Airside Load Calculator Airflow
The airside workhorse trio, solved in any direction.
qs = 1.08 × CFM × ΔT and its latent and total siblings tie the air a fan moves to the heat a coil transfers.
Solve in any direction — the load a measured airflow and ΔT imply, the airflow a reheat schedule needs, or the ΔT a healthy coil should be showing.
This is the airside twin of the waterside-load tool: same coil, other side.
Inputs are seeded with an example — edit them to your numbers.
Input
Enter ΔT as a positive difference — entering-to-leaving on a heating coil, leaving-to-entering on cooling; the math is the same either direction.
Sensible only — temperature change alone. A wet cooling coil is also moving latent heat; that’s the other two tabs. Tons live on the Total tab, because a ton is 12 MBH of total heat.
Output
Worked example
A hot-water coil takes 2,000 CFM of supply air and leaves it 20 °F warmer.
- The sensible equation:
qs = 1.08 × CFM × ΔT°F, in Btu/h. - Plug in:
qs = 1.08 × 2000 × 20 = 43,200 Btu/h = 43.2 MBH. - The same heat left the water. At a 12 °F water ΔT that’s
Q = 43,200 ÷ (500 × 12) = 7.2 GPM— run it on the Waterside Load Calculator. The two sides of one coil have to agree; when they don’t, one of the readings is lying.
Input
Δ is entering minus leaving humidity ratio — grains of moisture per pound of dry air; 7,000 grains make a pound. Read both states off the psych chart or the dew-point calculator.
Output
Worked example
A cooling coil wrings 10 gr/lb of moisture out of the same 2,000 CFM.
- The latent equation:
ql = 0.68 × CFM × Δgr/lb, in Btu/h. - Plug in:
ql = 0.68 × 2000 × 10 = 13,600 Btu/h = 13.6 MBH. - Every grain the coil removes is water condensing on the fins and giving up its latent heat — heat the compressor has to move even though no thermometer in the airstream sees it.
Input
Δh is entering minus leaving enthalpy. Starting from temperatures and humidity instead? The coil-sizing calculator computes the air states and splits the load into sensible + latent for you.
Output
Worked example
Same coil, all in: the entering and leaving air read 6.3 Btu/lb apart in enthalpy.
- The total equation:
qt = 4.5 × CFM × Δh, in Btu/h. - Plug in:
qt = 4.5 × 2000 × 6.3 = 56,700 Btu/h = 56.7 MBH. - In chiller language:
56.7 ÷ 12 = 4.7 tonsmoving through that one coil.
Cross-check against the other tabs: sensible 43.2 + latent 13.6 = 56.8 MBH against 56.7 here — the missing 0.1 is the rounded Δh (the chart really read closer to 6.31). Sensible + latent = total is the cross-check worth keeping — when the split misses by more than rounding, one of the three readings is lying.
Divide the airflow by the tons and you’ve got the DX sanity number — 2000 ÷ 4.7 ≈ 425 CFM per ton here. The trade’s floor sits near 400; the VAV Systems lesson walks the icing runaway below it.
Standard air, and where 1.08, 0.68, and 4.5 come from
All three constants are standard air — 0.075 lb/ft³, sea level, ~70 °F — folded into pocket-card form. 1.08 = 0.075 lb/ft³ × 60 min/h × 0.240 Btu/lb·°F (density × time × specific heat). 4.5 = 0.075 × 60 — it just turns CFM into pounds of dry air per hour. 0.68 = 4.5 × 1,060 Btu/lb of latent heat ÷ 7,000 grains per pound.
The US and metric constants are independently rounded trade conventions, not conversions of each other — the exact metric twin of 1.08 is 0.335, not 0.34 — so the two systems disagree by roughly a percent by construction. Each system is internally consistent, and the formula line always closes on the numbers shown. At altitude the air is thinner, each CFM carries less heat, and every constant on this page over-reads the load by roughly 3 % per thousand feet of elevation — call it 14 % in Denver; the psych chart carries a proper altitude input.
This page is the pocket-card arithmetic for when you already hold the deltas. Starting from full air states — dry-bulb and humidity in, dry-bulb and humidity out — the coil-sizing calculator does the psychrometrics properly and hands back total, sensible, latent, and SHR in one pass.