How is the reheat coil capacity calculated?

It uses the sensible-heat equation, Q = 1.08 x CFM x dT, where 1.08 combines air density and specific heat at standard conditions. The tool solves for the temperature rise the coil must produce at the minimum airflow to deliver your room heating BTU/h.

Why does the minimum airflow matter so much?

The minimum setpoint must satisfy ventilation and prevent the room from going stale, but it also limits how much heat the reheat coil can deliver without overheating the supply air. The tool picks the larger of your IAQ minimum and the flow actually needed to carry the heat without exceeding the allowed temperature rise.

What minimum airflow percentage should I use?

A common range is twenty to thirty percent of the maximum cooling flow, but the real driver is ASHRAE 62.1 ventilation for the zone and the maximum reheat discharge temperature. Very low minimums improve energy use but can starve ventilation and cause stratification on heating.

How do I convert reheat BTU/h to kW or GPM?

Electric reheat in kilowatts is BTU/h divided by 3412. Hot-water flow in gallons per minute is BTU/h divided by 500 times the coil water temperature drop, with a 20 degree drop being typical. The tool reports both so you can match either coil type.

Does a larger box always mean better control?

No. If the peak flow sits far below the box maximum, the damper rides near closed and low-flow control becomes inaccurate. The tool flags this and suggests stepping down a size so the operating range lands in the box's reliable pressure-independent band.

VAV Box Sizing Calculator

A variable-air-volume terminal box throttles conditioned air to a single zone and, when fitted with a reheat coil, adds heat on cold mornings or in interior-perimeter zones. Sizing it well means matching the inlet to the peak cooling airflow while keeping the minimum flow high enough for ventilation and heating, but low enough to save fan and reheat energy.

How it works

The peak cooling CFM comes from the zone’s Manual J sensible load. The minimum airflow is set as a fraction of that peak for indoor air quality:

min CFM = peak CFM x (min % / 100)

The reheat coil delivers heat into the minimum airflow. From the sensible-heat equation, the airflow needed to carry the heat without overshooting the allowed supply temperature rise is:

Q = 1.08 x CFM x dT
CFM_for_heat = heating BTU/h / (1.08 x max rise)

The controlling minimum is the larger of the IAQ minimum and the heat-driven minimum. The coil capacity in kilowatts is BTU/h / 3412, and hot-water flow in GPM is BTU/h / (500 x coil dT).

Selection tips and notes

Pick the smallest standard inlet whose catalog maximum covers the peak CFM, so the box operates inside its pressure-independent range. Avoid grossly oversizing: a box running near a closed damper measures flow poorly and hunts. Always confirm the minimum setpoint against ASHRAE 62.1 ventilation rather than a rule of thumb, and verify the coil selection on the manufacturer’s performance data at your entering air and water temperatures.