How Many CFM per Ton?
The common answer is 400 CFM per ton. Here is where it comes from, when to move off it, and how to share the air out to the rooms.
The short answer
350, 400 and 450 CFM per ton are a common rule of thumb, with 400 the usual starting point. ACCA Manual D ties the airflow to the sensible heat ratio of the load: roughly 0.70 gives 350, 0.80 gives 400 and 0.90 to 1.00 gives 450 or more. A humid climate asks the coil to remove more moisture, so the air is run slower across it. It also ties the airflow to the equipment maker’s performance data. That is why 400 is the default in the CFM per ton calculator.
The calculation itself is simple: total CFM = tons × CFM per ton. One ton is 12,000 Btu/h of cooling.
| Tons | 350 CFM/ton | 400 CFM/ton | 450 CFM/ton |
|---|---|---|---|
| 1.5 | 525 | 600 | 675 |
| 2 | 700 | 800 | 900 |
| 2.5 | 875 | 1,000 | 1,125 |
| 3 | 1,050 | 1,200 | 1,350 |
| 3.5 | 1,225 | 1,400 | 1,575 |
| 4 | 1,400 | 1,600 | 1,800 |
| 5 | 1,750 | 2,000 | 2,250 |
Nominal tons are not measured airflow
The tonnage on the nameplate is a nominal size. The airflow the blower really moves depends on the speed tap, the filter, the coil and the static pressure of the duct system. Before you trust a number, look up the blower table in the manufacturer’s data and read the CFM at the external static pressure you actually have. If the equipment instructions give an airflow range, stay in it.
When to move off 400
Treat 400 as the middle of a range. In a humid climate, a system that moves too much air across the coil can leave the house clammy, which is why the rule of thumb drops toward 350 there. Manual D warns that a set-point that reduces to 350 CFM per ton is not appropriate for most US homes, so do not go that low without a reason. In a dry climate there is little moisture to remove, so a higher airflow is fine. Variable speed and multi-stage equipment is often set up differently again, so read the installation instructions for the target airflow before you pick a figure. Whatever you choose, the duct has to be able to carry it at the static pressure the blower can handle, and that is what the duct sizing step checks.
Working from the load
If you have a sensible load for the space, you can get the airflow from it. At sea level: CFM = sensible load in Btu/h ÷ (1.1 × temperature difference in °F). For example, 24,000 Btu/h sensible with a 20 °F air temperature difference gives 24,000 ÷ (1.1 × 20) = about 1,091 CFM. The example is for illustration only. At altitude the 1.1 factor changes, and Manual D has a correction for it.
Splitting the air across rooms
Manual D shares the blower airflow out by room load: room CFM = blower CFM × room load ÷ total load. It does this separately for heating and cooling, and each room is designed for the larger of the two. If you only have percentages, the calculator splits the air by those instead. Use the CFM per ton calculator to do either one, then press Size on a row to open the duct size calculator with that CFM.
What to do next
- Get the total CFM for the system and check it against the blower table.
- Split it across rooms by load.
- Choose a friction rate. The friction rate guide shows how.
- Size each run in the duct size calculator and check the velocity.
Limits
Rules of thumb are a starting point. A Manual D and J calculation, the design documents, the manufacturer’s data and local code come first.