HVAC calculators

What these pages calculate

The HVAC set is three planning tools, and each one stops before equipment selection. Furnace BTU (HVAC-BTU-001) multiplies conditioned floor area by a BTU-per-square-foot factor you type, then scales that product by ceiling height divided by 8 ft. Room CFM (HVAC-CFM-001) multiplies room volume by air changes per hour and divides by 60. AC tonnage (GCC-COOLING-001) converts an entered cooling-load scenario into kilowatts, BTU/h and refrigeration tons. One refrigeration ton in that conversion is 12,000 BTU/h. None of these pages runs ACCA Manual J, Manual D or Manual S.

A number from this cluster is a way to start a conversation with the person who will actually size the equipment. It is not a model number, a duct diameter or a permit submittal. Insulation, windows, infiltration, design temperature, humidity, occupancy and internal gains are not inputs here, so two rooms with the same floor area can need very different equipment.

Work the heating example before you change the factor

The furnace tool’s own worked example is a 40 ft by 30 ft floor, an 8 ft ceiling and 30 BTU per square foot. Area is 1,200 ft². The height factor is 8 ÷ 8 = 1. The planning load is 1,200 × 30 = 36,000 BTU/h, which the page also shows as about 3.00 tons. That 30 BTU/ft² figure is the field’s default, labelled on the input as a mild-climate start. If the room is taller, the same area is multiplied by height ÷ 8, so a 10 ft ceiling on that floor becomes a factor of 1.25 and a higher planning load. Replace 30 with the factor you were given; do not treat the default as a climate recommendation for Doha, Minneapolis or anywhere else.

Keep garages, porches and unconditioned storage off the length and width unless those spaces are actually heated. Measuring the whole building footprint and then wondering why the load looks large is the usual mistake. If only one room is in question, measure that room. Adjacent rooms that share a different system belong on their own lines.

Read the method in how to plan a furnace BTU load and the cluster HVAC planning guide before you copy the result into an estimate. The guide states the same boundary: a planning BTU is not a furnace SKU.

Airflow is a different question from heating load

The CFM tool’s worked example is a 20 ft by 15 ft by 8 ft room at 6 air changes per hour. Volume is 2,400 ft³. CFM = 2,400 × 6 ÷ 60 = 240. The page also shows an equivalent in litres per second. Six air changes is the input hint’s “common occupied-room start,” not a ventilation-code minimum and not a shop, toilet or laboratory rate. Change the ACH when the use of the room changes. Do not reuse a heating BTU as if it were CFM.

Duct size does not fall out of CFM alone. Friction rate, equivalent length, fittings and the fan curve are Manual D work. A 240 CFM result is not a round-duct diameter. If you only needed the room’s geometric volume first, the area and volume calculator is the geometry check; it still does not apply an air-change rate.

Cooling scenarios, especially in a hot climate

The AC tonnage tool is a unit conversion of a scenario you enter, including an editable watts-per-square-metre factor. A published illustration on the tool uses 150 W/m² only to show the arithmetic: a 5 m by 5 m room at that factor is 3.75 kW, about 12,796 BTU/h, or about 1.07 refrigeration tons. That illustration is not a recommended load density for Doha, Dubai or Riyadh. Comparing 100, 150 and 200 W/m² on the same room shows how far the answer moves when the assumption moves. It does not establish a purchase range. The same boundary is written out in AC tonnage for a room in a hot climate.

Cooling kilowatts are not electrical input kilowatts. The electricity a machine draws depends on its efficiency and how long it runs. Floor area alone cannot answer “is one ton enough for a 20 m² room?” Outdoor design conditions, glazing, roof exposure and latent load are missing on purpose.

What to carry into the estimate

Write the factor or ACH you used next to the result. Equipment, refrigerant, registers, controls, electrical and labour stay as separate estimate lines with rates you type. Construclytic does not fetch equipment prices. If a designer later replaces the planning load with a Manual J total, keep the designer’s figure and do not average it with the rule of thumb.