A furnace that is too small for the space that it is installed in can cause all sorts of issues for your home or commercial property since it will struggle to properly heat the entire space and will drastically increase your energy bills. Understanding the most common warning signs associated with a furnace that is too small for the space that it is trying to heat can help you identify the problem early so that you can have your unit replaced as soon as possible.
In a similar vein to the above point, if your furnace is severely undersized for the space that it is trying to heat, you may find that the unit never turns off. This may be harder to notice than a constantly cycling unit, since there will be no change in the amount of noise that your furnace or fan makes, so you should monitor your furnace's operation at the thermostat over the course of the day to see if it ever shuts off. If the hot air never stops flowing, you likely need to talk to a heating professional about potential replacement furnaces.
In a logical extension from the above point, an undersized furnace will be in near-constant operation as it struggles to adequately provide heat to your entire home. As a result, there will be a marked increase in the wear and tear that the internal components, like the fan and heating elements, within your furnace experience. This can lead to frequent mechanical issues that require you to contact a heating professional to have the damaged parts in question repaired or replaced. If you find yourself constantly having to service your furnace for excessive wear, especially if it is a newer unit and it is exhibiting one of the other symptoms on this list, you may have an undersized furnace. Replacing the entire unit could be the smarter financial decision since constant repairs can quickly add up in excess of the cost of a replacement furnace.
Finally, an undersized furnace can drastically decrease the comfort levels within your home by heating certain areas of your house much less than others, simply because it is not powerful enough to move heat throughout the entire space. If you notice particular cold spots or drafts in your home and know that there are no insulation issues, your furnace is likely to blame, especially if you've noticed either of the two above points. Replacing your unit with a larger and more powerful furnace can help ensure an even temperature throughout your property.