Estimate furnace fuel use from heat demand and AFUE
A furnace installation quote and a future gas bill answer different questions. The quote measures the equipment and work needed to replace or install the unit, while seasonal fuel use depends on the heating demand of the building and the furnace's efficiency. This calculator keeps those results separate. Start with a professional heat-loss estimate, expressed as useful Btu per hour, rather than simply copying the nominal output of an existing furnace.
Annual Fuel Utilization Efficiency (AFUE) expresses the proportion of input fuel energy converted to usable heat over a standardized rating period. A furnace with 95% AFUE needs approximately 100,000 ÷ 0.95, or 105,263 Btu of fuel input to deliver 100,000 Btu of useful heat. Natural gas is commonly billed in therms; one therm represents 100,000 Btu. Fuel tariffs may include delivery charges and fixed fees beyond the energy rate entered here.
Furnace energy formulas
Seasonal fuel use (therms) = useful heating load × equivalent full-load hours ÷ (AFUE ÷ 100) ÷ 100,000
Scenario energy charge = fuel therms × entered price per therm
The full-load hours input represents an equivalent number of hours operating at the selected useful heating load; it is not the total number of hours that the thermostat calls for heat. Climatic conditions, setbacks, equipment cycling and duct losses will cause real consumption to differ. The estimate models gas energy use for comparison, not a utility-bill prediction or furnace-size specification.
Worked example: 48,000 Btu/h at 95% AFUE
Suppose a building has a useful heating demand of 48,000 Btu/h, and the scenario assumes 1,200 equivalent full-load hours. The total delivered heat is 57.6 million Btu. At 95% AFUE, input fuel energy is approximately 60.63 million Btu, equivalent to 606.32 therms.
At a hypothetical energy rate of $1.50 per therm, the scenario energy charge is about $909.47 before fixed bills, taxes or other charges. The installation budget is determined separately by equipment, professional labor, venting, removal and permits, with any contingency applied only to that quoted scope.
Build a furnace replacement budget from the actual scope
Equipment price alone does not cover an entire furnace replacement. A change in furnace type or efficiency may require venting changes, condensate management, return-air modifications, safety controls or revised combustion-air provisions. Record those items under their own quotations instead of hiding them inside a generic cost per square foot. The removal field applies only when an existing furnace must be taken out; new installations may not require it.
Heating-load calculations, fuel compatibility and venting must be checked by qualified installers under the applicable code. The calculator intentionally does not tell a homeowner what furnace capacity to install or provide gas-piping instructions. If a contractor supplies a firm project total, use its component breakdown to avoid entering the same charge under both labor and other work.
Furnace cost questions
Does 95% AFUE mean the furnace runs at 95% efficiency every hour?
No. AFUE is a standardized annual efficiency measure, not a guarantee of constant field performance. Actual runtime, cycling, weather, equipment condition and distribution losses influence delivered heating and fuel bills. Use the figure as a comparison scenario and check the furnace's certified rating.
Can I use house square footage instead of a heating-load estimate?
Floor area alone does not describe insulation, leakage, window area or climate. Two homes of the same size can have very different heating needs. For an equipment choice, obtain an appropriate heat-loss calculation. This calculator requires a useful Btu/h input rather than inventing capacity from house size.
Is the gas bill included in the installation total?
No. Seasonal therms and their energy charge appear as separate scenario results. The itemized project total includes quoted equipment and installation work plus the selected contingency. Keeping them separate prevents an annual operating expense from being mistaken for part of the one-time installation invoice.