What size furnace do I need? Why bigger is worse
Oversizing is the most common mistake in this trade, and it makes a house less comfortable, not more. Here is what actually decides the size.
Ryan DeRue · September 14, 2026 · 1 min read
Almost everybody assumes a bigger furnace means a warmer house. It is the opposite, and oversizing is the most common installation mistake we find in this area.
What an oversized furnace does
It heats the air near the thermostat quickly, satisfies it, and shuts off. Then the house cools unevenly and it fires again. That is short cycling, and it causes:
- Cold spots in the rooms furthest from the furnace, because the run is never long enough to reach them
- Temperature swings you can feel
- More wear, because starting is the hardest thing a furnace does
- Worse efficiency, because the system spends its life in the least efficient part of its cycle
What actually decides the size
A Manual J load calculation. It accounts for square footage, ceiling heights, insulation, window area and type, orientation, air leakage and the local design temperature. Two identical looking houses on the same street can need meaningfully different equipment.
What it is not: the size of the old furnace. That repeats whatever was decided the last time, by somebody who may also have been guessing.
The rough numbers, and why they are only rough
People want a number, so here is the honest version. In this climate a reasonably insulated home runs somewhere between 30 and 45 BTU per square foot. A 2,000 square foot house therefore lands somewhere around 60,000 to 90,000 BTU, which is a range so wide it proves the point: it is a starting sanity check, not a specification.
What a right sized system feels like
Longer, gentler run times. Steadier temperatures from room to room. Less noise, because it is not slamming on and off. And on the coldest night of the year it runs almost continuously, which is exactly what it is supposed to do.
Read it and still not sure?
Describe what the system is doing and we will tell you whether it is a five minute fix, a real repair, or a replacement conversation. That costs nothing.
Quick answers
My furnace runs constantly in January. Is it too small?
Probably not. A correctly sized system runs nearly continuously on the coldest days of the year, by design. That is the efficient way to heat. Constant running in mild weather is the symptom worth investigating.