The number on a furnace is its input in BTU per hour and the heat that reaches the house is that figure multiplied by the efficiency rating, so a 100,000 BTU furnace at a modest rating and an 80,000 BTU condensing furnace can deliver much the same heat while costing very different amounts to run.
A furnace calculator separates input from output
A furnace burns its input rating and delivers output at its annual fuel utilisation efficiency. Compare equipment on output, and be careful with replacement quotes that match the old input figure: on a more efficient furnace the same input delivers more heat than the house needs.
Get tonnage from a load calculation, not a furnace size chart
Cooling capacity should come from a room-by-room calculation with your glazing, insulation, orientation and air tightness. Square-foot rules exist because they are quick, and they are the reason a large share of American houses run oversized equipment that cools unevenly and dehumidifies badly.
Understand what a ton is
One ton of cooling is 12,000 BTU an hour, a figure that dates from the heat needed to melt a ton of ice in a day. It is a capacity, not a size, so a two ton system suits whatever house needs 24,000 BTU an hour, which depends on the fabric and the climate.
Check the airflow the size implies
Each ton of cooling needs roughly 350 to 400 cubic feet of air a minute, so sizing up means asking whether the ductwork can carry it. Capacity the ducts cannot deliver is capacity you paid for and will not receive, and it shows up as noise and uneven rooms.
Treat a heat pump vs gas furnace cost calculator as a check on the bid
Use the numbers to see whether a quoted size is plausible. If a contractor proposes a size well above the calculation, ask what in the house justifies it; if well below, ask the same in the other direction. Either answer tells you how carefully the bid was made.
Where this sits beside a loss
An adjuster prices the capacity that was destroyed, taken from the nameplate of the damaged equipment, so photograph the plate before anything is removed. Where a load calculation shows the destroyed system was the wrong size, the correction is a betterment you fund and it belongs on a separate line: the claim restores what was there rather than what should have been.
Questions people ask about furnace size calculator
How many BTU of air conditioning does a 1000 square foot house need?
Enough range that the honest answer is to calculate it: fabric, glazing, orientation and climate can move the same floor area by a whole size. Published per-square-foot figures are averages of houses nothing like each other.
What does a 100,000 BTU furnace deliver?
Its input is 100,000 BTU an hour and its output is that multiplied by its efficiency rating, so a high-efficiency unit delivers appreciably more useful heat than an older one at the same input. Compare furnaces on output at the rating, not on the input alone.
Is tonnage the weight of the unit?
No. It is a rate of cooling: one ton is 12,000 BTU an hour, from the heat absorbed by a ton of melting ice over a day. It says nothing about the physical size of the equipment, which is why sizing by appearance is meaningless.
Can I use my old system's size?
Only as a cross-check. The old size may have been wrong when it was installed, and the house has probably changed since through insulation, windows or air sealing. A calculation on the house as it is now is what the quote should be built from.
Is a heat pump vs gas furnace calculator worth running before a quote?
As a check, yes. It will not decide the job, but it tells you roughly what the two options cost to run in your climate and at your electricity and gas prices, which is the number a bid rarely shows. Take the size itself from a load calculation either way.
Does the cost of 4 ton heat pump installed work follow the tonnage?
Only partly. Equipment moves in steps as the size rises, and the rest of the price is the electrical capacity, the line set, the air handler or coil cabinet and whatever the ducts need to move that much air. Take the size from a load calculation first, because a tonnage chosen to match the old unit prices work the house may not need.