Tools · Equipment Airflow

Equipment Airflow Check Airflow

Is that airflow inside what the machine can live with?
A DX coil wants roughly 400–500 CFM per ton of running compressor; a gas heat exchanger wants its nameplate temperature rise. Both are pass/fail checks on a number you already measured — this page runs them. The free-form heat arithmetic (any q / CFM / ΔT, any direction) lives on the Airside Load Calculator; this page only judges the result against equipment limits. Both tabs are IP-native, like the K-factor — CFM per ton and °F rise are the frames the trade speaks.

Inputs are seeded with an example — edit them to your numbers.

Input

Capacity is the nameplate cooling rating. Equal stages are assumed in N-of-M mode; for unequal stages, a digital scroll, or an unloader, flip the staging entry to capacity %.

The airflow should be a measured number — a traverse or the blower table — not the design drawing. The check is only as honest as its worst input.

Output

Enter values to run the check.
CFM per active ton —
Active capacity (tons) —
Per nameplate ton (ignores staging) —

Worked example

A 10-ton rooftop on a mild afternoon: one of its two stages is running, and a traverse reads 2,400 CFM.

  1. The panic number first: 2400 ÷ 10 = 240 CFM per ton — read against the nameplate, that looks like an icing emergency.
  2. But only half the machine is on. Active capacity: 10 × 1 ÷ 2 = 5 tons.
  3. Per active ton: 2400 ÷ 5 = 480 CFM — inside the 400–500 window. The unit is healthy; the panic came from dividing by the wrong denominator.
  4. The direction that matters: if the supply fan turns down further while that stage holds on, 480 falls toward 350 — the coil floor, and past it the icing runaway.

Input

The rise window is printed on the furnace rating plate (a typical plate reads “RISE 30–60”). Measured rise is supply minus return air temperature, both probes out of the heat exchanger’s line of sight — radiant heat on the supply probe reads high.

Output

Enter values to run the check.
Allowable airflow band (CFM) —
Heat output (MBH) —
Implied airflow at measured rise —

Worked example

An 80 % furnace rated 100 MBH input, nameplate rise 30–60 °F. Supply minus return measures 44 °F.

  1. Output is what reaches the air: q = 100 × 80 % = 80 MBH = 80,000 Btu/h. The other 20 % went up the flue.
  2. The rise window is an airflow band in disguise — CFM = q ÷ (1.08 × ΔT). At the 60 °F max: 80,000 ÷ 64.8 = 1,235 CFM; at the 30 °F min: 80,000 ÷ 32.4 = 2,469 CFM. Note the inversion — the max rise sets the minimum airflow.
  3. The measured 44 °F turns the same equation into a flow measurement: 80,000 ÷ 47.52 = 1,684 CFM — inside the band, and you just measured airflow with two thermometers and no traverse.

Where the limits come from — and the check this page can’t do

The CFM/ton window is a design convention, not physics with a derivation. DX coils are engineered around roughly 450 CFM per ton of running compressor; the trade’s floor sits near 400. Starve a running stage much below ~350 and suction pressure dives, the coil surface drops below freezing, and condensate freezes on instead of dripping off — ice chokes airflow further, which starves the coil further. High-latent designs run 350–400 deliberately (slower air wrings out more moisture); past ~500 the coil runs warm, dehumidification fades, and face velocity starts blowing condensate off the fins into the supply duct — carryover. Metric aside: 400 CFM/ton is roughly 54 L/s per kW of cooling.

The rise window protects the heat exchanger from both directions. Too little air (rise above the window) and the exchanger overheats — the high-limit switch trips and short-cycles the burner, and repeated overheating is how heat exchangers crack. Too much air (rise below the window) holds the exchanger surfaces cold enough that flue moisture condenses inside it — on a non-condensing furnace that condensate is mildly acidic and eats the exchanger over seasons, not minutes. The window on the plate is the manufacturer’s statement of where neither failure is in play. Both tabs compute in IP; at altitude the same CFM carries less air mass, so a real rise runs hotter than the sea-level math predicts and the band’s CFM numbers read low by roughly 3 % per thousand feet.

The third check — static pressure vs. airflow — is deliberately not a tab here. For a fixed-speed blower, the mapping from total external static to CFM is the blower curve, and that is per-model vendor data no generic tool can reconstruct — the affinity laws rescale a known point between speeds, but they can’t conjure the curve from nothing. The field method: measure total external static (supply static plus return static across the unit), open the blower table in the installation manual at the running fan tap, and read the CFM. That number is a legitimate input to both tabs above.

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