I am installing an 800 cfm range hood, to be toned down with a speed control. I have a 400 cfm in my rental studio that pulls down bad juju from the gas fireplace. Obviously I am overfanned. What is a proper way to add make up air? With the whole house fan we obviously open a window, but with range fans, its a pia to open a window. I think I would need an 8″ duct with a damper to fix this! Ideas?
Discussion Forum
Discussion Forum
Up Next
Video Shorts
Featured Story

These plywood clips allow you to strengthen the walls around pocket doors to support shelves, cabinets, and other fixtures.
Featured Video
Builder’s Advocate: An Interview With ViewrailHighlights
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Replies
Check with your local BI dept. Around here we have 2 options - a hole from the o/s (not thru the basement or crawl) equal to the duct area, + either a) a supplementary fan switched with the main fan to introduce fresh air, or b) a carbon monoxide detector. Can you imagine a 10" dia hole in your wall, sucking freezing air around your feet?
IMNSHO unless one has 3 vats of french fries and 2 dozen burgers on a broiler, then anything above 150 cfm is overkill. Sure, it's a stupid trend to go that big - and people are being suckered into paying $3K for something that is totally unnecessary. Buy a cheapo, and get a car with the difference! One won't hear oneself talk with the noise, at least, and it'll suck the water out of the toilet at worst. (Who needs a douch? <G>)
ciao for niao
To those who know - this may be obvious. To those who don't - I hope I've helped.
Thanks, I appreciate the input. I must disagree a little however. When we remodeled, we put in new windows, etc and the space is really tight. With 150 cfm at a higher static pressure, nothing would move. The 400 cfm is just about right with all the pots boiling on the stove. If I had a big gas range I would need a little more. One thing I did find was that a 30 inch hood over a 30 inch range is in reality sort of small. Our friends did a larger 36 inch hood over a 30" gas Viking stove and that worked well as there was a bigger catch area. My fan obsession comes from being in Seattle and watching mildew grow on interior walls and having air so thick after cooking that you can't breathe. We have allergies and stale air is a killer, especially air full of cooking odors. At least we have relatively warm winters so make up air, while cold, won't be too bad. "Democracy is when the people know exactly what they want, and get it good and hard." HL Mencken
"With 150 cfm at a higher static pressure, nothing would move. "Then you wheren't moving 150 cfm.And you are in damager of backdrafting any fuel buring furance, water heater, and fireplace.
"the space is really tight. With 150 cfm at a higher static pressure, nothing would move."
That says it - you had no air coming in. In Seattle you can have windows open in the winter, and even with 150 cfm you'd get the cooking and stale air out. If that is a problem instal some kind of intake vent - under the fridge, in an entry closet, etc. And as cold air is less moisture-laden the mold is coming from your cooking.
And if you have a wood fire - watch for the backventing, and instal a carbon monoxide detector promptly
ciao for niao
To those who know - this may be obvious. To those who don't - I hope I've helped.
Edited 8/26/2005 11:03 pm ET by piko
That is correct. We have CO detector and without a window open, you backdraft. This is from the pilot on our gas fireplace. The furnace is in a tight room with makeup air(2 eight inchers) by code. I think I will indeed put a makeup air kit in.
One way is to pull the makeup air through an ERV. This approach probably won't give you 100% of your needs, especially at 800 CFM.
Another is a duct to the outside. Keep in mind that the exhaust has a fan assist, and the intake probably won't. Don't forget about the free area of the grille/louver. This means that your inlet must support 800 CFM at a lower static pressure.
An electronically interlocked 100% shutoff damper on the inlet will keep the drafts away when the fan is not in use.
An electric reheat coil will temper the air when it is cold outside.
If the outdoor intake is located in the mechanical area, and if that area isn't sealed off from the rest of the home, the air will migrate through the structure. At least it is less likely that the flues will backdraft when the fan is on.
Your household HVAC system is not sized to deal with this much additional load. This becomes important if the fan is used more than intermittently.
If you can buy a make-up air kit that is designed to work with the hood, then do it. Then the fresh air will be at the point of use, minimizing effects on the rest of the space.
It costs money to do it right. Haven't seen a correct install yet.
All good points. I am rethinking this big guy. Even with a speed control it may be way too big.
These are made for use on forced air furnace returns, but might help in your situation:
http://www.skuttle.com/216.html
They are, essentially, barometric dampers
You might try a barometric damper and a downward turned 90 degree L
Hey! Thats cool! Next question then. How much fresh air is needed for a tight furnace room? I have an older gas furnace(circa 1971) and a 40 gallon gas water heater. There are 2 seperate 6" intake lines from the outside to feed these. I assume that the volume of air they supply must meet or exceed the volumes up the flue if both furnace and tank were burning. In other words, I can't cheat and use one of the 6" inchers for the make up air kit.
Standard SMACNA rule is one square inch of free area for every 1,000 btu of input. Your code differ. Note "Free Area"... not the same as the hole size.