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Ventilation Req'd for Replacement AHU?

Discussion in 'Commercial Mechanical Codes' started by Mech, Apr 27, 2018.

  1. Mech

    Mech Sawhorse

    Oct 30, 2009
    Likes Received:
    2009 IMC

    An existing air handler in a split system is being replaced with a new one with larger cooling capacity. The existing air handler is cooling only, so ventilation is only provided during the cooling season. Heat is provided by radiators without ventilation. The outdoor unit will not be replaced as it can handle the additional cooling requirements. When the new air handler is installed, must it provide ventilation year round? I planned on providing year round ventilation but have now been challenged on whether or not it is required.

    102.4 Additions, alterations or repairs.
    Additions, alterations, renovations or repairs to a mechanical system shall conform to that required for a new mechanical system without requiring the existing mechanical system to comply with all of the requirements of this code. Additions, alterations or repairs shall not cause an existing mechanical system to become unsafe, hazardous or overloaded.

    Minor additions, alterations, renovations and repairs to existing mechanical systems shall meet the provisions for new construction, unless such work is done in the same manner and arrangement as was in the existing system, is not hazardous and is approved.

  2. Paul Sweet

    Paul Sweet Sawhorse

    Oct 17, 2009
    Likes Received:
    It should be OK as long as the windows are still operable to provide the natural ventilation that was required when the building was built.
  3. steveray

    steveray Sawhorse

    Nov 25, 2009
    Likes Received:
    809.1 Reconfigured or converted spaces. All reconfigured
    spaces intended for occupancy and all spaces converted to
    habitable or occupiable space in any work area shall be provided
    with natural or mechanical ventilation in accordance
    with the International Mechanical Code.
    Exception: Existing mechanical ventilation systems shall
    comply with the requirements of Section 809.2.
    This section is consistent with the basic premise for
    new building elements and materials given in Chapter
    7, Section 702—that replacing elements is required
    to comply with the applicable code for new construction.

    The amount of mechanical ventilation must be
    provided in accordance with the IMC as required for
    new construction. This could be that air-moving
    equipment would need to be altered, as well. However,
    the exception allows special minimum provisions
    for existing mechanical equipment that would
    not require increasing the capacities for existing airmoving
    equipment, as long as minimum ventilation
    rates are provided.
    809.2 Altered existing systems. In mechanically ventilated
    spaces, existing mechanical ventilation systems that are
    altered, reconfigured, or extended shall provide not less than
    5 cubic feet per minute (cfm) (0.0024 m3/s) per person of outdoor
    air and not less than 15 cfm (0.0071 m3/s) of ventilation
    air per person; or not less than the amount of ventilation air
    determined by the Indoor Air Quality Procedure of ASHRAE
  4. rgrace

    rgrace Sawhorse

    Feb 18, 2010
    Likes Received:
    steveray, your reference appears to come from the existing building code? (the rehabilitation code?). Anyway, along that same line, 801.2 states that in addition to the requirements of this chapter, all work shall comply with Chapter 7. 701.2 states that an existing building or portion thereof shall not be altered such that the building becomes less safe than its existing condition. With reference to 809.2 (and for example an office space), if the existing mechanical system was designed, permitted and installed per the 1987 mechanical code, you could not reduce the outdoor air down to 5 cfm nor the ventilation air to 15 cfm. This would cause the building to become less safe than its existing condition of 6.6 and 20 (conference rooms required even more, 11.55 and 35). Even the 1993 code jumped outdoor air up to 20 cfm (removing the ventilation air requirement but allowing recirculation through different means). Can't reduce to 5 cfm without causing the building to become less safe. Best to see if current code would work better on any buildings designed under the 1993 through 2006 code editions. 1971 through 1975 code was 7.5 cfm per person, and didn't separate occupant classifications. Can't reduce to 5 cfm.

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