It can be difficult to tell if a furnace is too small before it is installed and used, but furnaces that fail to keep a home sufficiently warm during cold times are likely too small. The need for space heaters often may be another sign.
Whether a furnace is sized correctly or is too small also depends on the homeowners' preferences. Some people are comfortable with furnaces that occasionally struggle to keep up during especially cold times, opting to use supplemental heating and wearing warmer clothes indoors. Other homeowners want furnaces that operate comfortably even during record-breaking cold spells. Furnaces that cannot keep a home at a safe temperature, however, are insufficiently sized.
While homeowners may be tempted to buy a larger furnace than calculations dictate, doing so is generally counterproductive. Large furnaces cost more to buy and install. They also typical turn on and off frequently, causing them to spend more time in inefficient "warm up" modes. Fortunately, there are often steps homeowners can take if their furnaces struggle to keep up. Adding better insulation saves money and can keep homes warmer, while detecting and sealing air leaks around doors and windows can lead to better operation. Ductwork or venting upgrades can also lead to better operation without demanding a furnace replacement.