by Jake Basnett and Anthony Kent

As an HVAC contractor, we’ve encountered all sorts of approaches to system sizing. Some contractors still rely on rule-of-thumb estimates, while others swap out like-for-like units without considering the evolving needs of homes or buildings.

But over the years, at Basnett Plumbing, Heating, AC & Electrical, we’ve learned that there’s no substitute for precision. That’s why we are passionate about load calculations — not just as a tool but as a philosophy that separates High-Performance HVAC™ contractors from the rest.

The Problem with Rule-of-Thumb HVAC Sizing

Basnett authors
Authors Jake Basnett (left) and Anthony Kent
of Basnett Heating and Plumbing

Many HVAC contractors hate doing load calculations. Why? It often comes down to a lack of knowledge or concerns about time. It’s easy to assume that a 3,000-sq.-ft. home needs a four- or five-ton system based on experience, but assumptions like that can lead to costly mistakes.

Oversizing is one of the most common issues. We can’t count how often we’ve walked into a home with a 120,000 Btu furnace that only needed 60,000 Btus.

Oversizing leads to short cycling, uneven heating and cooling, and higher energy costs. And let’s not forget about premature equipment failure — cracked heat exchangers and other issues arise because the original system wasn’t correctly matched to the home’s needs.

Undersizing, though less common, is just as problematic — especially in an era where heat pumps are becoming the standard. If we don’t know the home’s heat load, how can we confidently select a system that will keep customers comfortable year-round without excessive reliance on backup heat?

Why Every Contractor Needs to Do Load Calculations

Load calculations are about more than compliance — they’re about performance, efficiency, and long-term customer satisfaction.
At our company, we consider every factor: insulation levels, window types, building orientation, duct system design, and more.

That level of detail makes a massive difference. We once worked on a 3,000-sq.-ft. home that, by traditional standards, would have required an 80,000 Btu furnace. However, because of modern insulation and high-performance windows, it only needed 35,000 BTUs.

Windows alone can have a huge impact. A home with all south-facing windows will have significantly different cooling needs than one with shaded or north-facing exposures. These are the details that make or break system performance.

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