Hi Brulis,

The need for a MUA. Unit is based on the cfm of the hood and the tightness of the house. Your range plays a small role, mostly based on the range’s size and use case determining your hood. Even if you have a 1200 cfm hood, I don’t think you need a MUA unit, especially if your house is 30 years old.

if you’re worried about the risks, you can look at getting a leakage test done. When we built our house it was designed to be energy efficient with little leakage. Even with that we got a 1200 cfm hood and we ended up not needing the MUA.

Hope this helps,

We have a 48" range. We don't have MUA and we only have 600cfm max--and most of the time dialed to low. Our ducting is extremely short though, about 3 ft. Our house is not that old but not "tight" either as air enters through the gas fireplace vent when operating our kitchen fan on max. We just know not to have the fireplace on when we cook. The 48" is great. Lots of room for two people to cook and not get in each other's way. Our GE range ovens are fully capable with nice convection and powerful broilers. Love it.

This is a wall that can provide low pressure loss make-up air at 1200 CFM. Any reasonable house will have a significant pressure loss. Besides the potential safety hazard of back-drafting combustion appliances, the pressure loss causes a severe reduction in actual flow rate.

[Comment in image should read: A given fan at a particular power setting can deliver one flow at one pressure loss in a given system.]

This site has a vast amount of commentary on make-up air requirements and potential solutions. Searching and reading is your friend.

What others do, illegally, has no bearing on what you should do. You need a 54” hood and make up air both if you are going to go with a 48” range. Let others slowly poison themselves. You have 12 people in your family to watch out for their health and welfare.

I was talking with my husband and trying to calculate the other things we have in our house to factor in. Two furnaces (one in garage, one in attic), a hot water heater in the garage, and a gas insert fireplace that is direct vented on the opposite wall from the kitchen (About 35 feet directly across the room). I’ve been reading about these products like Broan. Are they sufficient? I see they are mounted on existing ductwork, but what I don’t understand is if they are separately vented to the outside, also. If not, this seems like the simple choice, as the ductwork running through the attic is easily accessed. Also, am I wrong in assuming that if we didn’t do this, the air needed would be pulled through the fireplace vent and be potentially harmful? Or would that just be cleanish air from the outside?

The code requirements derive from the need to avoid back-drafting carbon monoxide into the household. If all your combustion appliances derive their air from sources outside of your living space, then they don't count toward asphyxiation risk, but the code enforcement process may discount this because configurations can change in the future. In other words, if your cooktop ventilation blower is rated (in free air) over some value -- typically 400 CFM -- then deliberate MUA supplementing any house leakage is a requirement in many municipalities independent of the actual risk.

Unfortunately, fireplaces operate on very low differential pressure, and can be easily back-drafted back down the chimney.

MUA by definition takes outside air and somehow transports it to the kitchen. One might bring it through an attic into a room or to the kitchen, or into an entryway, or into a basement. In some cases it can be introduced into furnace ducting, but this should be approved by a HVAC engineer or furnace manufacturer. In many states the MUA will have to have provision for heating.

For general perspective, the MUA will need a large duct to flow sufficient air to keep the pressure differential where it will not back-draft combustion appliances. A smaller duct may do if there is also a blower in it to make up for pressure loss.

Here is one description of a product performing the MUA system task. It may clarify a few points. https://shop.fantech.net/en-US/muas--750--makeup--air--system/p111416