HVAC calculators for homeowners

Start with the load, not the equipment

Almost every expensive HVAC mistake is a sizing mistake wearing a different costume. A system that is 50 percent too large cools the house in four minutes, shuts off before the coil has pulled any moisture out of the air, and leaves the rooms cold and clammy at the thermostat setting. A furnace that is two sizes too big cycles the heat exchanger through more thermal stress than it was designed for and runs at a temperature rise the rating plate never specified. Neither failure looks like a sizing problem from the driveway, and neither is fixed by a better brand.

So this cluster starts at the beginning. The tonnage page, the room BTU page and the block load page all answer the same underlying question at different scales, and they share one model, which is why the numbers agree with each other. That consistency is not a flourish: a calculator site whose pages contradict each other is telling you the numbers were invented separately.

What the calculators have in common

Every load calculation here is a block load — the same structure ACCA Manual J uses before it splits a house into rooms. Conduction through the shell is area divided by R-value times the temperature difference. Infiltration is air changes per hour times volume times 0.018. Internal gains come from people at 230 BTU per hour sensible each and equipment at 3.412 BTU per hour per watt. Solar comes through the glass, weighted by orientation, and duct gain comes from wherever the ducts run. Add the terms, and the total is the load.

Showing the terms is the point. A single number with no breakdown cannot be argued with, checked, or corrected for a house that is unusual in one specific way. A table that says "your west glass is 41 percent of your cooling load" tells you what to do about it, and it tells you when the estimate is drifting away from your house.

Running cost and installed cost are different questions

Whether a heat pump beats a furnace is a fuel-price question with a weather input, and it has a definite answer that changes when either price moves. Whether it is worth installing one is a capital question that also depends on what the equipment replaces, what the electrical service can support, and what rebates exist at your address. Mixing the two is how a $19-per-million-BTU operating argument gets used to justify a $9,000 installation without anyone doing the second calculation.

The cost pages therefore itemise. Equipment, labour hours at a regional rate, materials, permit, disposal, and a visible overhead-and-margin line. The last one matters: a 3-ton job whose parts and labour come to $3,700 is not sold for $3,700, and a homeowner comparing three quotes deserves to see which portion of the price is the work and which is the company.

The three numbers people mix up

Tons, BTUs and CFM measure different things and get used interchangeably in conversation. A BTU is a quantity of heat; BTU per hour is the rate at which heat moves; a ton is 12,000 BTU per hour, so it is a rate wearing a different hat. CFM is air volume per minute, which is how the rate gets delivered to a room. A 3-ton system moves about 1,200 CFM at nominal conditions, which means it carries roughly 8,600 BTU per hour of sensible heat per 400 CFM at a 20 degree split. When someone says “the system is short of capacity”, they could mean any of the three.

SEER2, HSPF2 and AFUE are seasonal efficiency ratings for three different machines. SEER2 covers cooling, HSPF2 covers heat pump heating, and AFUE covers combustion heating. They are not comparable to each other and none of them is linear: moving from 14 to 15 SEER2 saves 6.7 percent, moving from 20 to 21 saves 4.8 percent. The 2023 test procedure also made SEER2 read about 4.5 percent lower than the old SEER for identical equipment, which is why a 16 SEER unit is a 15.2 SEER2 unit and why a comparison across the two ratings has to be converted before it means anything.

Design temperature and average temperature decide the size and the bill respectively. Equipment is sized at the 1 percent summer and 99 percent winter design temperatures — the hours a year that are more extreme than the design — while annual energy is computed from the average, weighted by hours. Confusing the two produces either a system that cannot keep up on the worst week or an energy estimate that is wrong by a factor of three.

What is deliberately not here

No lead forms, no brand rankings, no "average cost in your area" that resolves to a phone number. This site does not sell installation leads, which is why the pages can tell you that a 3-ton system is often oversized, that a $2,200 efficiency premium takes forty years to repay, and that the federal tax credit most contractors still quote on their paperwork expired at the end of 2025. Those are all things a lead-generation site has a commercial reason not to say.

Frequently asked questions

Disclaimer: These calculators provide estimates for planning and for checking quotes. They are not a substitute for a room-by-room Manual J, a Manual D duct design, or a licensed contractor's site assessment, and none of the cost figures is a quote.

Last reviewed: October 30, 2026 · Sources: ACCA Manual J, S and D, AHRI 210/240, DOE 2023 efficiency standards, EPA Technology Transitions rule, ASHRAE Fundamentals

Related: Materials calculators · Home tax and mortgage calculators · How these numbers are built