Sizing a Furnace: Some Pointers for When It's Time for a ReplacementWhen you’re selecting a new heating system, sizing a furnace is the crucial first step. If you skip this part of the replacement process, you could be facing higher energy bills, compromised indoor comfort and shortened system life. 

When furnaces are too large, they run in short cycles, driving up your heating costs and reducing indoor comfort. The furnace won’t run as long, leaving pockets of cold air in some of your rooms. Frequent starts and stops also wear the parts faster, which increases maintenance and repairs.

A furnace that’s too small won’t adequately heat your home when our weather turns from chilly to cold. The furnace will run for a long time, also increasing the stress and wear on the parts.

It’s important in our climate, especially, to choose the right size heating system because our heating load is low, except at the higher elevations in the Los Angeles area. The HVAC contractor needs to perform a load calculation that takes into account the many features of your home that contribute to its energy efficiency, including:

  • Number of family members and their ages;
  • Preferred indoor temperatures;
  • Amount of heat you generate in your home, including lighting styles;
  • Landscaping factors.
  • The cubic footage;
  • Insulation levels throughout;
  • Window quality, size and orientation to the sun;
  • Air infiltration rates; and
  • Floorplan design.

The software that HVAC contractors use for sizing a furnace is called Manual J. Its companion software is called Manual D and it sets forth the best size and configuration for the ductwork.

If the contractor you interview suggests that you don’t need a formal load calculation and that going by the Btus (British thermal units) of your existing furnace works to calculate the size, you may want to reconsider using that company. If you or prior owners of your home have improved its energy efficiency since the existing furnace was installed, you’re at risk of installing the wrong size.

To learn more about sizing a furnace correctly, contact Around the Clock Heating and Air Conditioning. We’ve provided HVAC services for the greater LA area since 1967.

Our goal is to help educate our customers in Los Angeles, North Hollywood and surrounding areas about energy and home comfort issues (specific to HVAC systems).  For more information about furnace sizing and other HVAC topics, download our free Home Comfort Resource guide.

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