Whole House Ventilation and Title 24

Whole House Ventilation and Title 24

Posted on 17. Nov, 2009 by in Ventilation

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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. 

Moisture Control

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.” 

A balanced ventilation system includes supply and exhaust. Images from www.energysavers.gov.

A balanced ventilation system includes supply and exhaust. Images from www.energysavers.gov.

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.

 

Supply ventilation creates a positive indoor air pressure.

Supply ventilation creates a positive indoor air pressure.

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.

 

Exhaust-only ventilation creates negative indoor air pressure that relies on infiltration rather than an intake fan.

Exhaust-only ventilation creates negative indoor air pressure that relies on infiltration rather than an intake fan.

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. 

Whole-house ventilation system sizing formula

Whole-house ventilation system sizing formula

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.)

 

The recommended best practice is to take the cubic feet of conditioned space times the number of air changes per hour, and then convert that to minutes in order to get the cubic feet per minute (cfm) of the house ventilation system.

The recommended best practice is to take the cubic feet of conditioned space times the number of air changes per hour, and then convert that to minutes in order to get the cubic feet per minute (cfm) of the house ventilation system.

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.

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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.


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10 Responses to “Whole House Ventilation and Title 24”

  1. Carson

    12. Jan, 2010

    Very informative article, I couldn’t agree more with your point on ventilating areas that are prone to high levels of moisture. Not only can this create an environment where mold grows but this can also cause oxidation and rust on pipes which can create additional problems. Overall great information!

  2. Lesson of the day: build tight, ventilate right. New residential T24 http://tinyurl.com/yloamq4

  3. Art Studio Furniture

    15. May, 2010

    Really interesting to read 😛 :N

  4. Steve

    28. May, 2010

    Lesson of the day: build tight, ventilate right. New residential T24 http://tinyurl.com/yloamq4

  5. Douglas Lopez

    07. Sep, 2011

    I could not agree more with the article. Build tight means a efficient envelope. Insulation, Windows (maybe Milgard windows), proper vapor barriers……….

    Douglas Lopez
    Certified Green Building Professional
    GreenPoint Rated Advisor
    LEED Green Associate

  6. D Trend News

    08. Oct, 2011

    I appreciated with your article. This tutorial about house ventilation is a good choice rather than air conditioning system that had a great effect for our earth.

    Global warming is a serious problem. We must care about it. May be with this ventilation house system reduce people use air conditioning.

  7. Plumbers

    16. Oct, 2011

    Failure to provide adequate ventilation may worsen and increase the levels of pollutants inside, does not make enough fresh outdoor air to dilute emissions from indoor sources. So this is a best guidance to how we can manage proper ventilation.

  8. Plumbers Gadsden AL

    15. Nov, 2011

    This is a great article… I will come back to read more in the future…

  9. Rod

    20. Jun, 2014

    I understand this is necessary but with the new requirements on the dual return air, whole house, access door and possible dump zone if your zoning a house…your hallway will be filled with grilles & access doors. Most homeowners will abandon these and not maintain them because they need to be educated as well as understand the importance of them.

    I’m also intrigued by the Author being a Sorcerer’s apprentice, maybe she could teach me a few things!!

  10. Chris

    05. Sep, 2014

    Yes, for those Californians not smart enough to open a window.

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