HVAC tonnage

The conversion

Tonnage is a rate: how much heat a system can move out of a house per hour. A ton equals 12,000 Btu per hour, which is the power needed to melt one ton of ice in 24 hours ¹. So the math between the two units is one step:

• Tons = Btu per hour ÷ 12,000
• Btu per hour = Tons × 12,000

Because a ton describes capacity per hour, it says nothing about how much electricity the system uses. That's what efficiency ratings such as SEER2 are for, and the energy itself shows up on the bill in kilowatt-hours.

Nominal tons vs real capacity

The tonnage people say out loud ("it's a 3-ton") is a nominal size class. The capacity the system actually delivers depends on the indoor and outdoor equipment combination, airflow and conditions. The AHRI certificate for a matched system shows its certified ratings for that exact indoor and outdoor combination ², so use that, not the size class, when you compare options or write a proposal.

Where the right tonnage comes from

The size of a replacement should come from a load calculation. ACCA's Manual J calculates the home's heating and cooling loads, and Manual S covers choosing equipment to meet those loads using the manufacturer's performance data ³. Tonnage is just the unit the answer gets rounded into.

Example: A Manual J load calculation shows a cooling load of 27,000 Btu per hour. Divided by 12,000, that's 2.25 tons. The old unit on the pad is 3.5 tons (42,000 Btu per hour). Replacing it like for like would install about 15,000 Btu per hour more than the house needs. You then use Manual S and the manufacturer's data to pick the matched system whose real capacity fits the load. The numbers are made up for illustration.

Common mistakes

• Copying the old unit's size. The original system may have been oversized from day one, or the house may have new windows, insulation or an addition since.
• Square-foot rules of thumb. A tons-per-square-foot shortcut ignores orientation, glass, insulation, air leakage and duct location, which are the things that set the load.
• "Bigger is safer." DOE notes that oversized equipment means lower efficiency, more wear and, for air conditioners and heat pumps, higher indoor humidity in summer ⁴. Those complaints come back as callbacks on a job you already priced.
• Changing size between tiers. When you present good, better and best options, keep the capacity matched to one load calculation and vary efficiency and features instead.

Why it matters for pricing

Tonnage drives the biggest line on a replacement quote, the equipment. Sizing from a load calculation can lower your equipment cost when the house needs less than what's there, and it surfaces duct changes before the price is set. The HVAC pricing guide walks through building a replacement price from the sizing up.