Room & Airflow
Result
Air changes per hour (ACH) measures how many times the entire air volume of a room is replaced by the HVAC system in one hour. It is a measure of ventilation intensity that works alongside CFM and tonnage to describe how a system performs in a specific room. The same 600 CFM air handler produces very different ACH values in a small bedroom versus a large open-plan space, because the room volume changes while the airflow stays the same.
| Space | Recommended ACH | Why |
|---|---|---|
| Bedroom | 4 to 6 | Quiet operation preferred; higher ACH causes draughts |
| Living room | 6 to 8 | Standard residential comfort |
| Kitchen | 8 to 12 | High heat and humidity demand rapid air exchange |
| Home office | 6 to 8 | Equipment heat balances lower occupancy |
| Retail or commercial | 8 to 15 | Higher occupancy and activity levels |
| Server or equipment room | 12 to 30 | Very high heat density needs very frequent exchange |
| Bathroom | 6 to 10 | Humidity removal is the primary goal |
A 15 by 12 ft bedroom with a 9 ft ceiling. The supply register delivers about 120 CFM.
What is air changes per hour?
The number of times the total room air volume is replaced by the HVAC system in one hour. A room with 6 ACH has all its air refreshed six times every hour. Higher ACH means faster cooling, better humidity control and fewer stale air pockets.
How many ACH for a bedroom?
4 to 6 ACH for comfortable, quiet bedroom cooling. Below 4 can leave the room slow to cool; above 8 creates draughts from registers.
How do I calculate ACH from CFM?
ACH equals CFM times 60 divided by room volume in cubic feet. For 200 CFM in a 1,500 cu ft room: 200 times 60 divided by 1,500 equals 8 ACH.
What ACH do I need to cool a room effectively?
For residential rooms, 6 to 8 ACH. Kitchens need 8 to 12. If a room is hard to cool despite the right tonnage, low ACH from restricted airflow or an undersized branch duct is often the cause.
Does higher ACH mean better cooling?
Up to a point. Beyond about 12 to 15 ACH in a residential room, air velocity becomes uncomfortable and register noise increases with little further benefit.
ACH figures are for guidance. Local building codes specify minimum ventilation requirements for occupied spaces; comply with those requirements in all habitable areas.