Ventilation Fresh Air Calculator

Purpose and model boundary

Calculates a whole-dwelling outdoor-air target from editable area and room rates, an entered target, or a CO2 mass balance, then checks installed airflow and a complete local-exhaust schedule.

Do not treat this result as an energy-code determination, Manual J, HVAC or ventilation design, moisture-control guarantee, structural or concrete design, permit approval, equipment selection, cost quote, or endorsement by a cited source. Site climate, assemblies, room use, ventilation, windows, material properties, geometry, manufacturer instructions, locally adopted codes, and approval thresholds remain the user's responsibility.

Inputs and units

• target basis: a listed route or text value.
• co2 occupancy basis: a listed route or text value.
• credit treatment: a listed route or text value.
• floor area: entered in ft^2.
• sleeping rooms: entered in rooms.
• area rate: entered in cfm per 100 ft^2.
• occupant rate: entered in cfm/person.
• entered target flow: entered in cfm.
• entered occupants: entered in people.
• outdoor co2: entered in ppm.
• indoor target co2: entered in ppm.
• co2 generation rate: entered in L/s-person.
• verified credit: entered in cfm.
• installed whole dwelling flow: entered in cfm.
• dwelling volume: entered in ft^3.

The local exhaust grid contains 10 fixed data rows plus one header row with these columns: Include, Location label, Space type, Operation, Installed flow (cfm), Required flow (cfm), Runtime minutes/day (min/day), Discharges outdoors, Source note. Numeric columns retain the units shown here; text, route, and include columns are intentionally unitless.

Governing relationships

The area-and-room route is:

Q_area = 0.01 A r_A + (N_bed+1) r_occ

Q_area is in cfm, A is floor area (ft^2), r_A is the entered area rate (cfm per 100 ft^2), N_bed is sleeping-room count, and r_occ is the entered occupant rate (cfm/person). The entered-target route uses its cfm input directly.

For the CO2 route, concentration difference and outdoor-air target are:

DeltaC = (C_in-C_out)10^-6

Q_CO2 = N_occ G_CO2 2.118880003/DeltaC

DeltaC is a fraction and Q_CO2 is in cfm. C_in and C_out are ppm, N_occ is entered occupants or sleeping rooms plus one, G_CO2 is generation per person in L/s, and 2.118880003 converts L/s to cfm. Net target is:

Q_net = max(0,Q_gross-Q_credit)

Installed margin and installed air changes are:

Margin = Q_installed-Q_net

ACH = 60 Q_installed/V_dwelling

Margin is in cfm and ACH is in 1/h, where dwelling volume is ft^3. A continuous local-exhaust row contributes its installed cfm to daily average; an intermittent row contributes:

q_avg,j = q_j t_j/1440

t_j is minutes/day. A row is deficient when installed flow is below entered required flow or it is not marked as discharging outdoors.

Calculation sequence

1. Validate every active selector, scalar bound, type, and every fixed-grid entry.
2. Apply the selected route and ignore values from inactive fields.
3. Normalize unit-dependent inputs into the workbook's calculation basis.
4. Evaluate every included row or candidate and aggregate the active schedule.
5. Calculate primary and supporting outputs, populate the authored chart series, and apply derived numeric checks.
6. Return the workbook status with the computed results; invalid input is never converted into a plausible engineering answer.

Outputs and interpretation

• selected gross target (cfm): Gross airflow produced by the selected target route before any verified credit.
• selected net target (cfm): Selected gross target less any active verified credit, floored at zero.
• installed flow margin (cfm): Installed whole-dwelling flow minus selected net target; negative means the entered target is not met.
• deficient exhaust count (rows): Included local exhausts below entered required flow or not marked as discharging outdoors.

The chart is an explanatory view of the workbook's named chart ranges. It does not add a separate calculation or replace the numeric outputs.

Validation and status logic

The workbook evaluates Model_Status in the order shown. The first matching condition wins.

Assumptions and limitations

• All shipped coefficients, exhaust requirements, labels, and rows are synthetic and illustrative.

• The CO2 route assumes steady state and perfect mixing.

• Installed flows are measured or independently verified.

• Pressure balance, makeup air, combustion safety, filtration, radon, smoke, duct losses, and commissioning are not modeled.

• The workbook is not a ventilation design or code-compliance determination.

Restrictions and non-computing states

Each numeric control offers a working range chosen for usable entry, which is not always the limit this calculator enforces; a value outside that limit is rejected with a message rather than quietly accepted. The calculator additionally enforces allowed choices, active-field types, complete fixed-grid rows, and conditional or cross-field rules before computing.

Unsupported route values, incomplete rows, out-of-domain inputs, relational failures, nonfinite derived arithmetic, and workbook error tokens are non-computing states. Hidden inputs remain in the values sent to the calculator but must not affect results while those fields are inactive.

Errors and warnings

A rejected entry means the submitted state failed the published input rules. Workbook NOT VALID identifies an invalid active domain or unsupported derived arithmetic. Workbook CHECK retains a finite result while flagging a planning condition that needs review. OK means only that the delivered workbook logic closed within the entered assumptions; it is not code compliance, professional approval, or a safety guarantee.

This version is checked against 107 saved test cases, covering both calculated results and entries that should be refused, run against this exact workbook.

• U.S. Environmental Protection Agency indoor-air-quality overview - Primary public context for indoor air quality and the limits of simplified ventilation screening. Official public U.S. government source; no copied standard, requirement table, text, or figure is embedded.
• U.S. Environmental Protection Agency indoor-air improvement guidance - Primary public context for source control, ventilation, and filtration as separate strategies. Official public U.S. government source; no copied text, table, figure, or endorsement is embedded.
• NIST amount-of-substance and gas-unit guidance - Primary public metrology context for gas concentration and unit conversion. Official public U.S. government metrology source; unit arithmetic is independently implemented without copied text, tables, or figures.

Licensing conclusion: No licensed, proprietary, manufacturer, utility, product-price, code-table, climate table, material table, forecast, or external dataset is embedded. The calculator independently implements general heat-transfer, moisture, airflow, energy, geometry, and quantity arithmetic from cited public and official sources; all defaults, labels, property rows, equipment assumptions, diagrams, and test values are synthetic illustrations. No ventilation-rate, local-exhaust, occupancy, or metabolism table is reproduced.