CHAPTER 4

1 CHAPTER contains requirements forventilation air supply, exhaust, and makeup-air requirementsfor occupiable spaces within a Ventilation General spaces listedin Table shall be designed to have ventilation (outdoor)air for occupants in accordance with this Construction outdoor airventilation rate and air distribution assumptions made inthe design of the ventilation system shall be clearly iden-tified on the construction Dwelling. Requirements for ventilation air ratefor single family dwellings shall be in accordance withthis CHAPTER or ASHRAE Natural of nNatural ventilationsystems shall be designed in accordance with this section andshall be permitted in lieu of or in conjunction withincludemechanical ventilation systems designed in accordance withSection and Section [ASHRAE ]Exceptions.

2 (1)An engineered natural ventilation system whenwhereapproved by the Authority Having Jurisdiction need notmeet the requirements ofcomply withSection and Section (2)A mechanical ventilation system is not required wherenatural ventilation openings comply with the require-ments of Section and are permanently open or havecontrols that prevent the openings from being closedduring occupancy.(3)A mechanical ventilation system is not required wherethe zone is not served by heating or cooling Location and Size of spaces shall be permanently open to andwithin 25 feet (7620 mm) ofby way of operable wall orroofopenings directly to the outdoors, the openable areaof which is a minimum of 4 percent of the net occupi-able floor area.

3 Where the openings are covered withlouvers or otherwise obstructed, openable area shall bebased on the free unobstructed area through the interior spaces without direct openings to theoutdoors are ventilated through adjoining rooms, theopening between rooms shall be permanently unob-structed and have a free area of not less than 8 percent ofthe area of the interior room nor less than 25 square feet(ft2)( m2). [ASHRAE ] Control and means toopen required operable openings shall be readily acces-sible to building occupants wheneverwherethe space isoccupied. Controls shall be designed to coordinate oper-ation of the natural and mechanical ventilation system.

4 [ASHRAE ] Mechanical natural ventilationis not permitted by this section or the building code, mechan-ical ventilation systems shall be designed, constructed, andinstalled to provide a method of supply air and exhaust ventilation systems shall include controls, manualor automatic, that enable the fan system to operate whereverthe spaces served are occupied. The system shall be designedto maintain minimum outdoor airflow as required by under any load outsidedoor-air inletsintakesshall be covered witha screen having not less than 1 4of aninch ( mm) openings,and shall have not more than 1 2 inch ( mm) : An outside-air inlet serving a nonresidentialportion of a building shall be permitted to be covered with ascreen having openings larger than 1 4 of an inch ( mm),but in no case exceeding 1 inch ( mm).

5 Weather Protections. Outdoor air intakesthat are part of the mechanical ventilation system shall bedesigned to manage rain entrainment, to prevent rainintrusion, and manage water from snow in accordancewith ASHRAE Ventilation design outdoor air intake flow rate fora ventilation system shall be determined in accordance withSection Section parameters shall bedetermined in accordance with Section throughSection [ASHRAE ] Breathing Zone Outdoor outdoor airflow required in the breathing zone ofthe occupiable space or spaces in a zone, , thebreathing zone outdoor airflow(Vbz)

6 , shall be deter-mined in accordance with Equation [ ]Vbz= RpPz+ RaAz(Equation )Where:Az=zone floor area: the net occupiable floor area ofthe zone in square feet (m2).Pz=zone population: The largest number of peopleexpected to occupy the zone during typical usage. Ifthe number of people expected to occupy the zonefluctuates, Pzshall be permitted to be estimatedbased on averaging approaches described in If Pzcannot be accurately predictedduring design, it shall be estimated based on thezone floor area and the default occupant densitylisted in Table MECHANICAL CODE35 CHAPTER 4 VENTILATION AIR SUPPLYPRE-PRINT36 UNIFORM MECHANICAL CODERp=outdoor airflow rate required per person as deter-mined from Table airflow rate required per unit area as deter-mined from Table [ASHRAE ] Zone Air Distribution air distribution effectiveness (Ez)

7 Shall be deter-mined using Table [ASHRAE ] Zone Outdoor design zoneoutdoor airflow (Voz), , the outdoor airflow that mustshallbe provided to the zone by the supply air distributionsystem, shall be determined in accordance with [ASHRAE ]Voz= Vbz/Ez(Equation ) one air handlersupplies a mixture of outdoor air and recirculated air to onlyone zone, the outdoor air intake flow (Vot)shall be deter-mined in accordance with Equation [ ]Vot= Voz(Equation ) Hundred Percent Outdoor Air one air handler supplies only outdoor air to one ormore zones, the outdoor air intake flow (Vot)shall be deter-mined in accordance with Equation [ ]Vot= all zonesVoz(Equation ) Recirculating air handler supplies a mixture of outdoor air and recircu-lated return air to more than one zone, the outdoor air intake flow(Vot)shall be determined in accordance with Section Section [ASHRAE ] Primary Outdoor Air is used to determine system ventilationefficiency, tThe zone primary outdoor air fraction(Zpz)

8 Shall be determined for ventilation zones in accordancewith Equation [ASHRAE ]Zpz= Voz/Vpz(Equation )Vpzis the primary airflow to the ventilation zone fromthe air handler, including outdoor air and recirculatedreturn air. [ASHRAE ] System Ventilation ventilation efficiency(Ev)shall be determinedusinginTable Section [ ] Uncorrected Outdoor Air uncorrected outdoor air intake(Vou)shall bedetermined in accordance with Equation (a1).Vou= D all zones RpPz+ all zones RaAz(Equation (a1))The occupant diversity, D, shall be permitted to be usedto account for variations in occupancy within the zonesserved by the system.

9 The occupancy diversity is defined asdetermined inaccordance with Equation (2):D = Ps/ all zones Pz(Equation (b2))Where the system population (Ps)is the total populationin the area served by the system. Alternative methods shallbe permitted to be used to account for population diversitywhen calculating Vou, provided that the resulting value isno less than that determined by Equation (a1).[ASHRAE ] Outdoor Air design outdoor airintake flow (Vot)shall be determined in accordance withEquation [ASHRAE ]Vot= Vou/Ev(Equation ) for Varying Operating Variable Load systemsshall be designed to be capable of providing the requiredventilation rates in the breathing zone whenever the zonesserved by the system are occupied, including all full and part-load conditions.

10 [ASHRAE ] it isknown that peak occupancy will be of short duration orthe ventilation rate will be varied or interrupted for ashort period of time, the design shall be permitted to bebased on the average conditions over a time period (T)determined byin accordance withEquation [ASHRAE ]:T=3v/Vbz(4-950v/Vbz) IP(Equation )Acceptable design adjustments based on thisoptional provision shall be in accordance with thefollowing [ASHRAE ]:(1)Zones with fluctuating occupancy: The zone popu-lation (Pz) shall be averaged over time (T).(2)Zones with intermittent interruption of supply air:The average outdoor airflow supplied to thebreathing zone over time (T) shall be not less thanthe breathing zone outdoor airflow (Vbz) calculatedusing Equation AIR SUPPLYPRE-PRINT37 UNIFORM MECHANICAL CODEVENTILATION AIR SUPPLY(3)Systems with intermittent closure of the outdoor airintake: The average outdoor air intake over time (T)shall be not less than the minimum outdoor airintake (Vot) calculated using Equation , ,or.