After January 1, 2010, all new homes in CA must include whole-house ventilation systems. Yes… we’ve made building envelopes so efficient, that now we have to in essence introduce highly controlled leakage. There are two mandatory ventilation features in the new Title 24:
- Intermittent exhaust fans for moisture control in all kitchens and bathrooms
- Whole-house continuous mechanical ventilation for indoor air quality (IAQ)
A good ventilation system will filter out indoor air pollutants (VOCs like formaldehyde from particleboard or acetone nail polish remover) as well as filtering outdoor air on the intake side; however, not all whole-house ventilation systems include outdoor air filtering. Here’s a summary of the ventilation course module from last week’s Title 24 update class.
Exhaust fans, which were common enough before, now required in all kitchens and bathrooms. Really, any “wet” room like a laundry room that has plumbing, should have an exhaust fan. One of the side effects of tighter building envelopes has been increased concentrations of mold in the indoor air, and keeping moisture down helps prevent the growth of mold.
Single-room exhaust fans can be intermittent (operating only when when occupied) or continuous with an override switch. They must meet minimum rated capacities for cubic feet per minute (cfm) or air changes per hour (AC/h) and be ducted to the outdoors. Look for exhaust fans with Energy Star ratings for low noise level and high efficacy.
You may need to educate the owner on how to use these fans, but unfortunately my notes trail off at this point. Required sizing for exhaust fans was roughly 1 cfm/SF, in 50-cfm increments, so a typical range might be 50-300 cfm based on the number of showers, hot tubs, toilets, etc.
Whole House Ventilation
The motto is “build tight, ventilate right.” The reason is that a tight home costs less for conditioning and ventilation combined than a leaky home does for conditioning alone.
Ideally, the indoor air pressure should remain balanced. A really effective whole-house ventilation system includes both intake and exhaust, in equal amounts, running all the time. Intake is filtered, from a controllable location. They can include heat-exchange features too. However, these balanced ventilation systems are harder to install, and are, in the words of the presenter, “susceptible to insulation neglect.”
In California’ relatively dry Mediterranean climate, supply-only ventilation is OK. In very cold climates, a positive indoor pressure can lead to moisture buildup inside the walls as moisture-laden warm air is forced outward through the walls until it meets with cold air and condenses. Advantages include filtering of outdoor air, and better delivery of air where it’s needed – including bedrooms behind closed doors. The presenter noted that they can range widely in energy-efficiency, but didn’t explain why.
When using forced-air fans for continuous ventilation in a supply-only system, it was recommended NOT to use a typical forced-air fan, but instead to use either a variable-speed forced-air fan or a supply fan that is separate from the forced-air system itself.
An exhaust-only ventilation system is cheaper, but brings in unfiltered air, draws in other outdoor pollutants, and air distribution is easily disrupted by closed doors. Exhaust-only systems are not good with tight homes and open fireplaces, either – the negative indoor pressure can actually draws smoke from the fireplace or carbon monoxide from the furnace back into the house.
To size the system, multiply the cubic volume of the conditioned space by the number of air changes per hour. Although the code minimum for air changes is 0.35 an hour, the presenter noted that most international standards are higher: 0.5 or even 1 AC/h, and some exhaust systems seem to specify a much higher number.
For example, a 2,000 SF house with 10 foot ceilings would have a cubic volume of 20,000 ft3. (Note the conversion of hours to minutes.)
Ventilation Energy Efficacy
Although there was a lot of discussion on this topic, the summary slide says to pay attention to the cfm/W – that’s the cubic feet per minute of air moved, per watt consumed. A lot of the efficiencies seem to be dependent upon how well the system is sized, designed and installed, and how well it’s suited for the particular home and climate.
A well-designed continuous mechanical ventilation system should consume the power equivalent of a single 20W bulb running in the background 24-7-365.
About the author
Rebecca Firestone has been working in the Bay Area since 1998 as a technical writer, business content developer, architectural filing lady, marketing director, and sorcerer’s apprentice.