Dave B plans on installing a range hood in the kitchen of his new house so any evidence of the blackened redfish or pan-seared scallops he might want to cook can be quickly whisked away.
The house will be tightly air sealed, and Dave wonders which of the two Broan kitchen fans he is considering—moving 250 and 390 cubic feet of air per minute respectively—would be best, and whether he should provide a source of makeup air for them.
“I seem to get conflicting information regarding if the unit needs makeup air,” Dave writes in a Q&A post. He wonders whether a clothes dryer also will need some kind of makeup air.
That’s the topic for this Q&A Spotlight.
Even some manufacturers don’t get it
Dave isn’t the only one who’s confused by what he’s heard. Martin Holladay points him toward two articles on this topic that GBA has published in the past (see the “Related Articles” sidebar below). One of them describes the frustrations of a homeowner whose 1,200-cfm range hood was so powerful that it pulled ashes out of the fireplace. She wanted to know what could be done about it.
In poking around for answers, Holladay learned that the manufacturer of the GE Monogram range hood offered no guidance on the topic of makeup air, and in telephone calls to the company, various representatives basically had nothing to offer.
So the problem is not new and not unique.
That said, Tom Wheeler notes that range hoods that exhaust less than 400 cfm do not require makeup air, and Dave’s own research suggests that even in a tight house—one with an air leakage rate of 1.5 ach50 or less—no makeup air should be needed for a vent hood with a capacity of 250 cfm.
“It’s hard to generalize,” Holladay replies. “If a house is extremely airtight—under 0.6 ach50—it’s certainly possible for a 250 cfm exhaust fan to be starved of makeup air. In all cases, this problem can be alleviated by opening a window when the fan is in use.”
Most clothes dryers should work fine even in a tight house, he adds, although it’s possible that makeup air might be a good idea in a house that meets the Passivhaus limit of 0.6 ach50.
Accounting for heat output and duct design
John Clark suggests that sizing a range hood depends on several factors, including the heat output of the range, and how the ductwork is designed.
A good rule of thumb, Clark says, is to allow 100 cfm for every 10,000 Btu of output. If, for example, the range is a four-burner model with a total output of 50,000 Btu when all of them are full-on, the fan should be able to move about 500 cfm. For electric ranges, the fan should be rated at 100 cfm for every 10 inches of width.
Other factors include the number of bends in the exhaust duct and the length of the ductwork. When the duct size is smaller, it will take more air movement to compensate; larger duct sizes mean smaller fans.
“Ideally,” he says, “the hood should be 6 inches wider (3-inch overhang on each side) than the width of the range.”
While production builders are finally starting to add range hoods, they often do it on the cheap, Clark stays, resulting in less-than-stellar outcomes.
“I know I sound annoyed,” he writes, “because it my opinion it is annoying. A typical homeowner doesn’t know to think about these things so they’re none the wiser. The homebuyer gets his/her range hood but most likely not as efficient as it should be.”
A 250 cfm blower will not cause problems
GBA reader Akos takes a look at the specs of the 250 cfm fan Dave is considering and concludes that it wouldn’t cause any problems.
“If you don’t add any combustion appliances, that is not something I would worry about,” he says. “Cracking a window open while cooking would boost the flow slightly (10%) if needed.”
What if Dave bumps up the blower to the next size? With an air leakage rate of 1.5 ach50, the house would become more depressurized, Akos says, but because Dave won’t have any combustion appliances there’s probably nothing to worry about.
He adds that a range hood with an axial blower, the standard, tend to be louder at higher air flows. A radial blower is significantly quieter, and sees a smaller drop in air flow as the inlet filter becomes clogged.
W Ramsay points out that real world performance can be different than the calculations Akos has done. Changes in pressure on various sides of the house can significantly alter the performance of a range hood— fans that are vented on the windward size of the house may hardly work at all.
And, he adds, performance also will hinge on the style of cooking that Dave and his wife are likely to do. Pay frying or stir frying need much better performance than boiling water for noodles, he says as he cites an online discussion that might be useful.
Does the outlet size matter?
Ramsay thinks the Broan model that Dave is considering has a small aperture and “zero” containment area, meaning that it won’t work very well even if it had a 1,200 cfm blower. He suggests looking at models made by Zephyr and Modernaire.
“If you want it to work very well at all, then make sure it’s at least 6 inches wider than your range and 27 inches deep,” he says.
Akos doesn’t think the intake dimensions of the blower make much difference unless the stove is very wide. A larger inlet with a larger grease filter, however, will be less restrictive as grease accumulates.
“Not necessarily,” Ramsay replies. “Hot effluent from cooking rises up and expands out a bit as it does so. It takes an enormous amount of airflow, like multiple thousands of cfms, to overcome that and pull the column into a smaller aperture. This is why the opening of hoods (aperture) should extend out at least 3-6 inches beyond the range…Anything that does not naturally rise up into the aperture and containment area will spread out throughout the house.”
Our expert’s opinion
GBA Technical Director Peter Yost added these thoughts:
I sure thought that the exhaust system we have in our kitchen does a decent job, but this exchange got me thinking. Here is what our home’s kitchen exhaust system looks like: It’s a 3 1/4-inch by 10-inch rectangular vent leading to a to 6-inch round duct that’s powered by a 200 cfm fan. I’m pretty sure it’s a Tamarack Technologies fan that’s quite efficient, but I can’t find the specs on it and it is not easy to access at this point.
But my good friend and former BuildingGreen colleague, Alex Wilson, has a real kitchen exhaust system: 3-speeds, a hood extending down to within 3 feet of the cooktop surface, 12-inch by 16-inch rectangular duct powered by a scroll fan with a 6-inch round outlet duct.
I decided that some Wingnut testing was in order. Here is the testing protocol:
Step 1: Determine home airtightness in ACH50.
Step 2: Measure negative pressure developed by kitchen exhaust fan.
Step 3: Measure air velocity at face of exhaust system (I used my 4-in-1 Omega Environmental Meter’s anemometer in feet per minute – FPM).
Step 4: Set up theatrical fogger 5 feet away from center of kitchen exhaust, in plane with the cooktop.
Step 5: Run theatrical fogger for 5 seconds and video-record until fog is no longer visible.
Results for Wingnut Testing 208 Yost kitchen exhaust
Step 1: 4 ACH50.
STEP 2: -0.7 Pa (all four interior doors closed); -0.3 Pa (interior doors open).
Step 3: Averaged roughly 900 fpm.
NOTE: If we assume uniform air velocity across the grille surface (and I realize that is a big, bold assumption), we can multiply the air velocity by the area of the vent (900 fpm x .27 ft x .83 ft = 202.5 cfm. Just about right.).
This video shows the fan at work with all interior doors to kitchen closed. You can see how the fog rolls across the cooktop and spreads around the kitchen taking about two minutes or more to completely clear visible fog.
Here’s a another view, this one with all interior doors to kitchen open. It takes a bit longer for the fog to clear, about 2 1/2 minutes.
Admittedly, the fogger is throwing the fog across the cooktop and this is quite different than vapor or smoke coming directly up and off the cooktop, but as you will see with the Leonard Farm testing below, it does reveal a bit more about overall kitchen airflow.
I chose to locate the fogger 5 feet away from the center of the cooktop based on trial runs of how the fog moved from the fogger to the cooktop.
Results for Wingnut Testing Leonard Farm kitchen exhaust
Step 1: 1.6 ACH50.
Step 2 results for each hood fan speed:
Low: -4.6 Pa
Medium: -9.1 Pa
High: -11.9 Pa
Alex says they never crack a window or door when running their kitchen exhaust fan, at any speed. The only combustion appliance in the house is a rarely-used wood stove, which has its own combustion air supply duct.
Step 3 results at three speeds:
Low: 330 fpm (439 cfm)
Med: 533 fpm (709 cfm)
High: 574 fpm (763 cfm)
This video shows the fan operating at low speed. An awful lot of fog makes its way past the cooktop and up to the ceiling, partially trapped by the ceiling beams.
With the fan running at medium, here’s what it looks like. The capture is a bit better but quite a bit of fog escapes the hood’s capture zone.
Now, the fan running on high. Even at high speed, we were surprised by how much fog escaped the hood capture zone. And of course, this Wingnut test throws an awful lot of fog at speed across the cooktop rather than up and off of the cooktop.
Alex Wilson and I decided to crack the back door open to see how that affected exhaust flow.
With the back door cracked open, here’s the result. We were surprised again that with the back door just just 6 feet from the cooktop there wasn’t much of an impact.
We then noticed that the grilles were pretty dirty and decided to take out the grilles to see how that affected exhaust flow. This is the result, with the fan running at high speed and, once again, the back door cracked open.
Another surprise: Quite a bit more fog escapes the hood capture zone and moves down the hallway off the kitchen. For all of these tests—my house and Leonard Farm—there was essentially no wind.
The best results for Leonard Farm proved to be back door closed, high speed, with grilles removed. Here’s what it looked like.
Air flow is complicated. If smoke or vapor makes its way past any range hood extending down to the cooktop surface, it will take quite a bit of time for that escaped smoke or vapor to clear from the kitchen. Keep your grilles clean. If you are cracking a window or door for pressure relief, it’s probably best to do it some distance from the kitchen exhaust so that wind does not affect the communication between the cooktop and the exhaust system.
Do your own wingnut testing to add to our understanding of how kitchen exhaust systems perform.
-Scott Gibson is a contributing writer at Lakesideca Advisor and Fine Homebuilding magazine. All photos by Peter Yost.