Sizing the Furnace Properly: Here’s How Your Installer Should Be Doing It

Posted On: October 10, 2013

One of the most important factors to pay attention to when buying a new furnace is making sure you get one that’s correctly sized. Few people end up with a furnace that is too small because the problem they’ll experience is pretty obvious – they will not be able to provide enough heat to keep their home warm during especially cold weather.

However, over-sized furnaces also have a number of problems.

Many of the problems over-sized furnaces experience stem from the fact that they only have to run for a short period of time before they fully heat your home. This short-cycling is hard on components, wastes energy, and causes increased wear. The long periods of time between kicking on can also result in cold and hot areas in the home, rather than the even heating you expect. Other reasons to avoid over-sized furnaces include their propensity to develop rust holes in the heat exchanger and their higher purchase price.

Sizing the furnace should not be based on the one that’s being replaced. Furnace efficiency has significantly improved over the years, so your new furnace will require less fuel that your previous one. Additionally, your home is likely more airtight and better insulated than it was when you bought your old furnace, so a smaller model likely will suffice.

To determine the size of furnace you will need, your HVAC contractor should use Manual J from the Air Conditioning Contractors of America to determine your home’s heating load. These calculations take into account your temperature preferences and comfort in a range of outdoor weather, and your home’s size and layout, as well as:

  • The amount of insulation in the walls and attic
  • How much air leakage there is
  • Any chimneys and their ventilation
  • Number of windows and doors

If you need help correctly sizing the furnace for your home, contact the professionals at Ernst Heating & Cooling. We’ve been providing quality HVAC services to Alton, Glen Carbon, Highland and other communities in southwest Illinois since 1951.

Our goal is to help educate our customers in Hamel, Alton, Glen Carbon, Highland, Greenville, and Troy, Illinois and surrounding areas about energy and home comfort issues (specific to HVAC systems).  For more information about sizing your furnace and other HVAC topics, download our free Home Comfort Resource guide. 

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