Choosing the Right Range Hood Makeup Air System – GWC Mag

Kitchen ventilation is critical to having good indoor air quality in your home. Cooking, as the research shows, produces a lot of indoor air pollutants. The best way to deal with that is to have an exhaust ventilation system that removes those pollutants at the source. But when you exhaust air from an airtight house, you can induce some serious negative pressure. So what’s the solution? Do you need a range hood makeup air system?

As part of our residential HVAC design services at Energy Vanguard, we design makeup air systems for some of our clients. To do that properly, we’ve had to dig in on this topic to find the most appropriate solutions. Here’s a brief summary of what you need to know about range hood makeup air systems.

One in, one out

The first thing to know is that we’re talking about air. Yeah, Alex Meaney of Mean HVAC once said, “Air is squishy and hard to predict,” but there’s one thing about air that’s easy to predict. If you turn on an exhaust fan inside the house, it pulls air from the house and sends it outdoors. But that doesn’t mean the house has less air in it.

When a volume of air, a cubic foot for example, goes from inside to outside, a cubic foot will come in through the building enclosure to make up for that loss. If this happens in one minute, we call it a cfm, which stands for a cubic foot per minute. So in the building science world, we have this saying:

A cfm in equals a cfm out.

And that brings us to range hood capacity. How many cubic feet per minute does it move?

What size is your range hood?

First, let’s start by eliminating recirculating range hoods from this discussion. In general, it’s best to have a range hood that exhausts to the outdoors. Yes, Passive House designers like recirculating range hoods because they eliminate a penetration through the air barrier as well as eliminating the potential for negative pressure and the need for makeup air. But let’s save that discussion for another time.

If you’re putting in a standard range or cooktop, whether electric or gas, you don’t need a hood with a huge exhaust rate. Brett Singer, an indoor air quality researcher at Lawrence Berkeley National Lab, says 250 cfm should be enough. Of course, that depends on hood design, location, and capture efficiency. But just put the range against the wall and have a hood that covers the whole thing. With that setup, a 250 cfm range hood should suffice.

Going with a commercial range or cooktop? That’s usually where we see people installing the super-duper range hoods with capacities of 1000 cfm and higher. If you’re dead set on having a range hood with a huge capacity, you may need makeup air.

Do you really need makeup air?

In my article on airtight homes and makeup air, I mentioned that we get a lot of requests for range hood makeup air systems in our residential HVAC design projects. Then I wrote, “The first thing we try to do is convince them to put in a smaller hood so they don’t need makeup air.”

But when do you really need a makeup air system? The HVAC Applications Handbook from ASHRAE (2019 edition) says this:

The exhaust rate of residential hoods is generally low enough and natural infiltration sufficient to avoid the need for replacement air systems.

That may be true for a lot of houses but not for many of the ones with big range hoods and an airtightness level that meets current building codes. Here’s what the 2021 International Residential Code (IRC) says:

Where one or more gas, liquid, or solid fuel–burning appliance that is neither direct-vent nor uses a mechanical draft-venting system is located within a dwelling unit’s air barrier, each exhaust system capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be mechanically or passively provided with makeup air at a rate approximately equal to the exhaust air rate.

As you can see, it’s not simply a matter of range hood capacity. You also have to have combustion equipment that’s not isolated from the indoor air. Put a 2000 cfm range hood in an all-electric home, and you won’t be required to install a range hood makeup air system. That doesn’t mean you might not have problems; you just don’t have to put in makeup air.

Passive vs. active systems

Let’s say you decide you can’t give up that 1200 cfm range hood, and you plan for a makeup air system. You have to decide whether to go with a passive system or an active one.

Passive makeup air system. With this type of system, you have a duct connecting the kitchen to the outdoors. There’s nothing to move air from outdoors into the kitchen except the negative pressure created by the range hood. You don’t want that duct to be open all the time, though, so it needs a damper that opens when the range hood comes on and closes when it goes off.

Deciding where and how to let that passive makeup air into the kitchen is where things get really tricky. The best solution we’ve seen is this system designed by Ross Trethewey, PE from TE2 Engineering for NS Builders. This isn’t a cheap or easy way to do makeup air, but it does seem to work.

Active makeup air system. Rather than using negative pressure created by the range hood to move air, an active makeup air system has its own blower to move the air. This is our go-to method for clients who hire us to do makeup air. We use the Fantech makeup system because it includes a complete package and works well.

The diagram above shows the various components in the Fantech system. It includes the wall cap, filter, damper, blower, silencer, and an optional heater.

Where to locate makeup air intake vent

Once you’ve decided on what type of system, you need to figure out where to introduce the makeup air. Of course, the obvious location is the kitchen. Somewhere near the range hood, right? Here’s what the 2021 IRC says:

Kitchen exhaust makeup air shall be discharged into the same room in which the exhaust system is located or into rooms or duct systems that communicate through one or more permanent openings with the room in which such exhaust system is located.

So it doesn’t have to go into the kitchen. You could put it in the great room, living room, or some other room that’s open to the kitchen. Or dump it into the heating and cooling system so that it gets conditioned on its way in. One advantage of putting at least some of the makeup air outside the kitchen is that the kitchen then will be under negative pressure. That keeps kitchen pollutants from spreading out into the rest of the house.

Joe Lstiburek wrote a great article on this topic titled First Deal with the Manure and Then Don’t Suck. In it he discussed a method that we try to use in our makeup air system designs. Rather than putting all the makeup air into the kitchen, he puts about 60 percent of it into the kitchen. Then he puts the other 40 percent into a heating and cooling system to be sent to rooms connected to the kitchen.

Other things to consider

This isn’t an All About Makeup Air article. I’m giving you the basics here, but there’s a lot more that goes into a good makeup air system design. One of the big considerations, for example, is whether or not to condition the makeup air. Having 5 °F (-15 °C) air blowing across your ankles isn’t so comfortable. Nor is having hot, humid air fill your kitchen.

Similarly, it’s a really good idea to filter the makeup air to keep outdoor air pollutants from degrading your indoor air quality. And of course, you need to plan for the best location of the makeup air intake. You don’t want it right next to the range hood exhaust, for example. Or the garage, dryer exhaust, or too near the ground.

Also, how will the makeup air system be activated? There are products that allow you to do it with a direct electrical connection, pressure sensors, or heat sensors. You’ve got to get this right because the makeup air needs to run simultaneously with the range hood.

So there you go. Once you’ve digested all this, you should know enough to be dangerous. Or at least what to look for in a makeup air system. But the big thing I hope you take away from this is that you should just stick with a smaller range hood.

_______________________________________________________________________

Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to the Energy Vanguard newsletter and follow him on LinkedIn.