Figuring out the right air conditioner size became harder for one homeowner because the system already in place had been behaving unpredictably. In a 2,720-square-foot house, they said the unit can hold 78 degrees, but trying to cool below that causes performance to unravel.

What happened?

In a Reddit post, the homeowner said the installed system weighs 3.5 tons, yet HVAC opinions have ranged from 4 tons to 5 tons. They described a two-story home with zoning, a living room with 20-foot ceilings, and 16 windows — 12 of them taking strong afternoon sun.

Their hesitation about sizing up was clear: "I personally want 5 tons, but dont want short cycles or poor humidity." Bigger is not always better, because an oversized system can turn on and off too quickly and make cooling less comfortable.

After multiple visits from technicians, the homeowner was still hearing two different explanations: replace the unit or suspect the thermostatic expansion (TXV) valve.

The original poster explained that the confusion followed some strange behavior: "Current AC keeps house relatively stable at 78, going below 78 on thermostate screws it up big time where it then doesn't cool below 80'F.....weird."

If you're in a similar situation of trying to sort through options, EnergySage can help you compare efficient heating and cooling solutions through its heat pump marketplace program and connect with trusted installers. For many homeowners, that can be a useful way to determine whether the better path is a repair, a properly sized replacement, or a more efficient upgrade that could slash energy bills.

Why does it matter?

Heating and cooling is one of the biggest drivers of household energy use, which means an HVAC upgrade can be one of the most effective ways to save money on utility bills and protect against rising energy prices. If your equipment is undersized, failing, or simply mismatched to your home, you could end up paying more each month while feeling less comfortable.

Square footage alone does not settle the 4-ton-versus-5-ton debate. Ceiling height, sun exposure, ductwork, insulation, window placement, and whether the current unit is dealing with a mechanical issue all play a role in determining the right answer.

The risk for homeowners cuts both ways. Too little capacity can leave a system running hard for long stretches, while too much can compromise humidity control and overall comfort. Either mistake can eat into the savings you were hoping to gain from a new system.

What can I do?

Ask for a full load calculation rather than relying on a rule-of-thumb estimate. A proper evaluation should account for windows, insulation, air leaks, sun exposure, ceiling height, duct condition, and whether a failing part is making the current unit appear weaker than it is.

You can also use EnergySage's heat pump marketplace to explore efficient HVAC options before you commit. And if you're not ready to spend upfront, Palmetto offers a $0-down HVAC leasing program that can lower your energy costs by up to 50%.

If you only need a lower-cost option for a specific area, Merino makes single-room ultraefficient HVACs. That type of targeted heating and cooling may be worth considering for the sunniest or hardest-to-cool spaces in your home.

You can also pair solar panels with electric appliances, including efficient HVACs, to push your utility costs even lower. EnergySage makes it easy to find the best solar system and installer for your home and budget.

Get TCD's free newsletters for easy tips, smart advice, and a chance to earn $5,000 toward home upgrades. To see more stories like this one, change your Google preferences here.