Hi,

We are in the midst of an extensive remodel to a 1950's mid century home in Northern California.  We have an existing 48 inch Wolf gas range stove with 6 burners.  We are in need of a custom wall mounted hood with an internal blower and plan to have a direct vent to the outside.  The hood we are planning to purchase is a ModernAire.  I am trying to figure out whether a 600CFM internal blower will work just fine for our stove or whether I need to get the 1200 CFM.  I know that manufacturer's typically recommend 1200 CFM for a 48 inch stove.  However, I've read conflicting information from industry professionals that say that 600 CFM will suffice for a 48 inch stove unless you are using your stove in a commercial facility where you use all 6 burners at once, which we never do.  At most, we use two of the burners at the same time.

Any insight is most appreciated.    

I'm using a 48" Blue Star which is similar, but I think the burners might have slightly higher output. I think we calculated we needed 1200 cfm, but we managed to get away with a 900 cfm 54" Vent-A-Hood because it does something special that allows the slight downgrade. Don't ask me what it is. It has three motors and you can step up the output by using a combination of one or all three.

One of the functions of a hood is to contain a rangetop fire and that helps account for the sizing of the hood as well as the capacity to move volume. It's supposed to help suck the fire up where it can't do damage.

Whoa - The last thing you want to do is suck a fire up into a grease ladened duct. That is unless you have used an "A" vent chimney for you duct work. Almost for sure you would start the wood next to the duct on fire otherwise.

Always cut off the oxygen from a fire.

The range hood has to be of non combustable material and offer thermal protection to adjacent combustable materials.

Whoa - The last thing you want to do is suck a fire up into a grease ladened duct.

They're also supposed to be cleaned regularly. Or they might become a fire hazard.

Before you turn on that range hood, either 600 cfm or 1200 cfm, you have some homework to do. That large a fan has a serious chance of depressurizing your house enough to cause a problem with your other combustion appliances. This would be a good time to hire an energy auditor and do a bit of testing, if you have not already. Once the hood is installed, they can test to see how negative the air pressure is where the heating systems is located ( assuming it is gas or oil fired) They can also check to see if they are drafting correctly and working safely. Have them test the location of the fireplace also (if you have one) for depressurization.
In most areas of the country, makeup air is a code requirement if you have more than a 400 cfm range fan.
While you are at it have them test the range oven for Carbon Monoxide output. It is scary how much CO some gas ovens put out.
More than likely you will need a passive or even powered makeup air supply that is set up to run whenever the range hood is on. This "Makes up” for the cfm of air that is being removed from the house so the pressure stays neutral. In some cases the makup can go to the return side of your forced air distribution system. If you don’t have forced air, it has to be ducted directly to a wall vent somewhere.
One other thing to think about, the hood should have a capture area that extends past the front burners an inch+ for good function. A lot of the designer range fans look cool, but don’t actually suck very efficiently.
Cheers,
Eric

A lot of the designer range fans look cool, but don’t actually suck very efficientl

Well, 1200 cfm is 1200 cfm. I put a skylight over the range. It can open when the range hood is in use, supplying make up air. At least it doesn't suck all through the house.

Posted By eric anderson on 27 Feb 2014 03:25 PM
Before you turn on that range hood, either 600 cfm or 1200 cfm, you have some homework to do. That large a fan has a serious chance of depressurizing your house enough to cause a problem with your other combustion appliances. This would be a good time to hire an energy auditor and do a bit of testing, if you have not already. Once the hood is installed, they can test to see how negative the air pressure is where the heating systems is located ( assuming it is gas or oil fired) They can also check to see if they are drafting correctly and working safely. Have them test the location of the fireplace also (if you have one) for depressurization.
In most areas of the country, makeup air is a code requirement if you have more than a 400 cfm range fan.
While you are at it have them test the range oven for Carbon Monoxide output. It is scary how much CO some gas ovens put out.
More than likely you will need a passive or even powered makeup air supply that is set up to run whenever the range hood is on. This "Makes up” for the cfm of air that is being removed from the house so the pressure stays neutral. In some cases the makup can go to the return side of your forced air distribution system. If you don’t have forced air, it has to be ducted directly to a wall vent somewhere.
One other thing to think about, the hood should have a capture area that extends past the front burners an inch+ for good function. A lot of the designer range fans look cool, but don’t actually suck very efficiently.
Cheers,
Eric

My builder is installing something that will be controlled by the hood.  We are waiting for the hood, so they can make all of the cuts at the same time.  We have an 8 ft limitation because we placed windows at 9 ft above the range, so we will have two exhaust holes.  We are using ventahood, but we will use the 1200 CFM, so that becomes 4 small motors.  We plan to cut some holes in the same wall, and we will use the toekicks with vents to provide make-up air.

We are even doing something for the dryer.  Similar concept with the dryer somehow connected to the makeup air.

A little worried about negative pressure.  We are ICF exterior walls, closed cell foam attic.  Our windows will be foamed around the edges.  We are using european tilt/turn windows.  We are using a european entry door which has a many point locking system like the windows.  Our other doors will also use multi point locking systems.  I think we are foaming any penetrations, so I think our house will be super tight.

As I write this, I realize, I probably need to connect it to the central vacuum because I think it also pulls >1000 cfm

As I write this, I realize, I probably need to connect it to the central vacuum because I think it also pulls >1000 cfm

We had to forego a central vacuum in consideration of trying to have a green house.
Vents at the toekicks are a great idea, but I'm not sure I would want to wear a kilt whilst operating it at maximum capacity.

Posted By ICFHybrid on 28 Feb 2014 09:41 AM

As I write this, I realize, I probably need to connect it to the central vacuum because I think it also pulls >1000 cfm

We had to forego a central vacuum in consideration of trying to have a green house.
Vents at the toekicks are a great idea, but I'm not sure I would want to wear a kilt whilst operating it at maximum capacity.

Don’t worry about it, test it- especially if you are not using sealed combustion appliances. If you are going to the trouble to build an efficient house, testing the house during commissioning makes a lot of sense. Generally speaking, you want to know that the house is safe, airsealed, and comfortable, that the heating and cooling system works as designed, and that there is fresh air for the occupants. At a minimum, you should be checking the following: Depressurization of the house under winter worst case conditions(when all the exhaust fans are running) and the effects that has on the combustion appliances in the house. Especially necessary in houses with large range fans and central vacs that exhaust to the outside. Testing combustion appliances for safe operation, make sure all combustion byproducts are removed from living spaces and that Carbon Monoxide production is minimized. The combustion efficiency should match the expected values. Total air leakage, using a blower door. ACH50 is within code specifications or expectations. Making especially sure any attached garage or chemical/paint/voc storage area is sealed off from living space. Duct leakage, both total and duct leakage to the outside if applicable are code compliant or meet expectations. That the measured flow rate of the air handler is correctly matched to the furnace and or AC size and that the temp change across the heat exchanger or A coil makes sense. That the room to room pressures in the house don’t exceed 3-4 pascals when the air handler is on. If it is greater than that that you need to work on the return air system pathways. Check that the flow rate and temp of the individual room registers matches what the manual J and D calculations are for the room. Flow rates of the bath fans are high enough (Within spec). Exhaust rate of the clothes dryer is within spec- this is important if you have long or convoluted duct runs. That Mechanical ventilation is accounted for and correct for ashre 62.2 2010. I know this is controversial, but It is code and makes sense to me. While you are at it test the water temp at the faucets and showerhead. Now you know the house is performing as expected. Relax. Cheers, Eric

I'm not too worried about it. I built it myself and there are no gas appliances needing venting save for the gimungous range. We also managed to stay away from forced air.