Range Hood Q & A

Q.

Peggy,
I stumbled across your blog when googling 'range hoods'. I have a few questions about range hood size and power.

1. I have not been able to find any clear directions/advice about what minimum cfms a hood needs to pull to safely and effectively vent a kitchen range.

A.
Directions and advice on recommended hood CFMs and size come from the manufacturer of your chosen range. If you don't see such specifications on their web site - call them and ask them. These are always minimums as range makers don't want to discourage purchase of their products by specifying larger and more expensive hoods than absolutely necessary (especially if they're not selling the hood).

Q.
2. Are there different recommendations about minimum cfms for gas and electric ranges? Removal of the various cooking odors, smoke, and grease aside, intuitively, I would think there might be a minimum expectation for gas due to the combustion process that occurs during cooking (as opposed to electric).

A.
You are correct that gas combustion creates byproducts that are best exhausted out of your home. However, since these byproducts are created along with the steam, grease and odors of cooking, they too are exhausted with a proper ventilation system.

Q.
3. What is the minimum/maximum distance above a range that a hood needs to be installed to function properly or is that a vent specific specification?

A.
Hood manufacturers specify this distance in their installation instructions. Optimum is usually stated at 24-27" above the cooking surface. Again, each manufacturer will specify for their own products.

Q.
4. Is there a rule of thumb about the proper size of the hood opening to effectively and safely vent all of the burners of the range below?

A.
We designers skirt the issue of the hood being too low at 24-27" by using larger hoods to capture the ever-spreading column of steam and grease coming off a pan on the range surface. I typically specify a 42-48" hood over a 36" range because of this issue. The rule of thumb I use is that a column of hot steam coming off a pot or pan on the cooking surface spreads at a rate of 3" in 30" of ascent. This requires a minimum 6" in extra width for each 3" of height over the recommended maximum.

I presently have a 30" standard opening in the counter top with room for a stand alone range. Above that is a microhood combo that vents outside at 90 degrees. Oddly, I also notice that the microhood also recirculates some of the exhaust back into the kitchen.
Thanks for your help!
Stacie

A.
With your present situation, you are limited to a 30" hood over a 30" range. In that case you should choose a hood with the deepest canopy possible and mount it at the manufacturers recommended height from the cooking surface to capture most all of the heat, steam, grease and odors of cooking.
If you plan to replace your cabinetry, then you can choose a wider hood to raise it higher.

I can't imagine why your present micro-hood is vented out the back and also recirculating at the same time, since it is always either/or when it comes to hookup. I suggest you turn on the hood and go outside to see if there is any air at all coming out of the vent cap. It may be that the installer simply set it up to recirculate and abandoned the existing exterior vent. A poor choice, but not uncommon, because it is easier.

Many installers simply take the easiest route when it comes to these things. You have to demand that products be installed in the optimum way for function rather than just the easiest way. You also need to check to make sure that your instructions, or your designer's specifications, are followed to the letter.

Ventilation pipes are almost always hidden in the walls and/or attic. That puts them in the province of "what you don't know can hurt you". Manufacturers always specify the type and size of ducts; how long they can be before function is impacted; and how many turns can be made. The performance of your carefully selected and properly mounted hood can be severely impacted by poorly installed ductwork. If you can't see what's in the attic or walls, ask your installer to furnish you with digital pictures showing installation as specified.

In your particular case, since the hood is being installed on an outside wall, a hood that has the option to vent directly out the back would be the best choice for funtion. Broan makes hoods with this option.

Peggy