Code Cycle: 2019 Standards
RCM 126.96.36.199 - Balanced Ventilation
Balanced systems use both an exhaust fan and a supply fan to move approximately the same volume of air into and out of the dwelling. In order to be considered a balanced ventilation system:
- The total supply airflow and the total exhaust airflow must be within 20% of each other.
- For determining compliance, the average of the supply and exhaust airflow's is equal to the balanced system airflow rate.
Balanced Ventilation Heat Exchange
Indoor Air Quality (IAQ) using a dedicated heat recovery ventilation system (HRV), 0.25 W/CFM used for exhaust fans is not the case for other whole-building ventilation systems such as heat recovery ventilation (See the excerpt from the RCM below).
- Some balanced systems are small packaged systems that include heat exchangers that temper incoming air with outgoing air, which reduces the thermal impact of ventilation on heating and cooling loads, but the dual fans also increase electrical energy use.
- They are most practical for use in very tightly sealed houses and in multifamily units where exhaust type systems have difficulty drawing adequate outside air due to limited exterior wall area. An example of a heat recovery ventilator is shown in figure 4-31.
Balanced Ventilation Air Filtration
Like supply ventilation systems, balanced systems are required to be equipped with:
- MERV 13 or better filter, which removes particles from the outside airflow.
- The outdoor air inlet should be located to avoid areas with contaminants such as smoke produced in barbecue areas and products of combustion emitted from gas appliance vents.
- Air may not be drawn from attics or crawlspaces.
Another balanced system configuration uses a standalone supply fan coupled with a standalone exhaust fan, both wired to a common switch or control to ensure they operate simultaneously. The controls must make it possible to adjust the speed of the fans for balancing airflow's.
If both fans supply the same airflow, the system is balanced, and the house has a neutral pressure.
RA188.8.131.52.2 - HERS Verification - Balanced Ventilation Systems
- A flow measuring device that meets the applicable instrumentation requirements given in Section RA3.7.2 and RA3.7.3 shall be used to measure the ventilation airflow's.
- Confirm that both the supply side and the exhaust side of the balanced system operate simultaneously in response to a shared system control.
- Measure the airflow rate for the exhaust side of the system.
- Measure the airflow rate for the supply side of the system.
- Calculate the percent difference between exhaust and supply airflow rates.
- Calculate the average of the exhaust and the supply airflow rates.
- If the exhaust and supply airflow rates are within 20% of each other, and the average of the exhaust and supply airflow rates is greater than or equal to the airflow rate required by the Standards or the Certificate of Compliance, the balanced ventilation system complies. Otherwise, the system does not comply, and corrective action shall be taken.