thanks -

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This is super complicated and has been discussed a lot in the Appliances forum here. First you should check your local codes. If I recall correctly, MUA is only required by code if the CFM of the hood is higher than a certain level, in certain jurisdictions. Opening windows can provide MUA, but that is subject to user error and may not be acceptable for local codes.

A typical guideline would be to use a 36" wide hood for a 30" range to increase capture area. I have often seen it advised on this forum to have extremely deep hoods (24" or more) and that does increase the capture area and improve function. But, 1. residential hoods tend to be shallow and 2. you have to work out the geometry of cook heights and head-bumping hazards. If you install a deep hood high enough to prevent head-bumping of a tall cook, it may end up being mounted high enough that it undermines the effectiveness of the hood, thus requiring higher CFM.

I am stalling on this issue myself. You can see how far I got in my thread "super-quiet 600+ cfm range hood for condo - mission impossible??" in the Appliances forum.

    Bookmark   March 17, 2014 at 2:10PM
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Determine what type of exhaust you need for your induction range. It will be quite different than a high BTU commercial gas range.

    Bookmark   March 17, 2014 at 2:15PM
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All the volume of air that the hood vents outside must enter the house from somewhere. In older homes, some of it will come from poor sealing around doors and windows. The rest will be pulled into the house from other openings, and these openings are usually the vents on gas-fired (and oil-fired) equipment (furnace, water heater, clothes dryer,etc). If enough MUA is needed, it can actually reverse the flow of the flue gas leaving the appliance and essentially bring the fumes into the house ("backdrafting"). Obviously, this is a very dangerous situation due to CO concerns. In newer highly-sealed homes this is an even greater problem.

Every house and vent setup is different so it has to be handled on an individual basis. One way is to run the vent with all windows and doors closed and check the airflow at all gas appliance vents to ensure there is no backdrafting.

Any MUA system will bring in outside air, which will be cold in winter and hot in summer. In some cases, a heat exchanger could be used to save energy costs.

There is no easy or cheap answer if you find you need MUA.

You can't count on always remembering to open a window when you run the vent. It really needs to be a foolproof system such as an automatic vent to let in MUA as needed.


    Bookmark   March 17, 2014 at 2:35PM
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Thanks for the replies. I'm convinced MUA is something I need to address. Where do I go to determine the BTU I need for my induction range? When I had a range & vent priced out last month at a reputable appliance store, it wasn't even broached by the rep who's been in the business 25yrs.

At what point do I contact someone to discuss the remedy for MUA? And who is that person? The only people I thought we needed to contract for this remodel (changing out cabs/replacing range & fridge & adding a venting hood) was a KD from whom we'll order the cabinets and a GC who will manage the new installations.

    Bookmark   March 17, 2014 at 4:09PM
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I don't know if it's appropriate for your situation, but here's a solution that I found:

Here is a link that might be useful: What Is Make-Up Air? (Definitions/Solutions)

    Bookmark   March 17, 2014 at 4:17PM
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Contact a local HVAC company to discuss MUA and the ERV that you will need in a cold climate. Many people find that proper venting ends up costing significantly more than the range they are buying.

    Bookmark   March 17, 2014 at 5:23PM
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Darn, the vent was originally seen as a P.S. to replacing a broken range. I had no idea putting a vent into an outside wall in a relatively new home would be so costly. Now I know why people opt to live with the less effective venting of an OTR MW if they, like we, rarely fry or do high sear cooking.

Bummer news, but I'm glad to know sooner than later.

    Bookmark   March 17, 2014 at 6:35PM
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Can you contact your appliance person to ask them? I was just in a showroom and they had a great demo of a hood with make up air and it was enclosed in a plexiglass cabinet so that you could see how it all worked...was pretty cool and informative

    Bookmark   March 17, 2014 at 6:37PM
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Fori is not pleased

Wait--is your OTR vented now? If so, do you know how much (many?) CFM it has? And does it vent well enough?

You might be able to get away with a lower CFM hood if you don't fry or do stinky greasy stuff. It's really more what and how you cook than what you're cooking it on (unless there is a grill involved...).

    Bookmark   March 17, 2014 at 6:44PM
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What you need to do is find out for sure if your county enforces the 2009 IRC, section M1503.4, which states that range hoods capable of exhausting in excess of 400 cfm must have makeup air. If 400 cfm is all you need, then MUA probably isn't necessary in practice. My county does not enforce that section of the IRC, but I wanted MUA for safety reasons, since our hood is 1000 cfm. My not-to-code but effective and cheap solution are linked below.

Here is a link that might be useful: My MUA solution

    Bookmark   March 17, 2014 at 6:48PM
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You may or may not need a high cfm exhaust fan. We opted to go low with a 400 cfm since we had a vented OTR MW in our previous house and it did an adequate job for our needs. I don't do a ton of high-temperature frying, induction doesn't build up as much heat when cooking, and we are okay with a little bit of remaining fumes since our house isn't a new build and isn't a tight envelope...we tend to have some fumes escape naturally.

You need to think of how you normally cook and what your goals are with exhausting to determine what size cfm might be adequate and if you will then need MUA.

    Bookmark   March 17, 2014 at 7:12PM
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I have an induction range and I got my hood off the shelf at the big box store. I can't remember its maximum CFM but I know it is 400 or less. It works fine for what we need. I asked our electrician, who installed it (he also does HVAC and knows -- and follows -- the codes) and he said there was no need for us to worry about MUA at all. This is also an older house, though over the years it's been increasingly well sealed (new windows and siding, basement encapsulation, etc). If you will have someone familiar with your local codes installing it, you could ask them in advance if you'll need to arrange for MUA.

    Bookmark   March 17, 2014 at 8:32PM
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