Properly Sizing Kitchen Vent Hood CFM and Makeup Air?
I'm in the midst of a kitchen remodel and have selected and ordered a hood with external blower. Sized per the salesperson's recommendation at 1200cfm (900cfm would have been the next size down). Their advice was you can always run the 1200cfm lower but you can't turn the 900cfm up. But after reading here and googled posts, I see that makeup air may be an issue with either one of these high cfm beasties. I've been unable to find good info on properly sizing the hood blower itself, let alone means of make up air. I live in Austin, TX, rarely does it dip below freezing, and we have our summers in the high 90's low 100's for several months, then several more in the 80's. So Just putting a big hole in the wall with a damper will probably murder me on cooling costs. Sure I can open a window when I run it, but we cook at home a lot and it will get mighty hot that way.
Hood and external blower are Viking (VCWH3648BK)
and External Blower (DEV1200)
They are being installed over a Thermador 36" 5 burner gas cooktop. (SG3365C13) Total BTU output 58,200. Single largest burner 16,000 BTU
I called Viking and am told I need a 10" duct either way (900 or 1200cfm). The rep wasn't able to help me size the unit. She was quoting some very loose guidelines. They had no suggestions about makeup air.
My vent run will be straight up from the hood, 90 degrees to exit exterior wall of house (duct will go up into ceiling and turn to run in between the floors, no attic access at this location).
The existing original exhaust looks to be a 6" diameter. (See photos). I was thinking I might just leave it as is and install a damper which opened in vs. out on the exterior portion to supply a good portion of the make up air directly behind the exhaust hood (It seems like from what I read commercial kitchen units often work similarly with the return air being ducted to the hood location). The existing duct will not be removed (as the new one will just go straight up to the ceiling going up through the 2x4 wall as the current one does).
Photo 1,3 - Cabinets which will be removed, existing vent duct here.
Photo 2 - close up of the existing duct, you can see what I believe is a 6" round pipe.
Last edited by mettius; 09-11-2008 at 08:50 PM.
I wouldn't get all crazy worrying a lot about makeup air. Makeup air is needed for commercial applications because they run their vent hoods whenever they are cooking, which is usually all the time. You will run yours a fraction of the time compared to theirs and when you do run it, it will be for short bursts. Makeup air on restaurants is generally accomplished by having a positive air source dedicated only to the vent hood and only runs when the vent hood does. When the vent hood stops, so does the makeup air. If you want to spend the $$$ to put something in to accomplish this, more power to you. It will not be cheap and IMO a waste of money. We've done a lot of homes with these "beasties" as you call them with no makeup air and have never had a problem. You can add air from your a/c/heater, but keep in mind that when you are not running your "beastie" you will have excessive heat or cool air being dumped into your kitchen.
You can vent range hoods and not bring in make-up air? Are these being inspected? Here anything over 300 CFM must have make-up air into the house somehow. The most commen way that we do it is to bring it into the furnace return air and interlock the blower and a damper (in make up air pipe) with the blower. When the hood is turned on the damper opens and the furnace fan starts.
To the op be carefull. If you have gas appliances (furnace and water heater) that uses a flue out of the roof when you run a huge hood like that you can cause a negitive pressure in your house. This could cause the furnace and water heater to backdraft and draw carbon monoxide into your house.
My opinion is that you need to do something. Call your local codes department and see what they suggest. Around here a permit is needed to do a range hood anyway.
Its a good Life!
I'm in an unincorporated part of the county. No permits or inspections other than for water/sewage are required. (I still want to be safe, however)
Originally Posted by m kilgore
I have two water heaters and two furnaces (all gas). One water heater is located in a closet in the garage (with vents open to the outside in the closet) the other water heater and both furnaces are in the attic.
Is there a test (post or pre install) or formula to determine needs for this sort of thing?
Without make up air, that 1200 CFM exhaust fan, WON"T exhaust 1200 CFM.
You can do a balance test
Run fan and see if it creates negative pressure in house. You want pressures to be slightly positive. You should at 900 CFMs have at least a 10" or bigger fresh air hood interlocked to exhaust hood and if you are in colder climate you should put heater element in fresh air. In my area anything over 300 CFMs you need to interlock to furnace and size according.
Do it right the first time.
For exhaust hoods over a range you should not need any fan bigger than the size range you have in your kitchen, Therefore a basic rule of thumb for range hood exhaust is 100 CFM's per foot of range/Hood width. Most Oven's/Range's in most homes are no wider than 36" Therefore your hood should not exhaust no more than 360 CFM's which will be plenty of exhaust air for that size hood and anything under 36" so you should not have any ducting larger than 6" as that is larger enough to handle 360 CFM's.I would take the Beasty back 1200 CFM exhaust hood and tell them to sell you something in the 360-400 CFM's for exhaust. Was this a inline exhaust fan or just a typical exhaust fan that yoru can buy at most home centers ? Go to Fantech's Website where you can read up on exhaust fans and there application's and look at the many types of fans they offer to the homeowners, you will find alot of usefull information.
Don't forget the other appliances that need make-up also. The bath fans and clothes drier are exhausting air regularly. In green grass climates, there is the issue of damp outside air loading your home with high humidity. I would suggest considering a fresh make-up air wholehouse dehumidifier. The larger units are capable of +400 cfm of make-up air. Typically, the units are setup to provide 75-100 cfm of fresh when the home is occupied. Upon recieving a signal from a large exhaust device the dampers can be set-up to introduce 400 cfm of outside air. During times of high indoor humidity, the dehumidifier removes upto 8 lbs. of moisture per hour. This assists your a/c to maintain <50%RH during wet cool weather. In addition dealing with the extra moisture from the fan use, this unit will maintain <50%RH during wet cool weather with or without a/c operation. Check the Ultra-Aire XT150H thermastor.com. Regards TB
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"
It is an exterior blower from Viking, controls will be variable speed (so I can dial in the speed I want vs. having 3-4 presets), so I can always run it lower.
Originally Posted by DanW13
The unit I had originally (cheapo GE unit) was rated at 200cfm as I recall, it was woefully inadequate, any serious cooking (braising/searing) would flood the kitchen with smoke.
I was looking at fantechs LD Silencer actually and did see the numbers they quoted for exhaust. They seemed pretty low. But then it seems that there is little consensus out there as to what is "right size".
How would such a device be triggered by the vent hood being activated? What about a variable speed device (such as the range hood/blower) I'll be using?
Originally Posted by teddy bear
Trust me 100 CFM's per ft of range space is more than adaquete. You can buy a variety of varible speed controller and powered dampers and have both wired directly in to your furnace blower if you have a ERV/HRV or some type of make up air when you turn the range hood on. This is something I would have your's or a compent HVAC person to wire up for you especially if your going to tie it in with your HVAC equipment. If your looking to vent the bathrooms along with your exhaust hood I would probably recomend keeping the bathrooms separate from the range hood as you do not want to have a constant exhaust in your kitchen but your bathrooms are a different story where you need to have the stale air removed from those areas on a more constant basis therefore only needing roughly 50-75 CFM's being circulated thru out those rooms.
Your 200 CFM fan, probably didn't work well due to lack or make up air.
And this one won't either without it.
I beg to differ about constant exhaust being desirable from the bathrooms or anywhere else in the Austin TX climate. I am a homeowner west of Houston, so am familiar with the hot-humid climate. Use the bath fans when needed but not constantly I say. In a hot-humid climate better to have positive inflow most of the time, to inhibit infiltration of humid outside air. Just the opposite from the rule in cooler climates.
We can exchange reference links if this is controversial, I believe it is well accepted wisdom backed up by Building Science Corporation.
Regards -- Pstu