HVAC Calculators

This HVAC cluster covers the early heating and air-distribution checks that usually happen before a final duct layout or equipment submittal. Start with the mini-split sizing calculator for a ductless room-by-room BTU/h screen from area, ceiling height, design temperatures, sun exposure, envelope quality, occupants, equipment gains and margin. Use the furnace BTU calculator when the question is central heating equipment size: it screens heat loss from floor area, BTU/h per square foot, design temperature difference, AFUE and sizing margin, then estimates supply airflow from temperature rise. For coil or airflow checks, the HVAC airflow and BTU load calculator uses q = ρ·cₚ·Q·ΔT — the same relation behind BTU/h ≈ 1.08 × CFM × Δ°F — and reports required CFM, required ΔT, cooling tons and CFM per ton. For water-side loops, the BTU/GPM Delta T calculator applies q = rho·cp·Q·Delta T and the 500 × GPM × Delta F water shortcut to return hydronic load, required GPM and required Delta T.

Once airflow is known, the duct size calculator turns CFM into round and rectangular duct dimensions by either target velocity or equal-friction sizing. It also calculates straight-duct velocity, pressure drop and friction rate, so a branch or trunk can be screened before fittings, transitions and terminals are added. The duct friction loss calculator then checks a selected round, spiral or rectangular duct run from airflow, duct size, straight length, fitting equivalent length, roughness and available static, returning velocity, friction rate, total pressure drop and static-pressure margin. The duct transition calculator checks reducer length, controlling taper angle, inlet/outlet velocity, velocity-pressure change and sheet-metal blank area for rectangular-to-rectangular or rectangular-to-round transitions. The grille size calculator sizes a register or diffuser from airflow, target face velocity and free-area ratio, and checks actual face velocity for an installed grille size.

For the pressure budget, the HVAC static pressure calculator sums return duct, filter, coil, supply duct, terminal and accessory drops into total external static pressure and compares it with available blower static. The air changes per hour calculator converts room volume and airflow into ventilation rate. For refrigeration-side planning, the refrigerant line size calculator screens selected suction and liquid line IDs from capacity, entered refrigerant properties, equivalent length, velocity windows and pressure-drop budgets. For installation arithmetic after the approved line size is known, the refrigerant line charge calculator estimates added charge from line-set length beyond the factory-included length, using either the manufacturer charge rate or a line-volume/density fallback.

These are screening tools for HVAC designers, contractors and building owners; final designs still need ASHRAE/ACCA/SMACNA methods, Manual J / Manual S, manufacturer fan and terminal data, refrigerant line-size tables, fittings, leakage, balancing, acoustics and local code checks.