In last weeks blog post near backdrafting water heaters, I mentioned that I might someday follow up with a blog post attempting to explain how to use section 501.3 of the Minnesota Mechanical Code (MMC), which is the code section that describes how to supply makeup air to dwellings.  Well, there'southward no time like the present.  This isn't the nigh exciting topic, but it's certainly a misunderstood topic.

There's a general consensus amongst those 'in the know' that makeup air is needed whatsoever fourth dimension a kitchen exhaust fan rated over 300 cfm is installed, however, that'due south not exactly true.  The real requirement is that makeup air must be providedif information technology's needed.If an frazzle fan rated over 300 cfm is installed, makeup air might be needed and a adding will need to be done.

The Minnesota Mechanical Lawmaking is a combination of chapters 2 to 15 of the 2006 International Mechanical Code and Minnesota'southward amendments.  The amendments tin can be found online hither:  https://world wide web.revisor.mn.gov/rules/?id=1346 .  Minnesota has amended department 501.3 in it's entirety, which means that this unabridged section is available online: https://world wide web.revisor.mn.gov/rules/?id=1346.0501.  Overnice.

The MMC has very specific language about makeup air that basically says makeup air must be installed when information technology's required, unless it's non required considering of an exception. Yeah, that's how lawmaking language is written.  Clear as mud.

I'm going to paraphrase this code section to hopefully make it a little easier to understand, because it's written in a very confusing mode. The easiest way to break information technology down is by the age of the home.  There are 5 historic period categories, but the requirements for two are identical so I lumped them together nether number two:

  1. New Homes
  2. Homes congenital after 1999 using the provisions of the Minnesota Energy Code, Minnesota Rules, chapter 7672 and homes congenital later 2003
  3. Homes congenital during or after 1994 under the Minnesota Free energy Lawmaking, Minnesota Rules, chapter 7670
  4. Homes congenital before 1994

New Homes

New homes require a calculation for  makeup air using table 501.three.1.

To complete this calculation, you'll need to know the number and blazon of combustion appliances (power vent /direct vent / fan-assisted / atmospherically vented / solid fuel ), the conditioned floor area foursquare footage, and the CFM rating of exhaust fans.  Once all of these numbers are plugged into the table and a little math is done, the final number will be the amount of makeup air needed.

If the number is negative, nothing has to be done.  If the number is positive, table 501.3.two determines how makeup air should be supplied. Footnote "Thou" at this table says that if flex duct is used (and flex duct is about e'er used), the diameter of the makeup air duct needs to exist increased past i inch.

Homes built after 1999 using the provisions of the Minnesota Energy Code, Minnesota Rules, affiliate 7672 and homes built after 2003.

Whatsoever time a vented combustion appliance is installed or replaced, or an exhaust system is installed or replaced, table 501.iii.ane needs to exist used to make up one's mind if makeup air is needed.  This means that if an 80 cfm bath exhaust fan is replaced with a 90 cfm exhaust fan, yes, a new adding needs to exist washed to determine if makeup air is needed.

Homes built during or afterward 1994 under the Minnesota Energy Code, Minnesota Rules, chapter 7670

When a solid fuel appliance is installed, use tabular array 501.3.1.  One exception is if the appliance is a closed combustion appliance and combustion air is installed in accord with the manufacturers installation instructions.

When an frazzle system with a rated capacity greater than 300 cfm is installed, employ table 501.3.3(ane).  Ane exception is if powered makeup air is electrically interlocked and matched to the airflow of the frazzle equipment.

Homes built before 1994

When a solid fuel appliance is installed, use table 501.three.3 (three).  One exception is if the appliance is a airtight combustion appliance and combustion air is installed in accordance with the manufacturers installation instructions.

When an frazzle system with a rated capacity greater than 300 cfm is installed, employ table 501.3.3(2).  One exception is if powered makeup air is electrically interlocked and matched to the airflow of the frazzle equipment.

Exceptions

There are two exceptions to all of the above makeup air requirements.  One is if a exam is performed in accordance with ASTM Standard E1998-02,Standard Guide for Assessing Depressurization-Induced Backdrafting and Spillage from Vented Combustion Appliances, and information technology's proven that makeup air isn't needed.  I don't know what those standards await similar considering I don't own them and I'yard non going to buy them.  The other exception is if a test is done that's approved by the edifice official which verifies proper operation of vented combustion appliances.

Sample Adding

Just for fun, lets go through a sample calculation.  Lets say I have an old house in Minneapolis and I want to install a 600 cfm kitchen exhaust fan.  This is an exhaust system with a cfm rated greater than 300 cfm and the domicile was built before 1994, so I'll need to use table 501.iii.iii(2) to determine if makeup air is necessary.

The business firm has a high efficiency, sealed combustion furnace, which is a 'direct vent' appliance.  The water heater is a powervent, and the fireplace has a sealed combustion gas insert (straight vent).  In that location is a total of 2,000 square anxiety of flooring area in the firm, which includes the unfinished basement.

To start, I need to figure out which which column I'thou working with.  The first column on this table lists "ane or multiple power vent or direct vent appliances or no combustion appliances."  That's what I have, so I'll utilize this cavalcade.  To determine the estimated house infiltration, I can either use the conditioned flooring area or use the results from a blower door test.  If all-encompassing air sealing has been done on an old firm it would be wise to utilize the results from a blower door test, only nosotros'll utilize flooring expanse instead to get in easy.

I plug in the numbers and get 500 cfm for my estimated house infiltration.

Section 2 says to plug in 80% of the largest exhaust fan, which will exist the 600 cfm kitchen fan, which gives me 480 (600 x .eight).

Department 3 subtracts office ane from part ii (480-500), giving me a negative number.  When the number is negative, no makeup air is required.   And then in that location y'all accept it.  Makeup air isn't always required when installing a big kitchen frazzle fan, but the adding is ever required.  The calculations are shown below.

Sample makeup air calculation

On the other hand, lets say I'thou working with the same house but I have an atmospherically vented water heater and a wood burning fireplace.  In that case, I would demand to use the cavalcade at the far right.  Once I plug in all of the numbers, my makeup air quantity would be...  380 cfm.  That'south some serious makeup air.  Next, I use table 501.3.2 to make up one's mind the size of my makeup air duct, which  would be a nine" rigid duct with a motorized damper, or a ten" duct if I use flex material.

Sample Calculation 2

In that case, a 600 cfm kitchen fan would just exist a stupid selection, dontcha think? Hopefully I've taken a little mystery out of the makeup air requirements for Minnesota homes.

Author: Reuben Saltzman, Structure Tech Abode Inspections