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TECHNICAL TOOL / 11
Air Changes per Hour Calculator
Convert between ACH, airflow in m³/h or CFM, and room volume. Separate outdoor-air exchange from recirculation before estimating warehouse, factory or greenhouse moisture load.
VOLUME FLOW RELATIONSHIP
- Air changes per hour
- 0.500 h⁻¹
- Room volume
- 300.00 m³
- Airflow
- 150.00 m³/h
- Cubic feet per minute
- 88.29 CFM
For the example, enter 300 m³ and 0.5 outdoor ACH in the sizing calculator. The link opens the calculator; values must be entered manually.
Check the active inputs. Volume must be positive; airflow and ACH cannot be negative. Inferring volume requires positive airflow and ACH.
Use in the sizing calculator01 / IDENTIFY THE AIR PATH
The same ACH can describe different air paths.
Supply and return circulation
A dehumidifier or air handler can circulate room air many times without replacing it with outdoor air. This circulation rate helps describe distribution, but is not an outdoor moisture source.
Outdoor air and infiltration
Only the outdoor-air or infiltration portion belongs in the sizing calculator’s air-exchange moisture load. Its effect depends on incoming versus target humidity ratio. Account separately for people, crops and processes.
Mixed-air systems
Separate the measured outdoor-air fraction from total supply flow. Do not add supply and return flows as two independent exchanges, or count door leakage again if it is already included.
02 / METHOD & EXAMPLES
Calculation method and sources
ACH = Q (m³/h) / V (m³)
Q = ACH × V; V = Q / ACH
1 CFM = 1.69901079552 m³/h
ACH = 60 × Q (CFM) / V (ft³)
Q = ACH × V; V = Q / ACH
1 CFM = 1.69901079552 m³/h
ACH = 60 × Q (CFM) / V (ft³)
ACH is a volumetric flow definition, independent of humidity or temperature when both volumes use the same air-condition basis. For a 300 m³ warehouse, 150 m³/h outdoor airflow is 0.5 ACH. A separate 1,800 m³/h recirculation loop is 6 ACH of circulation; it must not be entered as 6 outdoor ACH. These are illustrative flows, not recommended ventilation rates.
For the subsequent moisture balance, the sizing calculator uses ASHRAE air properties and PsychroLib reference equations: dry-air mass flow × humidity-ratio difference. Convert measured incoming volume to the room-condition volume basis used by that calculator. Use the actual incoming state after any upstream conditioning.
ASHRAE Fundamentals (2017), Chapter 1 · PsychroLib — air-property and moisture-balance references, not a ventilation-rate prescription.
What this tool does not decide
It does not specify a required ventilation rate, verify room mixing or contaminant removal, determine leakage from a fan rating, or select dehumidifier capacity. Actual airflow needs measurement; moisture sizing additionally needs air states and internal water sources.
NEXT / PROJECT REVIEW
Bring the operating conditions to an equipment review.
Share inlet and target air states, room volume, outdoor airflow, moisture sources and installation constraints.
Review project requirements