EXISTING HOMES
Mechanical Ventilation
MV 1.0—Introduction
All weatherization measures have the potential to impact the indoor air quality and overall tightness of a home.
Existing Homes has aligned the program threshold for building tightness with the Regional Technical Forum, or RTF, Energy Efficiency Programs. Air sealing for the Existing Homes program requires calculating the Minimum Ventilation Level (MVL).
MV 1.1—Measurement
An air leakage test using diagnostic testing equipment, such as a Blower Door and manometer, shall be used to measure the overall tightness of a home. Building airflow shall be measured in Cubic Feet per Minute at minus 50 Pascals of pressure (CFM50).
MV 1.2—Calculating Minimum Ventilation Level (MVL)
When applying for an air sealing incentive, the MVL for a house shall be calculated based on: (1) the known number of occupants; (2) an estimate of the possible number of occupants based on the number of bedrooms; or (3) the air change rate and volume of the house. Basements containing HVAC ducts or which have a direct access to the interior conditioned space of a home shall be considered conditioned space and should be considered in volume calculations.
The equation resulting in the highest CFM shall be compared to the ventilation potential.
Table MV 1.2 Calculations
- MVL based on known occupancy:
MVL = (# of occupants) x (15 CFM)
- MVL base on number of bedrooms:
MVL = (# of bedrooms + 1) x (15 CFM)
- MVL based on ACH50 and volume:
MVL = .35 ACHnatural x (building volume of conditioned space (ft³))
60 minutes
0.35 ACHnatural is roughly equivalent to 7 ACH50
Each of these calculations yields an estimate of the required ventilation (in CFM) to maintain good indoor air quality.
The following formula (4) is used to convert the measured CFM50 from the Blower Door Test to an estimate of the average ventilation potential (in CFM).
- Ventilation Potential = CFM50/N
Where N is a correlation factor: N = C x H x S
C = climate factor, a function of annual temperature and wind. C = 20 is an appropriate approximation for most areas in the Northwest
H = height correction factor (see Table MV 1.31)
Number of Stories |
1 |
1.5 |
2 |
3 |
Correction Factor "H" |
1 |
0.9 |
0.8 |
0.7 |
S = wind shielding correction (see Table MV 1.32)
Wind Exposure |
Well Shielded |
Normal |
Exposed |
Correction Factor "S" |
1.2 |
1 |
0.9 |
As long as the measured Ventilation Potential is greater than the MVL, additional air sealing should be possible without creating moisture and IAQ problems. If the MVL is greater than the Ventilation Potential, a mechanical ventilation strategy shall be installed.
Limitations: The CFM50 of a house is a measure of the effective leakage area. Estimating the amount of ventilation that a given leakage area will provide is affected by many factors and is at best an approximation averaged over a wide range of conditions for the entire year. Periods of over and under ventilation will certainly occur. A properly sized and controlled mechanical ventilation system installed in a tight house envelope is the preferred alternative to ensure adequate ventilation rates at all times.
Sections MV 1.3-1.7 define techniques for adding mechanical ventilation and should be used as a guide to designing ventilation systems.
MV 1.3—Non-Heat-Recovery Ventilation—Intermittent or Continuous Operation
A bathroom exhaust fan that is controlled by both a manual switch, crank timer or dehumidistat in the bathroom to provide spot ventilation AND a time clock or programmable timer to provide whole-house ventilation when called for by the timer is an acceptable form of mechanical ventilation.
Surface-mounted fans shall have a sone rating of 1.5 or less, or other rating as approved by the contractor. Existing fans that meet the minimum airflow rates are exempt from the sone rating requirement. The fan shall have both automatic and manual controls. Automatic controls shall include a time clock or cycle timers with a minimum of two on-periods per day and a manual control switch to let the occupant turn the fan on or off for spot-ventilation purposes. The timer shall be set to make up the difference between the minimum ventilation level and the ventilation potential defined in MV 1.2 based on the measured flow rate of the fan. The ventilation fan shall be wired to both the manual spot-ventilation switch in the bathroom and to a time clock or timer.
Fans rated by the manufacturer to operate continuously and which are wired to a designated circuit or directly to the electrical panel shall be used in situations where continuous operation is required.
MV 1.4—Balanced Flow Non-Heat Recovery Ventilation—Continuous Operation
Balanced flow non-heat-recovery air exchange units shall have fans capable of providing the intake and exhaust airflow determined in rates in MV 1.2, provide complete isolation of the intake and exhaust air, and have UL approval of all electrical components.
MV 1.5—Air-to-Air Heat Exchangers
Air-to-air heat exchangers shall provide the ventilation rates for minimum ventilation levels, as calculated in Section MV 1.2 and installed to manufacturer's requirements.
MV 1.6—Other Mechanical Ventilation Systems
Other mechanical ventilation systems may be used if approved by Existing Homes.
MV 1.7—Installation of Exhaust Fans
Exhaust fans shall be installed according to AT 1.7.