What size furnace for 2400 square feet in zone 3 at 96% AFUE
| Furnace to buy (input)the number printed on the box | 100,000BTU/hr |
|---|---|
| Heat actually delivered (output)100,000 × 96% AFUE | 96,000BTU/hr |
| Heat the house needs2,400 ft² × 37 BTU/ft² output | 88,800BTU/hr |
| Oversize | 8% |
| Same box at 80% AFUE would deliver16,000 BTU/hr less — why you resize when you upgrade | 80,000BTU/hr |
| Efficiencycondensing, the common high-efficiency tier | 96% AFUE |
Notes
- Input and output are different numbers, and the box shows the bigger one. A 100,000 BTU furnace at 96% AFUE puts 96,000 BTU/hr into the house. Compare furnaces on output, or you are comparing how much gas they burn rather than how much heat you get.
- Upgrading efficiency without resizing is the classic error. Replace an 80% furnace with a 96% one of the same input and the house receives 16,000 BTU/hr more than it did — roughly 20% extra. The new furnace short cycles, the house swings warm and cool, and the owner concludes high efficiency is uncomfortable. It was the sizing.
- 8% over is the normal rounding step — furnaces come in 20,000 BTU increments, so landing exactly on the calculated load is impossible.
- These BTU-per-square-foot figures assume average insulation. A tight house with good windows can need a third less; a 1950s house with an uninsulated attic and single glazing can need a third more. The figure above is where the conversation starts.
- A Manual J is what turns this into a specification. It accounts for window area and orientation, air leakage, insulation, ceiling height and duct losses. On a furnace it matters more than on air conditioning, because heating loads vary more between houses than cooling loads do.
- Do not size by what was there before. The old furnace was very likely oversized too — contractors have rounded up for decades — and copying its input perpetuates the error into another twenty-year appliance.
Two BTU numbers, and the salesman quotes the bigger one
The house needs 88,800 BTU/hr of heat delivered into it. At 96% AFUE that takes a furnace with 100,000 BTU/hr of input, because 96% AFUE of what goes in comes out as useful heat.
The number printed on the box is the input. Compare two furnaces on input and you are comparing how much gas they burn, not how warm they make the house.
The upgrade trap
Replace an 80% furnace with a 96% one of the same input, and the house suddenly receives 20% more heat than it used to. Same box size, same ductwork, and now the furnace runs in short bursts, the temperature swings, and the rooms furthest from the plenum never settle.
Owners conclude that high-efficiency furnaces are uncomfortable. What actually happened is that nobody resized when the efficiency changed. That same 100,000 BTU box would deliver 80,000 BTU at 80% AFUE.
Why oversizing costs more than it seems
A furnace running in ten-minute bursts never reaches steady state. The heat exchanger expands and contracts on every cycle, which is what cracks them. The blower spends a disproportionate share of its runtime ramping. And the rooms at the end of the duct run stay cold, because the cycle ends before the air gets there.
The fix for a house that already has an oversized furnace is a two-stage or modulating unit at the next replacement — those spend most of the winter at 40 to 60% of capacity, which is roughly what the house actually wants.
What this figure assumes
Average insulation for the age of the housing stock, 8 ft ceilings, and reasonable air sealing. A tight modern house in zone 3 can want a third less than 88,800 BTU. A 1950s house with an uninsulated attic and single glazing can want a third more.
Do not size from the old furnace. Contractors have rounded up for decades, so the unit being replaced is very likely oversized already — and copying it locks the error into another twenty years.
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Read more
- Water Dripping From the Furnace Vent — Normal on One Type, a Fault on the OtherThe same drip is expected on one furnace and dangerous on another. Which one you have is decided by the vent material, and you can see it from across the room.
- Gas Bill Went Up After a New Furnace — Five Reasons That Are Not the FurnaceCompare therms against degree days, not dollars against last year. Half of these complaints disappear the moment the weather is normalised out.
- New High-Efficiency Furnace Short Cycles — Usually the SizingThe upgrade from 80% to 96% adds a fifth more heat from the same size box. Keep the input the same and you have just oversized the house by 20%.