The Right Furnace: How to Find It

Published on: February 17, 2014

When it’s time to buy a new furnace, many homeowners just don’t spend a lot of time making the decision beyond considering the brand. This is unfortunate since size and efficiency ratings are crucial not only for performance, but also cost of operation.

While it’s best to work directly with a licensed HVAC professional when choosing a new furnace, there are some things you need to know to make certain you are receiving the best possible guidance.

Calculating Size

To calculate the size of your new furnace, the contractor should use the Air Conditioning Contractors of America’s Manual J heating load calculation software. This tells the contractor how much heat the furnace has to supply to keep everyone comfortable in the home.

A load calculation takes into account the following factors, among others:

  • The home’s square footage
  • How the building was constructed
  • The building’s orientation to the sun
  • How well insulated the house is
  • The number, size and location of rooms in the building
  • The number, size and placement of windows and doors
  • Whether windows and doors are thermally insulated
  • How many floors and their arrangement
  • The local climate

Sizing a furnace cannot be done properly with basic “rule-of-thumb” calculations. If a contractor doesn’t take measurements and gives you a quick estimate based solely on square footage, find another contractor. That contractor is not following industry standards. Systems based on “rules of thumb” often end up oversized, costing the homeowner more in installation costs and long-term energy bills.

Why Is Sizing So Important?

If you have a contractor install a system that’s too large, you’ll likely end up paying more for a furnace that won’t last as long as one that’s been properly sized. You pay more upfront for larger furnaces, and oversized furnaces cycle on and off more often than a properly sized one. When starting and stopping, a furnace uses the most energy and also suffers the most stress on parts. This wears the system out faster, which leads to higher maintenance costs and a shorter lifespan.

Should your contractor install a system that’s too small, it will struggle to heat your home during cold weather, and also will wear out quicker since it has to work harder to heat your whole house.

You will pay less upfront for the undersized system. However, the smaller system will not be able to keep your home at a comfortable temperature. It will run continuously during a cold snap, yet your home will stay uncomfortably cold. Your energy bills will be higher. The continuous running will shorten the life of the heating system, which means replacing it before you should have to.

When a system is sized “just right,” it will keep your home comfortable without either running continuously or cycling too often. This keeps the system running efficiently, which keeps energy costs down. It will last for many years and require less maintenance.

System Efficiency

Once the contractor determines the size of your new furnace, the next part of the equation is to decide the right efficiency level. The efficiency of furnaces is measured by the Annual Fuel Utilization Efficiency (AFUE) rating. This is a measurement of how much fuel gets converted into heat for our home. The closer the number gets to 100 percent, the higher the efficiency.

For a gas furnace purchased in the past few years, the minimum efficiency rating is 78 percent. To earn the federal Energy Star rating in the Southern tier of states, the furnace must have AFUE 90 percent or better (in the North, it’s a minimum of AFUE 95 percent). A highly efficient condensing furnace runs from AFUE 90 to 98 percent.

While an efficient system will lower long-term energy costs, upfront purchase price will be higher. In most cases, the more efficient the system, the more it costs.


Before installing any new furnace, your contractor should check your existing ductwork. It may need to be repaired or replaced. Leaking ductwork can increase operating costs because the system has to work overtime to keep up with heating the home. If your ductwork is sized too small, it also can decrease system efficiency.

Replacing your old heating system with a new one is not a one-for-one replacement. The new contractor does not know if the old system was properly sized for the home or if you have made efficiency improvements that will change the load calculation.

To get a professional load calculation on your home, or to discuss replacing your furnace, contact us to find a contractor. We provide referrals to contractors throughout the Southeastern United States.