HVAC BTU Calculator

It depends entirely on insulation, glazing, orientation and air-tightness, which is why the load per square metre is an input here. A proper heat loss calculation beats any rule of thumb.

It satisfies the thermostat quickly, shuts off, and never reaches steady-state efficiency. For cooling it also never runs long enough to remove humidity, leaving a room that is cold and clammy.

Repeated brief run periods. Starting is the hardest thing equipment does, so frequent cycles wear compressors and burners faster and waste energy in each start-up.

Multiply kilowatts by 3,412. A 3.5 kW unit is about 12,000 BTU/h, which is the size commonly marketed as one ton of cooling.

For a design percentile rather than the record extreme. Sizing for the worst hour ever recorded produces equipment that is oversized for every other hour.

Yes, through volume for ventilation loss and through the additional wall area. A room with a high ceiling needs more than its floor area alone suggests.

12,000 BTU per hour, from the rate of heat absorbed by a ton of ice melting over a day. It survives as a unit purely by convention.

Seasonal efficiency ratings that average performance across a range of conditions rather than a single test point. They are a better guide to running cost than a peak efficiency figure.

Yes — capacity falls as outside temperature drops, so sizing must consider output at the design temperature rather than the nominal rating. Many are also inverter-driven and modulate, which softens the oversizing penalty.

Usually because it is oversized and short-cycles. Dehumidification happens over a sustained run as moisture condenses on the coil; a unit that reaches setpoint in minutes never gets there.

Enormously. Leaky or undersized ducts can waste a large share of the system's output, and duct losses in an unconditioned attic are among the biggest hidden inefficiencies in a house.

Where different areas have genuinely different loads or schedules, zoning saves real energy. It adds complexity and cost, and a poorly commissioned zoned system can perform worse than a simple one.