Calculating the right furnace size to meet your home heating needs takes into account factors such as the square footage of your home and the average climate in the area you live in. Areas are given climate factors, such as 30 for areas that will experience mild winters. You then multiply that factor by your home's square footage to determine your furnace's required BTU. If you have a 2,500-square-foot house and live in a place with mild winters, you will need to look at furnaces around 75,000 BTUs.
Additionally, there are other factors to consider, such as the height of your ceilings, the efficiency of your doors and windows, and the amount of insulation in your home. For example, if your 2,500-square-foot home has vaulted ceilings throughout, you will want to consider increasing the BTUs needed by 25%, since you will need more power to fill up the space between the floor and ceiling. If unsure about the size you need, you can discuss your needs with an HVAC technician or see how these factors affect your options.
It is important to note that choosing the right furnace size is critical. When units are too small, they can be overworked and wear out faster. When they are too big for your home, they are less efficient.