Code Conflict Between 2012 IRC and 2012 IECC
It has come to my attention that there may be a building science-based conflict in the language of the 2012 International Residential Code (IRC) and 2012 International Energy Conservation Code (IECC).
The 2012 IECC sets the mandatory blower door tested and certified air infiltration rate at 5 ACH50 in CZ1 & 2 and an even tighter 3 ACH50 in CZ 3 and above. The problem lies in the fact that the new IECC/IRC also allows builders to install natural draft gas appliances in these homes with no safety testing. There is a significant danger that these appliances will not be always be able to create a positive draft up the flue and CO will be drawn into the home. The IRC says that if your home meets the ACH50 standards of the IECC, you must do 62.2 mechanical ventilation, but nothing about CAZ safety or depressurization. The IRC also says that if you have gas appliances in this home, you must comply with the Gas Section of the International Mechanical Code (IMC) and the IMC says something weird: If the house is less than 4 ACH (50 or natural, who knows) you must have combustion makeup air, but again nothing to address CAZ depressurization due to fans.
Now we have climbed through several codes, none of which makes this trail easy to follow, we still don't have the answer we need. Unless there is language in the IMC or someplace else that I'm not aware of at this time, we have a threat to public life and safety.
This oversight will allow the construction of homes with a CAZ pressure and CO related defect that is inherently dangerous, potentially even deadly. For almost two decades, building scientists have known that the tighter a home becomes the greater the negative pressure generated by exhaust fan flows including kitchen and bathroom exhaust fans, appliances and duct leakage. This discovery back in 1986 by John Tooley, and Dr. Neil Moyer then working at the Florida Solar Energy Center, was the beginning in many ways of the study of the interactions between airflows, pressures and building tightness. Mr. Tooley coined the term, MAD AIR to describe what had been discovered.
This research led to an understanding of how house tightness and airflows interacted to create unbalanced pressures and potentially deadly back drafting of combustion appliances. In response to this research, the ability of natural draft appliances to draft properly was investigated and safety limits for depressurization were established by nationally recognized bodies such as the Building Performance Institute. The following combustion appliance zone depressurization limits are incorporated in the standards of the Building Performance Institute:
CAZ Depressurization Limits
|Venting Condition||Limit (Pascals)|
|Orphan natural draft water heater (including outside chimneys)||-2 - 5|
|Natural draft boiler or furnace commonly vented with water heater||-3|
|Individual natural draft boiler or furnace||
|Mechanically assisted draft boiler or furnace alone, or fan-assisted DHW alone||
|Exhaust chimney-top draft inducer (fan at chimney top); High-static pressure flame retention head oil burner; and sealed combustion appliances||-50|
As you can see, building scientists found that natural draft gas appliances would experience backdrafting of combustion byproducts including CO into the home when placed under a negative pressure of as little as -2 to -5 Pascals (depending on how the flue was constructed) with reference to the outside. When a home is very tight, as is now mandated and test verified by the 2012 codes, even very small exhaust fan flows will exceed these safety limits. My concern is that it is very easy to exceed these depressurization levels in the very tight homes we are talking about in the 2012 code. Kitchen exhaust fans with flows under 400 CFM don't require make up air. In a home at 3.0 ACH50, the operation of this exhaust fan alone could very easily create depressurization of -10 Pascals and no natural draft gas appliance in the home could draft safely.
2,000 square feet x 9' average ceiling height = 18,000 cubic feet of volume
18,000 CF x 3 air changes per hour = 54,000 Cubic feet per hour of air flow to equal 3.0 Air Changes per Hour at 50 Pascals
54,000 CFH / 60 mins. = 900 CFM50
From these calculations, it is clear that an airflow of 900 CFM will yield 3.0 air changes per hour in our 2,000 sf example home. This is the highest CFM and leakiest that a new home can be built under the 2012 code. What pressure will a 300 CFM kitchen exhaust fan generate in a home of 900 CFM50? The following chart taken from the Operation Manual of the Minneapolis Blower Door of The Energy Conservatory graphically depicts the relationship between house tightness, airflow and house depressurization.
The vertical blue line represents the air tightness of the leakiest 2,000 sf home allowed under the 2012 IECC at 900 CFM50. Note that all homes built tighter than this will experience even greater house depressurization than the barely code compliant home as the blue line shifts to the left on the chart. The green box at minus 2.0 Pascals represents the level of depressurization at which a natural draft gas water heater reaches the BPI CAZ depressurization zone safety limit. The fan flow of a kitchen exhaust fan at 300 CFM will depressurize the home to minus ten Pascals (red arrow/box)! This is twice the safe maximum depressurization allowed for a gas furnace and two to three times what a gas water heater can safely operate against. Simply turning on a clothes dryer or two bathroom exhaust fans to generate an exhaust flow of 200 CFM will result in a depressurization of minus five Pascals (purple arrow/box). You can all fill in your own "worst case depressurization scenarios" here.
I feel strongly that we the building science community must have a discussion regarding this issue. The sooner the better. What are the solutions that are acceptable to the marketplace? Draft induced and sealed combustion appliances will work, but will all stakeholders be willing to sign on?
This oversight needs to be addressed before jurisdictions across the country begin to adopt these new codes.
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