HVAC ducts and airflow

Airflow — the furnace’s output and temperature rise give the heating airflow (1.08 × cfm × ΔT); cooling comes from the sensible load or the 400 cfm a ton rule of thumb. List the rooms with their heating and sensible cooling loads, how many outlets each has, where its branch leaves the trunk, the branch length and its fittings' equivalent length. Import rooms from the heat-loss tool brings in the rooms and loads you entered in heat loss and HVAC sizing. Each room gets its share of the airflow by heating load and by cooling load, and takes the larger.

Static pressure — the blower’s external static pressure at the design airflow, less the coil, filter, registers, grilles and dampers, is what the ducts can use. Divided over the longest supply run plus the longest return run (total effective length), it gives the design friction rate. Enter points from the blower table to see where the fan curve meets the system curve.

Ducts — every branch, each section of a reducing supply trunk and the returns, sized round at the friction rate and upsized where the velocity would be too high, with rectangular trunk sizes for the height you choose. The layout is drawn as a diagram. The table shows how much pressure each shorter run has spare — what its balancing damper takes up.

Registers & grilles — free and gross area for a face velocity, and a common nominal size.

Round ↔ rectangular — Huebscher’s equivalent diameter both ways, with friction and velocity.

Friction in round sheet-metal duct is 0.109136 q^1.9 ÷ d^5.02 in. w.c. per 100 ft (the curve fit to the standard friction chart). Rectangular ducts use Huebscher’s De = 1.30 (ab)^0.625 ÷ (a + b)^0.25. Velocity limits (supply trunk 900 fpm, branches 900, return trunk 700, return branches 700 at most) and fitting equivalent lengths are typical residential practice — check Manual D’s tables for the fittings you’ll use. The system curve is pressure ∝ airflow².

Related: plumbing sizing, fluid flow, dwelling load and generator sizing (running a furnace blower or AC on standby power).

Any current browser, on a phone or a desktop; it works offline once loaded.

Everything runs in your browser and your rooms and ducts are kept in its local storage. Importing from the heat-loss tool reads that tool’s saved rooms from the same browser storage. Nothing is sent anywhere; Share puts the design in the link itself.