Heating
Furnace input, output and AFUE: reading the nameplate
A "100,000 BTU furnace" usually means 100,000 BTU of gas in, not heat out. Knowing which number is which is the difference between a right-sized furnace and a guess.
Two numbers on every furnace
A furnace burns fuel and delivers some of the heat to the air in the ducts; the rest goes up the vent. That gives every furnace two capacity figures:
- Input is the fuel energy it burns per hour, in BTU per hour. It is what the model number usually encodes: a model with “100” or “100K” in it is a 100,000 BTU per hour input furnace.
- Output, sometimes called heating capacity or bonnet capacity, is the heat that reaches the house per hour.
The house’s heat loss is matched against the output. Sizing a furnace from its input figure, which is what most people see first, makes it look bigger than it is for an old 80% furnace and close to correct for a 95% one.
AFUE ties them together
AFUE, annual fuel utilization efficiency, is the share of the fuel’s energy that ends up as useful heat over a typical season:
| Input | Output at 80% AFUE | Output at 95% AFUE | Output at 98% AFUE |
|---|---|---|---|
| 40,000 BTU/hr | 32,000 | 38,000 | 39,200 |
| 60,000 BTU/hr | 48,000 | 57,000 | 58,800 |
| 80,000 BTU/hr | 64,000 | 76,000 | 78,400 |
| 100,000 BTU/hr | 80,000 | 95,000 | 98,000 |
| 120,000 BTU/hr | 96,000 | 114,000 | 117,600 |
The nameplate’s steady-state output is slightly higher than input × AFUE, because AFUE includes cycling and standby losses over a season. For sizing, input × AFUE is the safe figure. Federal rules require 80% AFUE for non-weatherized gas furnaces, and condensing furnaces, at 90% and above, recover heat from the water vapour in the exhaust and vent through plastic pipe. The guide to SEER2, HSPF2 and AFUE covers the rating in more detail.
Sizing a furnace from the heat load
The furnace size calculator estimates the heat load from the floor area and climate zone, divides by the AFUE, and picks the smallest common input size that covers it. Residential furnaces come in input sizes of 40,000, 60,000, 80,000, 100,000, 120,000 and 140,000 BTU per hour, with some 45,000s, 70,000s and 90,000s in between from some manufacturers.
A 2,000 sq ft house in Chicago with a 95% furnace
Climate zone 5, average insulation, 8 ft ceilings, average windows.
- Heat load: 2,000 × 45 = 90,000 BTU/hr of output
- Input needed: 90,000 ÷ 0.95 = 94,737 BTU/hr
- Furnace: 100,000 BTU/hr input, delivering 95,000 BTU/hr, a margin of 6%
- Blower airflow at a 50 °F temperature rise: 95,000 ÷ (1.08 × 50) = 1,759 CFM
The same house with an 80% furnace would need 112,500 BTU per hour of input, so a 120,000 BTU furnace delivering 96,000. Seen on the model numbers, the old furnace was “120” and the new one is “100”, yet the new one is not smaller where it matters: it delivers almost the same heat. Replacing an 80% furnace with a 95% one of the same input is a common way to end up oversized.
The calculator’s second example shows the opposite case: a poorly insulated 1,400 sq ft house in zone 3 needs 55,440 BTU per hour of output, so an 80% furnace needs 69,300 of input and lands on the 80,000 size, 15% above the load. Insulation and air sealing would bring it to the 60,000 tier with a 95% furnace.
How much margin is acceptable
ACCA Manual S allows a furnace up to 140% of the calculated heat load, because input sizes come in 20,000 BTU steps and a furnace has no moisture to remove. Beyond that, short cycles and uneven rooms follow, as the guide on oversized systems describes. The size check calculator compares an installed furnace’s output with an estimated load and applies the limit.
The blower and the temperature rise
A furnace’s nameplate also gives a temperature rise range, such as 35 to 65 °F. This is how much the furnace warms the air passing through it, and it depends on the airflow:
A furnace that is too big for the ducts cannot move enough air and runs at the top of its rise range, or trips its high-limit switch. The 1,759 CFM above is more than the 1,200 CFM a 3-ton air conditioner on the same ducts needs, so on that house the ducts, not the cooling, decide whether the furnace can breathe. A 60 °F rise brings it to 1,466 CFM.
Converting BTU, kW and therms
Furnace and boiler figures come in a mixture of units. The BTU, watts, kW and tons converter handles them all:
| Unit | Equals |
|---|---|
| 1 kW | 3,412.14 BTU/hr |
| 1 therm | 100,000 BTU |
| 1 MBH | 1,000 BTU/hr |
| 1 boiler horsepower | 33,475 BTU/hr |
| 1 ton of cooling | 12,000 BTU/hr |
A 100,000 BTU per hour furnace burns one therm of gas per hour of firing, which is 29.3 kW of fuel. At 95% AFUE the house receives 95,000 BTU of it. Delivered by electric resistance, the same 100,000 BTU would take 29.3 kWh, which at 17 cents is $4.98, against $1.50 for the therm of gas.
Reading a nameplate
The rating plate inside the furnace door gives the model and serial numbers, the input rating (sometimes high and low fire for a two-stage furnace), the output or heating capacity, the temperature rise range, the gas type and pressure, and the electrical supply. The serial number usually encodes the manufacture date, which matters for the guide to repair or replace. If the plate gives only input, multiply by the AFUE from the yellow EnergyGuide label or the manufacturer’s literature to get output.
Buying a furnace
A replacement quote should name the input and output, the AFUE, the number of stages and the blower type, and the contractor should be able to say how the output compares with your heat load. The furnace installation cost calculator gives sourced ranges by efficiency tier, and the guide to BTU per square foot by climate zone shows what size a house of your area typically needs.