ACCA Manual J (Heating Load)
How Many BTU to Heat 1,800 Square Feet?
Heating 1,800 sq ft takes about 54,000 BTU in a moderate climate, but climate swings it widely. Here is the range, the furnace it maps to, and the calculator.
The Numbers for 1,800 Sq Ft
| Heat load, moderate climate (~30 BTU/sq ft) | 54,000 BTU/hr |
| Mild climate (~20 BTU/sq ft) | 36,000 BTU/hr |
| Very cold climate (~45 BTU/sq ft) | 81,000 BTU/hr |
| Furnace size (90% AFUE input, moderate) | ~60,000 BTU |
Heat-loss basis. A furnace is sized to the load divided by its AFUE efficiency; a heat pump is sized at the winter design temperature with backup heat. Insulation and air-sealing lower all of these, so a Manual J calculation is the accurate method.
Estimate Your Exact Load
Enter your climate zone, insulation, windows, and ceiling height for a Manual-J-style estimate.
Frequently Asked Questions
How many BTU to heat 1,800 square feet?
About 54,000 BTU per hour in a moderate climate (roughly 30 BTU per square foot). The realistic range is 36,000 BTU in a mild climate to 81,000 BTU in a very cold one, because heating load is driven mainly by your winter design temperature. Insulation, ceiling height, and air leakage move it further, so use a load calculation for equipment you will install.
What size furnace for 1,800 sq ft?
About a 60,000 BTU input furnace in a moderate climate. A furnace is rated by input BTU, and at 90% AFUE it delivers 90% of that as heat, so to cover the ~54,000 BTU load you need roughly 54,000 / 0.90 = 60,000 BTU input. A colder climate pushes this toward the 81,000 BTU load and a larger furnace.
Does climate change how many BTU I need for 1,800 sq ft?
A lot. The same 1,800 sq ft space needs about 36,000 BTU in a mild southern climate and up to 81,000 BTU in a very cold northern one, more than double. Heating load tracks the winter design temperature, so a square-foot rule of thumb is only a starting point; the accurate number comes from a Manual J heat-loss calculation.