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The Ventilation Rate page is based on Manual J's Table 8A.  It helps you determine the amount of ventilation air that may need to be added to the return side of the air handler.  If the amount of fresh air being brought into the building through infiltration is insufficient, it is recommended that ventilation air be introduced.  This page evaluates three types of requirements for outside air (see a, b and c in the picture below), takes the largest of those three, then compares that to the unadjusted amount of infiltration (cooling infiltration is used as a base for comparison).  Then if the infiltration is insufficient to meet the requirement, a CFM of ventilation is suggested to make up for the difference.

Ventilation requirements may be specified by a code or regulation.  If there are no local mandates, guidance is provided by industry standards and model codes.  The ventilation rate shown on the Ventilation Rate page is the Manual J default rate for heating and cooling load estimates, but it should never be used to determine the ventilation rate for health and safety.

Return-side mechanical ventilation is still usually not installed on residential systems.  If no ventilation will be installed on the current system, enter zero.  If ventilation will be installed, enter the amount that will actually be installed (do not simply use the Manual J default rate).

Note that the amounts you enter for ventilation and exhaust will affect the amount of infiltration you entered for this system.  If ventilation is greater than exhaust, the system will be under positive pressure and the infiltration will be reduced.  If exhaust is greater than infiltration, the system will have negative pressure and the amount of infiltration will be increased above the amount that you specified.

Above Grade Volume of Conditioned Space:  Allows you to override the volume of the conditioned space, which defaults to the larger of the heating or cooling above grade volume.

Number of People in System:  Allows you to change the number of people in this system if it is different than the total number you have entered for the rooms in this system.

Heating with Atmospheric Combustion:  If this system is heated by a furnace or boiler with a burner where the combustion air comes from the conditioned space, enter Yes in this input.  The amount of combustion air required will be equal to 0.50 times the input Btuh of the furnace or boiler divided by 1000.

Input Btuh of Furnace or Boiler:  If this system is heated by a furnace or boiler with a burner where the combustion air comes from the conditioned space, enter the input Btuh of the unit in this input.  The amount of combustion air required will be equal to 0.50 times the input Btuh of the furnace or boiler divided by 1000.

a)  General, 0.35 AC/hr:  Displays the amount of outside air required in order to have 0.35 air changes per hour, an amount considered by ASHRAE to be the minimum required amount of fresh air needed for a home.  The volume used in the calculation of this value is that of the Above Grade Volume of Conditioned Space input in the top section of this page.

b)  20 CFM/Person, Based on [x] People:  Displays the amount of outside air needed in order to provide 20 CFM of fresh air for each occupant in the building.  The number of people used in this calculation is the value entered in the input in the top section of this page.

c)  Combustion Air CFM Required:  Displays the amount of fresh air required for the furnace or boiler if its burner uses atmospheric combustion from within the conditioned space.  This value is equal to 0.50 times the input Btuh of the unit, divided by 1000.

d)  Required Outside Air (largest of a, b and c):  Displays the largest of the three preceding values, which is the amount of outside air required from both infiltration and ventilation.

e)  Cooling Infiltration CFM:  Displays the unadjusted cooling infiltration CFM calculated from the above grade cooling volume and the value you entered in the Summer Infiltration input on the System Data page.

f)  Half of Heating Infiltration CFM:  Displays one-half of the unadjusted heating infiltration CFM calculated from the above grade heating volume and the value you entered in the Winter Infiltration input on the System Data page.

g)  Smaller of e and f (if non-zero):  Displays the smaller non-zero number of lines e and f, directly above.

h)  Ventilation CFM Suggested (d minus g):  Displays the amount of additional fresh air needed to make up for insufficient infiltration.

Ventilation, Winter and Summer:  Specifies the amount of ventilation to use for this system.  The amount you enter for ventilation and exhaust will affect the amount of infiltration you entered for this system.  If ventilation is greater than exhaust, the system will be under positive pressure and the infiltration will be reduced.  If exhaust is greater than infiltration, the system will have negative pressure and the amount of infiltration will be increased above the amount that you specified.