Balanced Ventilation

Balanced Ventilation Is Appropriate for All Climates

Bird's eye view

Supply and exhaust fans working in cooperation

Balanced systems typically have two fans and two duct systems, one for supply air and one for exhaust air. Ductwork can be extensive, with supply ducts to provide fresh air to bedrooms and living areas and exhaust ducts to remove stale air from bathrooms, laundry rooms, and sometimes kitchens.

See below for:
ABOUT BALANCED VENTILATION
MORE ABOUT BALANCED VENTILATION


Key Materials

Avoid flex duct if possible

The best duct material for ventilation supply ducts is galvanized steel; flex duct should be avoided or minimized. Some installers use PVC pipe for exhaust ducts.


Design Notes

Heat recovery systems do a better job

While ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. ventilation requirements can be met with an exhaust-only or supply-only ventilation system, the performance of these lower-cost ventilation systems will not equal the performance of a properly installed HRV.

  • Compared to an exhaust-only system, a heat-recovery ventilation(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. system will do a better job delivering fresh air to bedrooms.
  • Compared to a supply-only system, a heat-recovery ventilation system will usually have a lower operating cost and will do a better job removing odors.
  • The energy penalty associated with a heat-recovery ventilation system will be lower than that of any other type of ventilation.

  • Builder Tips

    Getting the most out of an HRV

    If the budget allows, HRV installations should always include dedicated ventilation ductwork. Such systems are routinely installed in New England homes with hydronic heating systems. Ventilation ducts usually measure 4 inches in diameter. Because air flows and pressures are much lower than those encountered in a forced-air heating system, meticulous duct sealing is essential.

    Homes equipped with an HRV do not need bathroom exhaust fans, since the HRV draws its exhaust air from bathrooms and laundry rooms. Each bathroom can be equipped with an override switch to operate the HRV at high speed for odor removal. An HRV should never pull exhaust air from a range hood, since grease can clog an HRV.

    When an HRV is installed in a home with a furnace, the existing forced-air ductwork is usually used for the mechanical ventilation system. Although such systems work, they involve technical compromises that may degrade the performance and energy efficiency of the HRV. Forced-air ductwork is much larger than necessary to handle the needs of a ventilation system. If the HRV is wired to use the furnace blower motor to distribute ventilation air, energy use will be much higher than it would be if the HRV fans were used.

    The Code

    Keep intake air clean

    Ventilation of living space is covered in Section 303 of the 2006 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.. Natural and mechanical Intake openings should be at least 10 feet from any potential source of contamination like exhaust vents, plumbing vents, streets, alleys, chimneys, parking lots, and loading docks. When the contaminant is within 10 feet, the intake vent must be located at least to feet below the source of contamination (303.4.1). Intake and exhaust openings must be protected with corrosion-resistant grills, screens or louvers with a hole size between 1/4 and 1/2 in.

    OTHER CONSIDERATIONS

    PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. ventilation. Balanced ventilationMechanical ventilation system in which separate, balanced fans exhaust stale indoor air and bring in fresh outdoor air in equal amounts; often includes heat recovery or heat and moisture recovery (see heat-recovery ventilator and energy-recovery ventilator). systems — specifically, heat-recovery ventilators — are considered essential for houses conforming to the German Passivhaus standard. Passivhaus buildings are very tight, with fewer than 0.6 air changes per hour at 50 Pascals, so mechanical ventilation is essential. The use of an HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. minimizes the energy penalty associated with any ventilation system. Because space heating requirements in Passivhaus buildings have been pared to a minimum, any need for supplemental space heat can be supplied through ventilation ductwork by means of a small electric resistance element, a coil heated by an air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps., or a coil heated by a water heater. Passivhaus builders consider an HRV to be the key appliance in any home — more essential than optional appliances like a boiler or furnace.

    For more information on the Passivhaus standard, see "Passivhaus for Beginners."

    GREEN POINTS

    LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. -H EQ4 (Environmental Quality) offers up to 3 points for above-code ventilation practices.

    NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. Under Ch. 9 — Indoor Environmental Quality: up to 17 pts. for whole building ventilation per ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. (902.5).

    ABOUT BALANCED VENTILATION

    Supplying fresh air while exhausting stale air

    A balanced ventilation system uses a supply fan to introduce the same volume of fresh outdoor air that is being simultaneously removed from the house by an exhaust fan. These systems are more complex than supply-only or exhaust-only ventilationMechanical ventilation system in which one or more fans are used to exhaust air from a house and make-up air is supplied passively. Exhaust-only ventilation creates slight depressurization of the home; its impact on vented gas appliances should be considered. systems, but provide better fresh air distribution and in some cases lower energy bills than alternative systems.

    MORE ABOUT BALANCED VENTILATION

    Heat-recovery ventilators and energy-recovery ventilators

    The most sophisticated types of balanced ventilation systems incorporate either heat recovery or energy recovery, offsetting some of the energy penalty associated with bringing in fresh air from the outside. In winter, fresh air pulled into the house is warmed by outgoing air in a heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. that is capable of capturing 60% to 80% of the heat. If the house is air conditioned, summer exhaust air may cool incoming fresh air before it’s circulated in the house.

    The two systems are similar, although an energy-recovery ventilator (ERVEnergy-recovery ventilator. The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV.) has the additional capability of transferring some of the moisture in the air as well as heat. A heat-recovery ventilator (HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. ) transfers only heat. ERVs help retain humidity in indoor air in winter, potentially making a house more comfortable. However, research has shown that ERVs do not provide lower levels of indoor humidity in air-conditioned houses when compared to HRVs.

    HRV and ERV systems are more complex, more expensive, and require more maintenance than balanced ventilation systems without heat recovery. Moreover, not all HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractors know how to install them.

    However, they offer significant energy savings compared to ventilation systems without heat recovery.

    HRV or ERV?

    If you're set on installing a balanced ventilation system, which type of equipment should you choose, an HRV or an ERV? For more information on solving this dilemma, see "HRV or ERV?"


    Image Credits:

    1. / Fine Homebuilding
    2. Chris Green/Fine Homebuilding
    Tags: , , , , ,
    10.
    Apr 6, 2012 1:10 PM ET

    Response to Jerry
    by Martin Holladay, GBA Advisor

    Jerry,
    Your proposed ventilation system would work, if that's what you want to do.

    You can call it whatever you want. I don't think the phrase "balanced ventilation" is regulated by law.


    9.
    Apr 6, 2012 12:37 PM ET

    30 CFM exh 100% & 60 CFM Supply 50%
    by Jerry Krieg

    Thanks for the reply and link to the article.

    Do you have an opinion on running an exhaust / ventilation system at 30 CFM 100% of the time, and bringing in fresh air from the outside (supply) at 60 CFM, 50% of the time?

    Would that be considered "Balanced?"


    8.
    Apr 6, 2012 4:37 AM ET

    Response to Jerry Krieg
    by Martin Holladay, GBA Advisor

    Jerry,
    The amount of fresh air introduced to a building by a 60 cfm balanced ventilation system (60 cfm in, and 60 cfm out) is the same as the amount of fresh air introduced to a building by a 60 cfm exhaust ventilation system. It's impossible to exhaust 60 cfm without 60 cfm of makeup air finding its way indoors.

    In either case, a ventilation system designer needs to follow the ASHRAE 62.2 formula. For more information on determining ventilation rates, see Designing a Good Ventilation System.


    7.
    Apr 5, 2012 9:32 AM ET

    Response to Jerry Krieg
    by Jerry Krieg

    Martin,

    Thanks for the quick reply. So do you know of any formula or minimum ventilation rate that is required when utilizing a "balanced ventilation" system?

    Would ASHRAE expect HVAC contractors to use the same 62.2 - 2007 formula to determine necessary CFM levels for mechanical ventilation (supply)? Then expect the same CFM levels for the exhaust?

    If we bring in 60 CFM 50% of the time and exhaust 30 CFM 100% of the time...is that an acceptable balance? I'm trying to find some guidance on CFM levels from ASHRAE, but I cannot locate a formula dedicated to "Balanced Ventilation."

    Thanks again!


    6.
    Apr 4, 2012 1:08 PM ET

    Response to Jerry Krieg
    by Martin Holladay, GBA Advisor

    Jerry,
    If you "do not want to bring in fresh air continuously (100% of the time) due to our climate," switching to 60 cfm of exhaust ventilation for half of the day will not solve your problem, because an equal volume of outdoor air will enter your house through cracks to make up for the exhausted air when the exhaust fan is running.

    Whether you choose a balanced ventilation system or an exhaust ventilation system, you are introducing outdoor air into the house in either case.


    5.
    Apr 4, 2012 11:34 AM ET

    Balanced Ventilation - formula or minimum CFM?
    by Jerry Krieg

    Martin,

    I work for an HVAC company in Houston, TX, and we have developed a focus on installations for LEED and E-Star 3.0 homes.

    I have a question regarding "Balanced Ventilation." Is there a formula or minimum ventilation rate that is required when utilizing a balanced ventilation system? For example, we have a home that requires 60 CFM of continuous mechanical ventilation (per ASHRAE 62.2 - 2007 formula).

    We do not want to bring in fresh air continuously (100% of the time) due to our climate. Is there a formula or quantitative method to define us bringing in 60 CFM for 50% of the time, and exhausting (via continuous running exhaust fans) 30 - 60 CFM 100% of the time?

    I know this is a stretch, but I cannot find any type of formula that sets minimum supply / exhaust rates for a "balanced ventilation" design.

    Thank you for your time!


    4.
    Nov 14, 2011 4:29 PM ET

    Response to David Mackenzie
    by Martin Holladay, GBA Advisor

    David,
    An HRV provides balanced ventilation, as you point out, not makeup air. If you want to have a 600-cfm range hood, you need to install a source of makeup air -- and an HRV can't do that.

    Here's more information: Makeup Air for Range Hoods.


    3.
    Nov 14, 2011 4:10 PM ET

    600 cm/h range hood fan
    by David Mackenzie

    In a well sealed house with minimised air infiltration and HRV system provides for air exchange and heat recovery. My concern is when a commercial rated kitchen range hood is operating - this removes a significant volume of air (say 600 cu m per hour). If there is a wood fire in operation, is it possible to create a negative pressure inside the house which might draw CO gases down the fireplace flue and into the house. How can the system be set up to ensure a balanced ventilation when the fireplace and range-hood are operating?


    2.
    May 3, 2011 3:39 AM ET

    Edited May 3, 2011 3:56 AM ET.

    Response to Sean Wiens
    by Martin Holladay, GBA Advisor

    Sean,
    I agree that exhaust ducts should not terminate at soffits. Fortunately, this article neither recommends nor illustrates that practice.

    The HRVi ducting illustration was pulled from a different source; I agree that connecting an HRV to forced-air ductwork is an inferior installation compared to an HRV with dedicated ventilation ductwork.

    Here is a simplified drawing showing dedicated HRV ductwork; the illustration appeared on a Web site maintained by the Office of Energy Efficiency of Natural Resources Canada (http://www.oee.nrcan.gc.ca/Publications/infosource/Pub/hrv/hrvsystem.cfm...):


    1.
    May 3, 2011 12:28 AM ET

    Edited May 3, 2011 12:29 AM ET.

    Balanced Ventilation
    by Sean Wiens

    Hi Martin,

    Kind of surprised your graphic does not show independently ducted system considering your recommendations on this regard.

    Can you also caution your readers against terminating any type of interior air flow to soffit areas (especially when soffits are perforated like here in the Pacific North-West). The warm moist air is just sucked back into the attic cavities where it condenses on the cold surfaces and causes moisture problems including mould.


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