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What Size Furnace Do I Need?

If your furnace is old and needs to be replaced soon, you may wonder what size furnace you need to heat your home.

Unfortunately, we cannot answer that question in this article. Here’s why: our HVAC technicians base your furnace size on a heat load calculation that is specific to your home.

In this article, we explain:

Need immediate help?

If you would like to find out what size furnace you need right away, contact Michael & Son to talk to a trustworthy, Nate-certified technician.

How furnaces are sized

Furnace size is based on two factors:

  1. The amount of heat a furnace provides per hour (measured in BTUs)
  2. How efficiently the furnace creates that heat (measured by the AFUE rating)

We’ll explain both factors in more detail….

BTUs––BTU output measures the amount of heat a furnace provides. Residential furnace sizes range from 30,000 BTUs to 120,000 BTUs.

The higher the BTU output, the more heat the furnace provides.

Efficiency––Some of the heat produced by a furnace is lost into the chimney or is vented outside in the form of flue gases (for gas furnaces only). AFUE (Annual Fuel Utilization Efficiency) is a rating given to every furnace that measures the amount of heat it can produce that will NOT be lost.

Efficiency ratings range between 80% and 98% AFUE for gas furnaces. The lower the efficiency rating, the more it impacts the actual heat (in BTUs) that a furnace provides.

For example, a 100,000 BTU furnace that runs at 80% efficiency actually only provides 80,000 BTUs directly to the home.

The bottom line? The higher the AFUE rating, the more efficient the furnace and the less money you will waste in the long run on wasted heat.

Why the right size furnace matters

Installing the wrong size furnace will result in:

  • An uncomfortable home. Your home will heat unevenly if the furnace is too large. Your home will never heat to a comfortable temperature if your furnace is too small.
  • A higher utility bill. A furnace that is too large will cycle on and off quicker than the system was designed to. Because starting a furnace motor uses more energy than running a furnace, your utility bill will be higher. On the other hand, a furnace that is too small will run constantly to try to meet your heating needs (but it will never succeed), thus increasing your utility bill.
  • Shorter furnace lifespan. Cycling on and off or running constantly is hard on a furnace. Not only will your furnace wear out sooner than expected, but you will pay more for repairs along the way.

A lesser-experienced contractor may provide a rule of thumb estimate by calculating furnace size solely based on square footage or the size of the previously installed furnace. If this happens, we recommend a second opinion as these shortcuts can result in the wrong size furnace.

How we determine the right size furnace

Before you purchase a new furnace, you will need a certified HVAC technician to perform a heat load calculation.

Your home’s heat load calculation will determine the exact number of BTUs required to heat your home.

Here are just some of the factors a professional will take into account during a heat load calculation:

  • Your heating goals
  • Your climate
  • The size of your home
  • The direction your home faces
  • How well-insulated your home is
  • The number and type of doors and windows
  • The number of people living in your home

With a heat load calculation, an HVAC professional can recommend the right size furnace that will sufficiently heat your home without wasting energy.

Want to ensure your furnace is properly sized?

If you’re ready to replace your old furnace, ensure you get the perfect size furnace to meet your heating needs!

Contact Michael & Son to receive a free heat-load calculation and installation estimate from a NATE-certified HVAC technician.

Michael & Son is a home services company with over 40 years of reputable service and 3,000+ five-star reviews and that speak to promises kept and unmatched integrity. We also offer special financing with approved credit on installations.