Formula
Output is input times AFUE, as a fraction. The input you need is the output you need divided by that fraction. The default band is 100 to 140 percent of the design heating load. Wes Davis, writing in the ICC Building Safety Journal (January–February 2009), summarizes ACCA Manual S Table 1: furnaces at 100–140 percent of total heating load, section 2-2 of that manual. The example in that article compares output capacity, not the input rating. The band on this page is editable. Applying it here is not a Manual S equipment selection. An estimate, not Manual J.
output = input × AFUE
kW = BTU/h ÷ 3,412.14
CFM = BTU/h ÷ (1.08 × ΔT°F)
1.08 = 0.075 × 60 × 0.24
AFUE is the seasonal ratio of heat delivered to fuel supplied. The DOE Energy Saver consumer guide says it measures the heat actually delivered compared with the fuel supplied, and that a higher number is a more efficient system. The department’s residential-furnace purchasing page defines it as useful energy output divided by energy input over the heating season. Presets are 80, 90, 95, 96, and 98 percent. Electric resistance has no AFUE step: the heating rate in kilowatts is the load divided by 3,412.14. Electric furnaces are sold in element steps this page does not list.
Airflow uses the standard-air sensible equation, the same 1.08 as the CFM calculator (1.08 = 0.075 lb/ft³ × 60 min/h × 0.24 BTU/lb·°F). It is a sea-level room-temperature factor. The nameplate rise range is the one to type. Fifty degrees is the worked example, not a recommendation. Altitude is not applied. A single-stage furnace delivers one output. A two-stage or modulating furnace can run lower for much of the season. That is a description, not a selection. Oversizing past the band is what the Manual S limit is trying to avoid: short cycles, and a blower that may not match the cooling coil. Buy from a Manual J and a contractor, not from this table.
Worked examples
60,000 BTU/h, 96 percent AFUE, band 100 to 140 percent. Output 60,000 to 84,000. Input 62,500 to 87,500. A 60,000 input gives 57,600 output, 96 percent of the load, undersized. An 80,000 input gives 76,800, 128 percent, inside the band. A 100,000 input gives 96,000, 160 percent, oversized. At a 50°F rise, 76,800 BTU/h is 1,422 CFM.
The same load at 80 percent AFUE. Input 75,000 to 105,000. An 80,000 input gives 64,000, 106.7 percent, inside the band. A 100,000 input gives 80,000, 133.3 percent, still inside.
Electric. 60,000 BTU/h is 17.58 kW at the bottom of the band. Area sketch, not a recommendation. 2,000 ft² times 35 BTU/ft², a factor you would have to type, is 70,000 BTU/h. This page does not offer that 35 as a preset.
Reference
The fit words compare furnace output with the band. Input is the burner rating. Electric mode hides AFUE and reports kilowatts instead of this table.
Questions
What size furnace do I need?
Start from the design heating load, not from the square footage. Output should cover that load. A fuel furnace’s input is the output divided by the AFUE. This page is an estimate, not Manual J, and it does not pick a model.
How many BTU furnace for a 2,000 square foot house?
There is no honest single number. Area times a factor you type is only a sketch, and this page does not publish a climate factor. Use a Manual J, or the load calculator on this site, and bring that BTU/h here. Two thousand square feet times a factor you invent is not a size.
What is the difference between input and output BTU?
Input is the fuel burned. Output is the heat delivered. Output is input times AFUE. A 80,000 BTU/h input furnace at 96 percent AFUE delivers 76,800 BTU/h.
What does AFUE mean?
Annual fuel utilization efficiency. The DOE Energy Saver guide calls it a measure of heating equipment efficiency. The department’s residential-furnace page defines it as the ratio of useful energy output to energy input over the heating season, as a percentage.
Is a bigger furnace better?
Not past the load. A furnace that short-cycles spends more time starting than heating, and the air can feel uneven. The default band here is 100 to 140 percent of the heating load, the furnace limit summarized from ACCA Manual S. It is editable, and it is not a Manual S selection.
How do I size an electric furnace?
Divide the heating load by 3,412.14 to get kilowatts. Electric resistance delivers about what it draws, so there is no AFUE step. Furnaces still come in element sizes. This page shows the kilowatts, not a catalog.
Do I need a Manual J?
Yes, before you buy. This page is an estimate, not Manual J. Have a licensed contractor run the load and confirm the equipment, the venting, and the gas or electrical supply.
Sources
- U.S. Department of Energy, Energy Saver, “Consumer Guide to Home Heating and Cooling” and “Energy Saver: Tips on Saving Money and Energy in Your Home.” AFUE measures the heat delivered compared with the fuel supplied. The FEMP residential-furnace page defines AFUE as the ratio of useful energy output to energy input, as a percentage, over the heating season.
- Wes Davis, “Reviewing HVAC Designs for Compliance with ACCA Manual S,” ICC Building Safety Journal, January–February 2009, Table 1. Furnaces: 100–140 percent of total heating load, cited there to Manual S section 2-2. The article’s example uses output capacity. This page is not that selection procedure.
- Sensible airflow reuses this site’s CFM factor, 1.08 = 0.075 lb/ft³ × 60 × 0.24. ASHRAE’s terminology defines standard air as air weighing 0.075 lb/ft³. The 0.24 BTU/lb·°F specific heat is the dry-air figure used with that factor on the CFM page. No altitude correction.