Trade Math: Load and Airflow

Idea one

BTUs per hour, and the ton

One BTU is the amount of heat required to raise one pound of water by 1 degree Fahrenheit. But in HVAC we almost never deal with single BTUs. We deal with rates: BTU per hour. That “per hour” is the difference between an amount of heat and a heat flow, and it matches the real world, because heat is a continuous stream in and out through walls, windows, people, appliances and air leakage.

The field anchor is the ton. One ton of cooling equals 12,000 BTU per hour, a term from the old ice trade: the heat it takes to melt a ton of ice over a day. A “three-ton unit” does not cool three tons of anything; it moves heat at a rate, and that rate changes with conditions. Load calculation decides what rate a specific space actually needs.

Idea two

Where the load comes from

Load has two halves you met in lesson one: sensible load changes temperature. Latent load changes moisture content. A system must handle both. Load comes in by conduction through walls, ceiling and windows, solar gain through glass, infiltration and ventilation, and internal gains from people, lights and equipment. For conduction the textbook writes it as Q equals U times A times delta T: bigger area, bigger temperature difference or poorer insulation all mean more heat flow. You must pick indoor and outdoor design conditions, because load is driven by differences.

Solar gain is often the bully in the room. A west-facing window can turn late afternoon into a daily peak, and the customer hears it as “the unit can’t keep up after 4 p.m.” Infiltration arrives as an energy package carrying heat and water vapour, and on humid days it can dominate latent load.

That is why square-foot rules like “one ton per 500 square feet” can be dangerously wrong: they ignore climate, insulation, glass area, ceiling height, shading, air leakage and latent load. Trade math is about replacing luck with evidence.

Idea three

Oversized, sticky, and changed loads

Many comfort complaints exist because a system meets the sensible load but struggles with latent. An oversized system may satisfy temperature quickly and shut off before it runs long enough to pull moisture out. The space hits setpoint but feels sticky, the customer lowers the thermostat, and you get longer runs, higher bills and still poor humidity. Size only by sensible load and you can build that complaint into the job.

The load is also a diagnostic reference. When a homeowner says “it used to keep up, now it doesn’t,” ask the other half of the question: did the load change? New windows, lost shade, a darker roof, disturbed insulation, a return leak pulling attic air, someone now working from home. Sometimes the building started demanding more. Use runtime as a clue, not the only evidence.

Idea four

Airflow math

Airflow is measured in CFM, cubic feet per minute. A common starting target in comfort cooling is roughly 400 CFM per ton, closer to 350 in humid climates for more moisture removal and closer to 450 in dry climates. The exact number is not the point. Airflow is an intentional design and setup value, not a feeling like “it’s blowing pretty strong.”

Airflow connects straight to capacity: sensible BTU per hour equals 1.08 times CFM times the temperature change across the air handler. The chapter’s example: a 20°F drop at 1,200 CFM is 1.08 × 1,200 × 20 = 25,920 BTU per hour, a little over two tons of sensible cooling.

That is why a split alone can fool you. Low airflow often makes the split look better while total heat moved drops, and it pushes the coil toward icing. And “it’s blowing cold, but the house won’t cool” is perfectly consistent with low airflow, duct leakage or poor distribution.

Idea five

Static pressure, ducts and software

Total external static pressure is what the blower must overcome through the filter, coil, ducts, registers and grilles, measured in inches of water column with a manometer. High static is the air-side version of a dirty condenser coil. Break it into parts: a high filter drop points to restriction at the filter or a return too small; a high coil drop to a dirty or loaded coil; supply-dominated static to undersized trunks, crushed flex or closed dampers; return-dominated static to undersized or blocked returns.

Ducts are sized to carry the needed CFM without forcing the blower into an unhappy operating point. A sharp elbow can behave like many feet of straight duct, and flex duct that is not stretched tight has dramatically higher friction: a run draped like a hammock can cut a room to a trickle. A leaking return in an attic becomes a hidden load generator.

Load software such as ACCA Manual J tools speeds the math, but garbage in, garbage out. If it says a drafty house with single-pane windows needs a tiny system, challenge it: check infiltration, window specs and duct location before you believe it.