Short #32 - "It's Undersized"

HVAC School - For Techs, By Techs

When we replace equipment, we sometimes wonder if the old unit was undersized. Here are some things to consider before replacing that old A/C with a bigger one.

When we do load calculations, we figure out how much heat to remove or add to a home based on the building's design. We need to account for how much heat is entering or leaving a building and heat gains on the inside of a space. Heat gains can come from human body heat or electronics running, and heat losses are quite rare. Those factors are perhaps even more important for correct sizing than mere square footage.

In general, I don't recommend putting a bigger unit in. Focus on getting the equipment to work properly before considering an upsize, as the improper cooling could be caused by a mechanical issue and not an undersized unit. If you want to dig deeper and consider upsizing a unit, you have to consider a few things.

First of all, you want to look at the sensible and latent loads. Is the unit too small on the sensible or latent side? In either case, you can adjust the blower to try to address these first. If humidity is the issue, you do NOT want to oversize the unit.

Is leakage a factor? Check the integrity of the duct system and if you have cracks, can lights, or other sources of leakage. How's our ventilation? Attic ventilation is also a huge factor that will determine how well an A/C unit works.

We also discuss:

• Shade and impact on radiant gains
• Ductwork, wire, and copper pipe sizing
• Heat load reduction (lighting, ventilation, etc.)

Learn more about Refrigeration Technologies HERE.

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