HVAC Guides
Plain-English explanations of the concepts behind HVAC sizing - the standards, the formulas, and the how-tos - written to pair with our free calculators.
What Is a Manual J Load Calculation?
The ACCA standard behind every correctly sized HVAC system: what it measures, how it's performed step by step, what professionals charge ($79-$400), when building code requires it, and how Manual J relates to Manual S, T, and D.
Read the guideHow to Find AC Tonnage by Model Number
Read the size of your existing air conditioner straight off its data plate: where the model number hides, how the capacity code works (divide by 12), brand-by-brand quirks, and a decoder tool that does it for you.
Read the guideAC Not Working or Not Turning On
Find the cause by symptom with the AC symptom checker: not turning on, fan not spinning, tripping the breaker, short cycling, a new noise, and how to reset the unit. The checks that cost nothing come first.
Read the guideAC Not Blowing Cold Air
Why an air conditioner runs and does not cool: the free checks in order, a frozen coil and how to thaw it, warm or hot air from the vents, the signs of low refrigerant, and what changes on a window unit, a mini split or a heat pump.
Read the guideAC Leaking Water
Why an air conditioner leaks water and how to stop it. Pick the type of unit and where the water is: on the floor, from a ceiling vent, at the furnace, inside from a window unit or down the wall under a mini split.
Read the guideFurnace Troubleshooting
Why a furnace is not working, not turning on, blowing cold air, short cycling or leaking water. A symptom checker gives the likely cause, the free check first and the part price, plus how to reset a furnace and read its blinking light.
Read the guideAC Unit Sizes by Tonnage
What a 2, 3, 4 or 5 ton AC unit actually means: square footage covered, installed cost, airflow, duct size, watts and amps for every size from 1.5 to 5 tons, plus what a condenser, an air handler and a package unit are.
Read the guideWindow Heat Pump vs Window AC With Heat
A window unit that heats and cools. How to tell a real heat pump from a window air conditioner with a resistance heater, what each costs to run by BTU size, which ones work on a 110 volt outlet, and how cold a heat pump keeps working.
Read the guideSACC vs ASHRAE BTU, Explained
Why the same portable AC lists two BTU ratings - and why the DOE's SACC number is roughly half the ASHRAE figure. What each test measures, a conversion table for common sizes, and which rating to trust when sizing.
Read the guideOur Calculation Methodology
The exact formulas behind every calculator on this site - AC tonnage, SEER savings, mobile home sizing, and more - with the ASHRAE-inspired assumptions spelled out step by step.
Read the guideJump Straight to an Answer
- AC not turning on: the six free checks
- Outside AC unit fan not spinning
- AC blowing warm or hot air
- Quick fix for an AC freezing up
- Water dripping from a ceiling AC vent
- AC keeps tripping the breaker
- How to reset an AC unit
- Furnace leaking water when the AC is on
- 110 volt vs 220 volt window air conditioner with heat
- Window unit with heat by BTU size
- Condenser, coil and air handler explained
- Manual J load calculation cost and example
- What SACC means on a portable AC
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