Mechanical Ventilation Systems – 2012 IECC
Date Published: February 13, 2019
Starting with the 2012 IECC and IRC and carrying through to the 2015 and 2018 versions of these codes, there is a maximum air infiltration rate allowed, and measurement of air infiltration and blower door testing is mandatory. Due to the very tight air infiltration rates required, Mechanical Ventilation systems are also mandatory.
The definition of infiltration in the 2012, 2015, and 2018 IECC is this:
The uncontrolled inward air leakage into a building caused by the pressure effects of wind or the effects of differences in the indoor and outdoor air density or both. Maximum allowable infiltration rates are defined in the 2012-IECC code with testing required to validate infiltration rates.
For homes built in the Colorado Front-Range, climate Zone 5, the maximum infiltration rate allowed is 3 Air-Changes-per-Hour, measured with a blower door at 50 pascals of pressure. This is a very tight house and attention is needed to sealing all penetrations well in order to meet this mandatory requirement.
Blower door testing is a process whereby a metal frame unit is fit into a door frame of the home, and a blower fan is attached to exhaust air from the home while measuring the pressure difference between inside and outside. Air enters the home through any leaks or un-sealed penetrations and the leakage rate is calculated based upon the area and volume of the home when the blower door reaches the target 50 pascal pressure difference.
The definition of mechanical ventilation in the 2012, 2015, and 2018 IRC is this:
An exhaust system, supply system, or combination thereof that is designed to mechanically exchange indoor air with outdoor air when operating continuously or through a programmed intermittent schedule to satisfy the whole house ventilation rates.
Because of the mandatory, very tight allowable infiltration rates, there may not be enough fresh air exchange that naturally occurs in a home. The IRC dictates that when a dwelling has an infiltration rate lower than 5 ACH50, then the home must have a whole-house mechanical ventilation system installed. The code further outlines that a whole-house ventilation system must exhaust air and provide a supply of fresh air, and have controls to regulate the operation of the system. The system can be continuously run, or if intermittently run the airflow rates must be increased to ensure the required ventilation rates and air exchanges are met.
Ventilation Solutions: The typical solutions to meeting the mechanical ventilation rates are to install one of the following devices:
- Heat Recovery Ventilation Unit (HRV): this is a whole house ventilation unit that connects to the ducting system of the home and is typically installed in the mechanical room near the furnace. Two penetrations are needed to the exterior, one for supply air and one for exhaust air. Two penetrations are also made into the ducting system, one on the supply side to inject fresh air and one on the return side to exhaust stale air. A filter box is present within the unit to mix the air and recover heat from the duct system to minimize injection of cold air into the home. Maintenance of the unit is needed to clean the filter system.
- Mini-Energy Recovery Ventilation Unit (Mini-ERV): in homes where no duct system is present, such as a boiler system, or as an option on a ducted home, mini-ERV units can be installed in place of bathroom exhaust fan units, to act as spot ventilation. When an appropriate number of these units are installed in a home, the minimum ventilation rates can be achieved. The units still need to operate continuously, or on an intermittent timer system.
Note: Exhaust-only and supply-only systems are not recommended as solutions to meet the 2012 code requirement for a mechanical ventilation system. Both supply and exhaust ventilation is needed, and direct injection of cold, outside air into the return duct system is effectively considered as duct leakage and is not recommended.
Code References: Mechanical ventilation systems and maximum air infiltration rates are referenced in the 2012-IRC and 2012-IECC in the following sections:
2012-IECC R402.4 and R402.4.1.2. Air Leakage (Mandatory). The building thermal envelope shall be constructed to limit air leakage… and … The building or dwelling unit shall be tested and verified as having an air leakage rate of not exceeding 3 air changes per hour in Climate Zone 5. Testing shall be conducted with a blower door at a pressure of 50 Pascals.
2012-IRC R303.4 Mechanical ventilation. Where the air infiltration rate of a dwelling unit is less than 5 air changes per hour when tested with a blower door at a pressure of 0.2 inch w.c (50 Pa) the dwelling unit shall be provided with whole-house mechanical ventilation in accordance with Section M1507.3.
2012-IRC M1507.3: Whole-house mechanical ventilation system. Whole-house mechanical ventilation systems shall be designed in accordance with Sections M1507.3.1 through M1507.3.3.
M1507.3.1 System design: The whole-house ventilation system shall consist of one or more supply or exhaust fans, or a combination of such, and associated ducts and controls. Local exhaust or supply fans are permitted to serve as such a system. Outdoor air ducts connected to the return side of an air handler shall be considered to provide supply ventilation.
M1507.3.2 System controls: The whole-house mechanical ventilation system shall be provided with controls that enable manual override.
M1507.3.3 Mechanical ventilation rate: The whole-house mechanical ventilation system shall provide outdoor air at a continuous rate of not less than that determined in accordance with Table M1507.3.3(1).
Exception: The whole-house mechanical ventilation system is permitted to operate intermittently where the system has controls that enable operation for not less than 25-percent of each 4-hour segment and the ventilation rate prescribed in Table M1507.3.3(1) is multiplied by the factor determined in accordance with Table M1507.3.3(2).
TABLE M1507.3.3(1) CONTINUOUS WHOLE-HOUSE MECHANICAL VENTILATION SYSTEM AIRFLOW RATE REQUIREMENTS
TABLE M1507.3.3(2) INTERMITTENT WHOLE-HOUSE MECHANICAL VENTILATION RATE FACTORS
Here is a link to the full 2012 IECC Code for reference.
Scott Home Inspection is your source for energy-code compliance verification and testing. We provide HERS ratings, Performance-Path compliance services, along with Prescriptive path testing services, including blower door testing and duct leakage testing. For more information check out our Energy Services page.
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