Hey y’all, watch this: I can flush all the toilets in my house by turning on my range hood! I don’t think any of you have built a house that tight yet. If I can accomplish that feat, then consider this my claim for the exclusive YouTube rights.
Most houses have all sorts of exhaust appliances (bathroom fans, clothes dryers, wood stove, power vented water heater, etc.), but kitchen range hoods are typically the most powerful, pulling between 100 to 1200 cfm of air out of the house. As Isaac Newton told us in elementary school, for every action there must be an equal and opposite reaction. As such, every time air is removed from the house, an equal amount of air must enter.
Typically, this air enters through gaps and cracks in the building envelope. However, the concern now is that the combination of homes getting tighter and hood fans getting more powerful will lead to bigger problems than impromptu toilet flushing. Specifically, that problem would manifest in the form of air back drafting through water heater flues and through wood burning chimneys, bringing carbon monoxide with it when the range hood is activated.
In an attempt to address this issue, the 2009 IRC requires that range hoods pulling more than 400 cfm need to be provided with approximately the same amount of makeup air through means of an automatic damper that operates as soon as the hood is activated. Though this provision is well-meaning, the cure may be more harmful than the disease in this instance.
For starters, the makeup air has to come from the outside, which in our area usually means that air is hot and humid or nice and cold like it is now. That places a greater load on HVAC systems, and could result in systems being oversized in their design (a classic building science no-no), and may make the house an uncomfortable place to live, at least when the range hood is running.
I am really not sure why 400 cfm is the magic number chosen by the code. I have not been able to find a building science article explaining how the IRC drafters came up with it. The cynic in me says it is because most range hoods on the market are rated higher than 400 cfm. Assuming though that 400 cfm is indeed the magic number, the code allows you to have a 400 cfm fan with no makeup air.
However, if you have one rated at 600 cfm, you would need to provide 600 cfm of makeup air from the outside. Again, if 400 cfm is the magic number, it would seem to me that the code should require only 200 cfm for a 600 cfm fan and so forth.
Manual dampers (long recognized by the code) do quite well at introducing fresh air into the house without the added expense and impracticality of an automatic system. Additionally, sealed combustion appliances and fireplaces have their own air supply and are not at risk of back drafting dangerous gases into the house.
In the coming months, the HBA will be discussing this code provision and possible solutions that would provide greater flexibility with local code officials. We will also make this a topic of discussion at the North Central Texas Council of Governments’ Regional Codes Coordinating Committee that we serve on. That group has begun evaluating the 2012 codes, which will provide an opportunity for the industry to propose a long-term solution.