Hi, Venexiano,

The majority of homes in the U.S. operate at negative pressures due to improperly sized return air systems. This results in higher levels of air infiltration--leakage around windows, doors, electrical receptacles, etc. than the natural air infiltration rate.

You can duct make up air to the return side of your air handler. We call it the "poor man's make up air" approach. While it will introduce some quantity of make up air, the flow rate is not controlled. The amount of make up air entering your home when the HVAC system is not operating and the hood is will be different in the two alternative arrangements you described.

You will likely see an increase in your electric bill, but that's the trade off for better indoor air quality.

Due to location (Florida) you also add humidity drawing from outdoors so there's much more to consider.

You can duct make up air to the return side of your air handler. We call it the "poor man's make up air" approach.

If that's the "poor man's" approach, opening a window must be the homeless & destitute approach?

Things like: What? -- were you born in a barn, why did you open this window with the AC on?

Need more than what a window will provide? "shut the front door!"

OP- you may not understand this response --- most of it is ribbing our resident Builder Grade pro here on Houzz. But there's info in there for you to consider in an ironic sort of way, as your problem can be solved by simply opening a window when you cook. (Obviously, permits and codes might challenge you if you live in the city limits or something... as they may require make up air outside of just using common sense. Cooking in a home is a function that takes a small amount of time unless you are cooking commercially and if you are doing that you're probably violating some code, or HOA restriction anyway.)

Thank you everyone

Charles I am still confused, why do you call it the poor man approach? Are you saying that it is worse than having a separate makeup air system that releases air at a separate location (i.e. not in the AC duct)? Why is the latter approach better? And what does it need to be better? A filter and? That's it? I feel like a heater and/or dehumidifier on the make up duct would be overkill for North Florida. I am trying to make sure that what you are saying is that the second approach (separated from AC) is better than the first (connected to AC). What I am worried is: if the HVAC creates negative pressure all the time, then the separated make up system would still continuously activate and let unfiltered humid air in. So wouldn't the first approach (connected to AC) be better? Final question: is there a cheap tool to measure air pressure in my house with AC on and with my 400 CFM underdraft on, to see which values I get?

Hi, Venexiano,

A provision to supply make up air is required by the code when high-capacity range/cooktop vent hoods are installed. The IRC requires make up air when the vent hood capacity exceeds 400 cfm, but depending on how tight a home is, you may need to provide make up air even at lower vent hood exhaust flow rates.

Installing a duct directly into the return of an HVAC system is a method that many production builders use because it's the simplest and cheapest method to meet the code requirements for make up air. It's not the best make up air system, but it is the budget option and it's better than no system at all.

The better way to meet the make up requirement, in my opinion, is with a properly engineered make up air system--independent of your home's heating and air conditioning system. A couple of companies manufacture pre-engineered make up air systems. Here's a link to one such manufacturer which explains the need for make up air and how to address it:

https://www.fantech.net/our-solutions/kitchen/makeup-air-systems/

In my experience, introducing make up air in proximity to the range/cooktop vent hood (but not so close as to interfere with normal air flow) eliminates the need for conditioning the make up air.

Hope this helps.

Awesome. Connecting with the AC system is not something I like either. A few questions:

1) I am buying a Miele cooktop and Hood (this one), because I have an open space and we often forget to turn on the hood until it gets all foggy. Since they are wirelessly connected, hood will turn on when cooktop turns on. This is to say I don't wanna change the brand. Can I use the Fantech make-up-air system with Miele?

2) Which damper should I pick? My Hvac guys said that he can go with a gravity (aka atmospheric) damper, and he quoted me the system for $400. Would that be a good option? I am afraid that damper would open also when mild negative pressures are created when the AC/heat system is on, and we are back to square one, I might as well have added the make up air duct to the AC duct. The other option would be to have an automatic damper, that automatically opens when the hood starts and close when the hood stops. That would require some wired connection with the Miele hood. Is that even possible? Should I just go for gravity damper then?

3) See below a picture of my old kitchen, and the pic with the model of the new kitchen (it does not have the hood yet but it will go above the cooktop). Where would you put the opening for the make up air system. I also thought that "introducing make-up air in proximity to the range/cooktop vent hood (but not so close as to interfere with normal air flow) eliminates the need for conditioning the make up air", as you said. Which do you think is the best spot in my design? I just don't wanna have a draft of cold (40-45 F) air hitting my head/back when cooking, or hitting my lady or guests who usually sit on the stool in front of the cooktop so we chat and have a drink while I cook. Should I put it in front of the left most pantry? Or maybe just on the left of the island?

Hi, Venexiano,

The Fantech make up air system is a package system with everything you need including a motor-operated damper. It varies the speed of the make up air fan based on the speed of the exhaust hood blower (it senses the current to the exhaust hood blower) so there is wiring between the two systems.

You could ask the folks for Fantech for a recommendation of where to introduce the make up air. I think the space above the refrigerator and cabinets would allow for some mixing with room air and shouldn't give the cold shoulder, so to speak, in your climate zone.

Thank you. I should have said that the only thing that is not correct in that model is the height of the ceiling. Cabinets run all the way to the ceiling so no space there. Anyways, I think left of Island should be fine, I'll ask fantech. So are you saying that going for the $400 cheap option with a make up system with gravity vent that goes next to the island is not a good idea? Why? Is a automatic damper that connect to hood as the fantech really necessary?

Hi, Venexiano,

The Fantech system is a complete, pre-engineered make up air system which provides a controlled flow rate of make up air to match the range/cooktop vent hood exhaust air flow rate. That's as good as it gets. The system includes a motorized damper which provides a positive shutoff when the system is not in use. By contrast, a simple duct to the exterior with a damper doesn't provide a regulated flow rate; you get what you get.

A system to supply make up air to mitigate the effects of undersized return air ductwork and/or stack effect is a different application. I believe there are systems which can do both, but they will be more sophisticated and more expensive. My preference would be to design them as separate systems, each optimized for the specific application.

Question from another Range Hood installation regarding Make Up Air - I live in South Florida and the home is on a concrete slab and the kitchen is located in the middle of the home (no exterior walls available). I am planning on installing the external make up air vent (corrosion/size per code) in the roof eave. The code is silent on this being acceptable. Any thoughts? Beyond that my intent is to run the make up air in rigid duct to the interior supply vent beneath the range in the wall - but I did read somewhere about a distance of 10 feet between the two.

Thanks in advance for your help.

Hi Venexiano, I was wondering if you could provide an update on what you ended up choosing and how it worked out for you? Thanks so much.

Hi, I get the AC guy coming on Tuesday. He will do a make up air into the AC return. $450.

Hi, how has it been glikg through the AC return so far? Any issues? I’m kn Florida also and dealing with similar situation. Thanks tor sharing.

Hi, how has it been going through the AC return so far? Any issues? I’m in Florida also and dealing with similar situation. Thanks tor sharing.

I'm facing a similar situation! Island cooktop across from ovens. Installing a 900 cfm hood above the cooktop and want to invest in the FanTech MUAS.

Seems to me that the ideal setup would be to have dedicated 10" ducts for the FanTech, with a large register in the ceiling right above the ovens. Makeup air would flow from there to the hood, and maybe it's wishful thinking but that might even help with effluents from the lower oven when that broiler door opens.

So do I read these comments correctly -- that this system design would perform better than integrating with the HVAC system?

I think my only concern is with humidity (mold, condensation) and air temperature -- but it seems like with an active system, these bursts of humid or cold air would be restricted to the short travel area between the MUAS duct and the hood, and wouldn't really affect the whole house. Am I wrong?

The extent to which the introduction of outdoor (make-up) air affects indoor conditions depends on the relative flowrates of make-up air vs. exhaust air and the degree of mixing. The degree of mixing depends on where the make-up air is introduced. In the case where outdoor (make-up) air is introduced close to the exhaust point, mixing, and the attendant effect on indoor conditions is minimized. In the case where outdoor (make-up) air is introduced into the return side of the air handler, mixing, and the effect on indoor air conditions, would be more significant.

Thank you.

So I guess I've read conflicting advice:

1) Makeup air should not be too close to the hood, and should be diffused into the room.

2) Makeup air should be close to the exhaust point to minimize the effect on indoor air.

Is #1 only relevant if I'm ducting the makeup air through the HVAC ducts?

I see a video like this where the intake air is directly in front of the hood aperature, and what I see is the hood pulling up all of the cooktop effluent AND all of the intake air, with none of the intake air mixing into the room ... This looks pretty ideal!

(I'm not going to build anything like that, but my hood will be close to the center of the house, with an 8' ceiling, directly below a large, open attic. I could easily place a dedicated MUAS above the hood, with a Wolf remote blower and a FanTech MUAS supply hood mounted at the proper distance to either side of the roof. Is this as ideal as I think it is?)

I'm already going overboard with ventilation so if I can avoid complicated engineering, duct heaters, and dedicated dehumidifiers, that would be great.

Well, you've got to draw the right balance between the two: make-up air should be close to the exhaust hood, but not so close as to cause turbulence.