Thursday, February 3, 2011

Making Up Is Hard to Do

Illustration credit:
Some more ventilation discussion for your consideration:  It wasn't long ago that hood fans rated at over 200 CFM were considered "powerful". Today it’s not uncommon to see range hoods rated at 800 CFM or better.  CFM stands for cubic ceet per minute, and its the measurement of the rate the air inside your house is being moved outside your house.

The CFM rating of your hood fan isn’t just going to affect how quickly the smell of a fish dinner will be cleared from the kitchen.  It’s going to have an impact on your entire house.  As the fan takes air from the house, the air pressure inside the house will drop, causing air outside the house to attempt to get inside.  It will find its way through cracks in the exterior of your house, around poorly sealed windows and doors.  If you’ve gone to the effort of sealing those entrances (well done!) the air is going to find its way in through any chimneys or exhaust vents.  Along with the outside air, fumes from your water heater or furnace (for example) will be drawn back down the exhaust flue and into the house.  This is called backdrafting and presents a very dangerous situation; Carbon monoxide poisoning.

The good news is that this can all be prevented simply by replacing the air you exhaust from the inside of the house with air from outside the house; make-up air.  Make-up air prevents backdrafting, but it also allows the hoodfan you purchased to work at exhausting cooking odours and fumes rather than sucking the air in from the outside. 

Unfortunately, there’s no “standard” to determine what size fan will require make-up air.  It will depend on the house, the fan you wish to use, and many other factors.  Newer homes (e.g. R2000 rated) with tighter envelopes may require make-up air with a fan rated at only 450 CFM.  An older home may be fine with a 650 CFM fan.  Building codes are not much help either.  Bruce Manclark , co-owner of Delta-T, an energy services company in Eugene, Oregon, offers this observation:
With few exceptions, residential codes are silent on the question of providing makeup air. The Uniform Mechanical Code (section 706, 1994 edition) states vaguely that "operation of exhaust fans, kitchen ventilation systems, clothes dryers, or fireplaces shall be considered in determining combustion air requirements to avoid unsatisfactory operation of installed gas appliances."
The only way to be sure how much (if any) make-up air you will require is to have an HVAC engineer test your home.  It’s important to consider this, as well as the cost of adding make-up air to your project when determining the project’s budget.  You’ll also need to consider the possibility of having to heat the make-up air, especially in cooler climates. 

An easier solution is to limit the size of the fan you wish to use.  A 1200 CFM fan is overkill for the average cook. A correctly positioned and sized fan (wider than the cooking surface!) rated at 600 CFM and under is more than adequate unless you plan on running an underground restaurant out of your house. 

For a more detailed look at make-up air, this article from provides a detailed overview.  Your kitchen design professional is also an excellent source of information.
I wrote this post for the Paradigm Kitchen Design blog and thought I'd share it here as well.  Enjoy!


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