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Get the HVAC numbers right before you spend the money

Load calculators, efficiency explainers and fault-finding guides built on ACCA, ASHRAE and DOE methods, with the formulas shown so you can check the work.

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Your HVAC system, explained

Most comfort and cost problems come down to a handful of ideas: how heat gets moved out, how efficiently it's done, the climate you're in, and how much moisture is in the air.

How a central split air conditioner moves heat Cutaway of a house. Indoors, the air handler blows room air across a cold evaporator coil and sends cool air through the supply duct; warm room air returns through a return grille. A cold suction line and a warm liquid line connect the coil to the outdoor condenser, which releases the heat outside. ATTIC 55 °F supply 75° Thermostat 78 °F return 1 2 3 4 1 Evaporator coil ~40 °F refrigerant absorbs room heat 2 Line set cold suction line · warm liquid line 3 Condenser compressor + fan dump heat outside 4 Ducts supply out, return back: a closed air loop Heat out

An air conditioner moves heat; it doesn't make cold

Refrigerant boils in the indoor coil at around 40 °F, soaking up heat (and condensing moisture) from the air the blower pushes across it. The compressor pumps that heat-laden vapor outside, where the condenser coil and fan release it. A heat pump runs the same loop in reverse to heat your home in winter.

Diagnose an AC that isn't cooling →
SEER2 efficiency scale A scale from 10 to 26 SEER2. Units built before 2006 rate about 10. Since 2023 the minimum for split air conditioners is 13.4 SEER2 in the North and 14.3 in the South; ENERGY STAR starts around 15.2. Single-stage units sit around 13 to 15, two-stage around 15 to 18, and variable-speed inverter units from about 18 up to 26. Pre-2006 ≈10North 13.4South 14.3ENERGY STAR 15.2+101214161820222426Single-stageTwo-stageVariable-speed / inverter Each +1 SEER2 cuts cooling electricity by roughly 5–7%

Efficiency is measured in SEER2

Since 2023, new split ACs must reach at least 13.4 SEER2 in the North and 14.3 in the South. Going from an old 10 SEER unit to 16 SEER2 typically cuts cooling electricity by about a third.

How SEER2 ratings work →
US climate zones and what they mean for HVAC IECC climate zones 1 to 7 from hot to cold, with example cities, the share of the year's HVAC load that is cooling versus heating, and the 2021 IECC attic insulation target: R-30 in zone 1, R-49 in zones 2 and 3, R-60 in zones 4 to 7. ZONE · EXAMPLE CITIES COOL vs HEAT ATTIC Zone 1Miami, HonoluluR-30Zone 2Houston, PhoenixR-49Zone 3Atlanta, Dallas, LAR-49Zone 4Nashville, NYC, SeattleR-60Zone 5Chicago, DenverR-60Zone 6Minneapolis, BurlingtonR-60Zone 7Duluth, Fargo areaR-60 Load split is illustrative; attic targets per 2021 IECC Table R402.1.3.

Your climate zone sets the targets

Hot zones are dominated by cooling and humidity; cold zones by heating. The zone decides insulation levels, equipment sizing and whether a standard or cold-climate heat pump makes sense.

R-value targets by zone →
Indoor humidity comfort band Relative humidity scale from 0 to 100 percent. Below 30 percent is too dry. 30 to 50 percent is the ideal range. 50 to 60 percent is acceptable. Above 60 percent encourages mold and dust mites. Too dry static, dry skinIdeal 30–50%OK 50–60%Too humid mold, dust mites0%20%40%60%80%100% Winter: stay near 30–40% to avoid window condensation. Summer: a right-sized AC holds 45–55% on its own. Basements: a dehumidifier set to 45–50%.

Humidity matters as much as temperature

At 50% humidity, 76 °F can feel as comfortable as 72 °F at 65%. Keeping indoor air between 30% and 50% protects your health, your house and your energy bill.

Indoor humidity guide →

Most-used calculators

AC Tonnage Calculator: What Size Air Conditioner Do You Need?Most homes land between 1.5 and 5 tons. Start near 20 BTU/h per square foot, then adjust for climate, insulation, glass,…Heating BTU Calculator: Estimate Your Home's Heat LossHeat loss is roughly floor area × a building loss factor × the design temperature difference. An average 2,000 sq ft hou…Heat Pump Sizing Calculator: Cooling Load, Balance Point and Backup HeatSize a heat pump on the cooling load first, so it dehumidifies well in summer, then check the heating side. Find the bal…Mini-Split Sizing Calculator: BTU per Zone and Indoor Head SizeSize a mini-split one zone at a time. Start near 20 BTU/h per square foot, adjust for ceiling height, sun, climate and p…Duct Size Calculator: Round Duct Diameter from CFM and VelocityDivide the airflow in CFM by the design velocity in feet per minute to get the duct's cross-sectional area, then convert…Dew Point Calculator: Find Dew Point from Temperature and HumidityDew point is the temperature at which air becomes saturated and moisture starts to condense. At 75 °F and 50% relative h…Cost to Run an Air Conditioner: Hourly, Daily and Monthly CalculatorRunning cost = BTU/h × hours ÷ (SEER2 × 1,000) × your electricity rate. A 2-ton (24,000 BTU/h), 14.3 SEER2 central AC th…Tankless Water Heater Size Calculator: GPM, Temperature Rise and BTUA tankless heater is sized by peak flow and temperature rise, not by tank gallons. Add the flow of fixtures used at the …

Something not working?

AC Not Blowing Cold Air: Causes and a Step-by-Step DiagnosisWhen the AC runs but the air is warm or only slightly cool, check the thermostat mode and fan setting, the filter, and w…AC Short Cycling: Why Your Air Conditioner Turns On and Off FastShort cycling means the compressor starts and stops every few minutes instead of running steady cycles of roughly 10–20 …Furnace Blowing Cold Air? Causes and Step-by-Step FixesMost often the thermostat fan is set to ON instead of AUTO, the filter is clogged, or the furnace has locked out after a…AC Breaker Keeps Tripping: Causes, Safe Checks and RepairsA breaker that trips repeatedly is telling you the AC is drawing more current than the circuit allows, or that there is …Window AC Freezing Up: Why the Coil Ices Over and How to Fix ItA window AC freezes when its indoor coil drops below 32 °F. The usual causes are a dirty filter, running on low fan when…Thermostat Not Reaching Set Temperature: Causes and FixesIf your system runs and runs but the house never reaches the setpoint, the cause is either too little delivered capacity…

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HVAC CalculatorsSize air conditioners, furnaces, heat pumps, ducts, fans and generators with calculators that show every step of the math.19 pagesTroubleshootingStep-by-step diagnosis for the most common cooling and heating faults, ordered from the cheapest check to the one that needs a technician.16 pagesEfficiency RatingsSEER2, EER2, HSPF2, AFUE, COP and CADR explained in plain terms, with what each rating means for your bills.6 pagesInstallation & SetupPractical setup guides for window and portable units, vents, filters, fans and thermostats.11 pagesInsulation & Air QualityR-values, insulation targets by climate zone, and air purification that actually moves the needle.5 pagesEnergy CostsWhat it really costs to run air conditioners, heat pumps, heaters and dehumidifiers, with your own utility rates.6 pagesRefrigerantsRefrigerant types, pressure-temperature charts, superheat and subcooling, and how charge is verified.4 pagesIndoor ClimateHumidity, dew point, thermostat settings and comfort: the numbers behind a home that feels right.8 pagesWater HeatersTank and tankless sizing, recirculation and the trade-offs between fuel types.3 pages

How the numbers are built

Standard methods

Sizing follows the logic of ACCA Manual J and S; psychrometrics follow ASHRAE Fundamentals; efficiency figures follow the DOE 2023 test procedures (SEER2, EER2, HSPF2).

Formulas in the open

Every calculator page lists the equation and the assumptions behind it, so a contractor can see exactly where a number came from.

Honest limits

A quick estimate is not a full load calculation. Each tool says when a professional Manual J or on-site check is the right next step.