Home ventilation, according to ASHRAE

The industry's new ventilation standard is here.

"We keep beating ourselves, but we're getting better at it."
- Pro football coach

"You can't get ahead while you're getting even."
- B-Sculls, a Canadian

Back in the summer of 2003, the American Society of Heating, Refrigerating and Air-Conditioning Engineers passed a revised new ventilation standard for residences- ASHRAE 62.2.

Before it passed, NAHB and GAMA (Gas Appliance Manufacturers Association) fought the revision tooth and nail. Once it passed they appealed to have that standard overturned, but that appeal was rejected. So like it or not, a well-defined standard exists to guide homebuilders and the HVAC industry.

Conventional wisdom says the Inter-national Residential Codes won't adopt ASHRAE 62.2-2003 anytime soon. Yet given the realities in our litigious society, the mere publication of this standard warrants your attention. As you gradually modify or upgrade your cur-rent ventilation strategies, 62.2 is the standard you should scrutinize.

At present, codes don't require much mechanical ventilation in homes. Beyond needing to install cheap bath fans in bathrooms with no operable windows, the code largely ignores mechanical ventilation. The new standard turns that world upside down.

A silver lining to this seemingly dark cloud? When you upgrade your ventilation to meet the 62.2 standard, you're one step closer to meeting the Tier 2 (and this is not the eventual market-friendly nomenclature) level of Built Green.

CRITICAL RATIONALE

In the one-page "Foreword" to 62.2, the authors word their rationale for the standard very carefully. Some key points from that wording frame the discussion about indoor air quality:

"Residential ventilation was traditionally not a major concern because it was felt that between operable windows and envelope leakage, people were get-ting enough air ... Now there is] an increasing level of public concern about residential indoor air quality and ventilation ... [T]here is now a desire to define levels of acceptability and performance..."

"The standard appears to be principally about ventilation, but the purpose of ventilation is to provide acceptable indoor air quality."

The authors clarify that 62.2 does not address specific pollutant concentration levels (e.g., radon, carbon monoxide), certain potential pollutant sources (e.g., unvented space heaters), outdoor pollution levels, or credits for air cleaning or for pollution detection devices (e.g., carbon monoxide sensors). They state that those issues "are not yet ready for inclusion in a minimum standard."

62.2'S REQUIREMENTS

The standard describes three primary sets of requirements and a number of secondary items. The primary requirements include whole-house ventilation, local exhaust fans and pollutant source control. The secondary items include sound and flow ratings for fans, plus labeling requirements. Key items are summarized below.

1. Whole-house ventilation rate. The whole house must have ventilation and the requirements are laid out in Table 1. Figure that the average house will require the equivalent of between 45 and 75 cubic feet per minute (cfm) of continuous mechanical ventilation; only large custom homes should require more than 90 cfm.

TABLE 1

* = most likely ventilation requirement

Instead of selecting a number fromthe chart above, you can size your ventilation according to this formula: fan flow (cfm) = 0.01 X your floor area + 7.5 X (your number of bedrooms plus 1). For a 2200 square foot 4-bedroom home, that's (0.01 X 2200) + (7.5 times 5), or 60 cfm.

2. System type. You can use either a supply-air, an exhaust-air, or a combination system to achieve this ventilation rate. Outside air ducts connected to the return side of the air handler and combined with a special air-handler controller - a system installed by a few Colorado production builders do today - make up one allowable approach, as long as manufacturer requirements for return-air temperatures are met.

3. Duty cycle. Ventilation does not have to operate 24/7; however, intermittent systems must be proportionately larger as the amount of run time declines. For example, a home requiring a constant 60 cfm exhaust fan can operate with a 120 cfm fan run 50 per-cent of the time or with 180 cfm of fresh air operating 33 percent of the time. Any intermittent system must operate at least one hour out of every 12.

4. Local exhaust. A local exhaust sys-tem must be installed in each kitchen and bathroom. If these exhaust systems operate intermittently, they must be able to exhaust at least 100 cfm from kitchens and 50 cfm from baths. When the exhaust runs continuously, kitchen exhaust must provide at least 5 air-changes per hour and bathrooms must exhaust 20 cfm.

5. Transfer air. Air that is exhausted from a home must be drawn in from the outdoors and not in through attached garages, unconditioned crawl spaces or unconditioned attics. Measures must be taken to prevent in-migration of contaminated air from those spaces. Such measures include air sealing, management of air pressures, use of airtight recessed light cans, etc.

6. Clothes dryers. These appliances must be exhausted directly to the outdoors.

7. Combustion appliances. When the net exhaust air flow of the two largest exhaust fans is greater than 15 cfm per 100 ft2 - e.g., most down-draft kitchen exhaust fans - and the home includes atmospherically vented combustion appliances (e.g., a standard water heater with a draft hood), you have two choices. First, you can elect to reduce the net exhaust rate by either downsizing the fans or introducing more fresh air from the outdoors. (Note: with large exhaust fans, this could cause serious comfort problems.) Second, you can switch over to sealed combustion appliances (e.g., direct-vented, power-vented or draft-induced water heaters) and then install your larger exhaust fans.

8. Habitable spaces. All habitable spaces must have ventilation openings with a minimum openable area of 4 percent of the floor area, not less than 5 ft2. Even toilets (except toilet compartments in bathrooms) and utility rooms must meet the 4 percent threshold, but not less than 1.5 ft2.

9. Air inlets. Air inlets that are part of the ventilation design must be located at least 10 feet from a plumbing vent, an exhaust hood, vehicle exhaust, or other known pollution sources. (Exception: they can be located within 3 feet of roof or dryer exhausts.)

10. Sound rating. Fans that run continuously must be rated for sound at a maximum of 1.0 sone. (Typical bath fans rate 4 to 5 sones.) Intermittent fans can be rated at 3.0 sones or less.

11. Duct sizing. A table in the standard defines the duct sizes required with certain ventilation equipment. For example, a 50 cfm can be exhausted out of 5 feet of 3-inch duct, 105 feet of 4-inch duct, or 70 feet of 4-inch flex duct, assuming you subtract 15 feet for each elbow used. Alternatively, you can test airflows to prove that the system as designed and installed meets the minimum ventilation flow rates.

12. Cold climate limitation. Mechanical systems that supply 7.5 cfm per 100 ft2 of "habitable space" are not allowed in "very cold climates"-those mountain communities with over 9,000 heating degree-days (Breckenridge, Fraser, Leadville, Aspen, Gunnison, etc.).

The requirements described above have been simplified by this writer. When you start evaluating your current system to determine if you would need to upgrade, you should obtain a copy of the complete published version. Additionally, you should know that while there are some exceptions to the above rules, most don't apply to Colorado climates or to continuously occupied homes.

RECENT ADDENDA

Two addenda to ASHRAE 62.2 published within the last few months made minor changes to the original document. For the sake of simplicity, I've included those measures in the requirements listed above. But for your own clarity, if you obtain the original standard you'll need to request the two addenda.

WHICH SYSTEM TO PICK?

Supply systems. Today, most Colorado builders with upgraded ventilation use a supply system. Typically, a single air intake brings air into the home, dumps it into the forced-air system's return trunk line, mixes it with house air, and distributes it through standard ductwork. If the home requires fresh air but no heating or cooling, an upgraded controller operates the air handler intermittently, drawing in out-side air.

Supply systems allow filtration of outside air and reduce the tendency to bring in air from attached garages. A supply system adds slight positive pres-sure to the home. On the upside, this reduces potential problems with atmospherically vented combustion appliances. One possible downside: warm moist air can be forced into building cavities, slightly increasing the chances for moisture problems. Additionally, moving cooler air within the home can cause minor comfort problems during cold weather.

Exhaust systems. A system of centralized or localized exhaust fans draw in air from the outdoors through natural air leakage sites. This approach usually reduces the minor moisture and com-fort risks listed above. However, the in-coming air from outdoors can't be filtered. Furthermore, exhaust systems can exacerbate the potential for back drafting of atmospherically vented combustion appliances.

The easiest exhaust system to install combines a couple of low-sore fans with controllers to operate them at fixed intervals, or a single quiet fan run 24/7 or nearly continuously. A short-coming here: this doesn't assure that the incoming air is well-distributed throughout the home.

Balanced ventilation. These systems typically use either dedicated or shared ductwork to removed stale indoor air and distribute fresh outdoor air. While heat recovery is optional, most balanced systems provide it. Heat recovery adds notable cost, slightly improves comfort, and provides minor energy savings along the Front Range (major savings in cold-climate homes heated with propane or electricity).

COSTS VS. BENEFITS

Costs range across the board. The central-integrated-fan supply systems typically add $250 or more. An exhaust-only approach could run as little as $150 extra per fan and controller. A balanced system with heat recovery could run somewhat under $1,000 installed for a system tied to the furnace's ductwork to $2,000 for a separate, fully ducted system.

Keep in mind that there's a very marketable benefit here. How large a benefit? Down in New Mexico, Max Wade with large affordable builder Artistic Homes reported a couple of years ago that their buyers liked Artistic's energy and comfort guarantees, but post-occupancy surveys showed that the fresh-air systems were bigger factors pushing up their sales.

Steve Andrews consults with builders for E-Star Colorado and writes on energy issues (sbandrews@att.net). E-Star (www.e-star.com), is a nonprofit home energy rating system that works with both new and existing homes statewide.

2008 Built Green Colorado

Home Builders Association of Metro Denver, 9033 E. Easter Place, Suite 200, Centennial, CO 80112
(303) 778-1400 fax: (303) 733-9440  info@builtgreen.org

Last Updated: 10/05/2007