Size maters, and bigger isn’t better when it comes to a furnace’s heating capacity.
You’d think that given your furnace’s purpose in life is to add heat to your home, having a bigger furnace should be better, as it can add more heat, and that’s what it’s there to do, but that actually isn’t the case. A furnace that is too big will heat the air in the home too quickly, and yes, that’s a thing. The notion behind heating a home with a furnace is that we’re not so much trying to heat the air itself, rather we’re trying to use the air as a delivery vehicle to get the heat from the furnace into the dense material in the home; the walls, floors, ceiling, furniture, etcetera. The air can’t hold much heat, the dense material in the home can, so the air is fairly quick and easy to heat, and the rest of the home is not, which is precisely why an oversized furnace is a bad thing.
An oversized furnace will heat the air too rapidly, raising the air temperature quickly, satisfying the thermostat before the heat in the air has a chance to get soaked up by the dense material in the home. Then, once the furnace shuts off, the dense material in the home will cool the air down nearly as quickly as the furnace heated it, the thermostat brings the furnace back on, and the cycle continues. This causes the furnace to turn on and off more frequently than it was designed to and is referred to as “short-cycling”. As all mechanical systems and their components are designed for X many actions, X many “opens and closes” of their switches, X many “ons and offs” of their motors, short-cycling wears out furnace components much quicker than they were designed to. Short-cycling also causes a wave-like rise and fall of household temperatures, with the temperature rising rapidly when the furnace is on and falling rapidly when it’s off.
An oversized furnace will also be louder, as it’s trying to move more air through the same ductwork, will cost more to purchase and operate, as there’s more to, and in some situations, may be perpetually problematic for its owner, with it unable to run for long with anything other than a see-through furnace filter in place without shutting down because it’s overheating, frequently leaving the home without a functioning furnace.
Bigger isn’t better when it comes to furnaces.