Maximum CFM for range hood without makeup air
ranson
| Posted in Mechanicals on
In a tightly sealed house, what’s the maximum CFM that you could consider in a range hood without supplying makeup air. I know that the IRC allows up to 400cfm without makeup air, but is that actually advisable? Any personal experiences about 200-400cfm fans in tightly sealed houses?
I’m considering putting in a fan that goes up to 350cfm, and opening a window if I need to run it at full speed.
-John
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Replies
John,
Your approach will work, as long as you make sure that you don't have any atmospherically vented combustion appliances (like a water heater, furnace, or wood stove) in the house. All of your combustion appliances should be sealed combustion appliances.
For more information, see Makeup Air for Range Hoods.
-- Martin Holladay
I searched your last name on GBA and looks like you're building in zone 5, opening a window in the middle of winter doesn't sound appealing or a good solution, even for short periods of time.
Turning on any exterior vented range hood without makeup air in a tight house (< 1 ACH50) will depressurize the house, but I suppose that may not be an issue for short timeframes.
What kind of range are you installing, electric or gas?
I'd get the lowest power hood you can or even consider a recirculating hood if you're electric and not doing extreme cooking.
I've killed recirculating microwave hoods with condensation from boiling pots on an electric stove. I've been driven from my kitchen by tear gas from roasting pounds of garlic in a low oven. I'm not skimping on hood flow.
It strikes me that opening a window, even in winter, is probably better under most circumstances than having a makeup air damper. The makeup air dampers are essentially uninsulated extra wall penetrations that loses energy all the time. I'm going to have windows anyway, and when closed they seal, insulate, and even have solar gain. I expect I would only need to open the windows when I need the fan running at full blast.
I will have no combustion appliances. I will have an electric range, but I also occasionally use a 15kbtu countertop butane stove when I need a lot of heat. Brian is correct, we're in Zone 5, Rochester.
--John
John,
Why not use a window? In a well insulated and air-sealed house there shouldn't be much of a pressure differential between the exterior and inside until you turn on the hood. The window will bring in as much air as necessary to equalize, just as a make-up air vent would.
John, under the personal experiences category:
We have a Fantech SGHL30 range hood insert with a Fantech VF6XL remote fan on the exterior side of the kitchen wall.
Fantech says the remote fan is capable of 381 cfm with no obstructions, and 330 cfm with a typical "system with 20' of duct, two 90 ĚŠ elbows, a backdraft damper, ... and filters." We have one elbow with a few inches of ductwork either side and a backdraft damper, so I suspect we are closer to the 381 cfm when the fan runs full tilt.
Our last blower door test showed us at .9 ACH50. Someday I want to retest that, as we turned the HRV off, but didn't realize we could seal it up during the test. Anyhow the house is pretty tight.
We have a passive 4" gravity make up air damper in our mechanical room on the ground floor. Diagonally across the house and a floor below, it's at least 50' away from the range hood.
- The damper opens when we turn on the range hood fan.
- It also opens when our HRV goes into defrost and turns its supply fan off.
- And it will open if I close the door to the mechanical room and run the clothes dryer in there.
- It doesn't open when I operate our masonry heater.*
- It doesn't open when I run the upstairs clothes dryer.
- We have no bath fans. Exhaust grills in each bathroom are ducted into the exhaust/stale air input of the HRV. There is a control damper and timer switch for each bathroom.
Yes, I usually try to open a window if we are cooking and have the range hood on full. Sometimes we forget though, and then the mechanical room make up air damper opens; it's sort of a safety relief valve.
The only significant back draft events we have ever had occurred if it was a really cold day and I opened the door to the masonry heater to poke at the embers; it was unpleasant if the HRV went into defrost then. I have since learned how to build the fire and manage the MH intake air dampers so poking is unnecessary. (It is fun to stir the embers and put air back to them, as it creates kind of a blast furnace effect for a few minutes. I guess after a few years I have grown out of that.)
BTW, I also have a manometer on the ductwork around our HRV. You can actually see the pressure balance shift when you turn on the range hood.
*We put a "Cape" backflow prevention damper in the masonry heater's main combustion air input duct work. (We use air from the mechanical room.) At a point that is flame and heat safe distance away from the firebox, it's one more "fuse" in case somehow we simultaneously turned on every appliance with a fan.
Our house is air tight at 0.37 ACH50 with a 300 cfm range hood. We don't have make up air so we must open a window. Let's just say it's not for everyone... You may be able to make it work and maybe have your spouse learn not to use the range hood when the masonry heater is going. Even without a combustion appliance you'll notice that without a path for the air, there is no exhaust from your range hood unless you open a window. Same for the dryer. I understand why Passive Houses are equipped with a recirculating range hood.