HVAC: Heating, Cooling and Fresh Air

What this course is

Written for teens and adults with no HVAC experience. It picks up where the furnace-filter and alarm guides in Being an Adult 101 leave off. Each lesson explains the “why” as well as the “what”: why a liquid boiling at low pressure can cool a house, how a heat pump can deliver three times more heat than the electricity it uses, why humid air drips on a cold coil. Facts and numbers come from the U.S. Department of Energy, EPA and ENERGY STAR, the Consumer Product Safety Commission, the CDC, the U.S. Fire Administration, the Bureau of Labor Statistics, NIST and OpenStax physics textbooks, and every lesson lists its sources.

How to use it

1. Start with Lesson 1 for the big picture and Lesson 2 for the physics, then go in order or jump to the system you’re curious about.
2. Each lesson has diagrams, a “parts and what they do” table, a short quiz and links to go deeper. Badges show the level.
3. Play with the interactive pieces: the refrigeration cycle, the furnace start-up, the dew point lab, the airflow model, the thermostat and the heat-loss calculator.
4. Use the troubleshooting finder when something seems wrong, and climb the skill ladder one rung at a time.

Map of a home’s HVAC

Every system, drawn in one house. Point at a system or its name to see what it does; click to open its lesson. Switch views to compare a gas furnace with central air against a heat pump.

The lessons

1. 1 🗺️ The Big PictureHeat moves from hot to cold on its own; HVAC moves it where we want it. The four jobs, the house as a system, and a map of every part. Beginner7 min✓ Done
2. 2 💧 Heat, Humidity and ComfortConduction, convection and radiation; BTUs, watts and tons; sensible and latent heat; relative humidity, dew point and why an air conditioner drips. Beginner9 min✓ Done
3. 3 🔥 Furnaces and BoilersHow a gas furnace starts up and proves each safety, the heat exchanger and venting, AFUE and condensing furnaces, electric furnaces, boilers and radiant heat. Intermediate12 min✓ Done
4. 4 🔄 The Refrigeration CycleCompressor, condenser, expansion device and evaporator: how a refrigerant boils where it’s cold, condenses where it’s hot, and carries heat uphill. Intermediate9 min✓ Done
5. 5 ❄️ Air ConditionersSplit systems, the indoor coil, the condensate drain and pan, SEER2 and EER2, and EPA’s rules on refrigerants and certification. Intermediate8 min✓ Done
6. 6 ♨️ Heat PumpsThe reversing valve, why a heat pump’s COP beats 1, HSPF2, cold-climate models, backup heat and defrost, ductless mini-splits and geothermal systems. Intermediate10 min✓ Done
7. 7 🌀 Ducts, Airflow and FiltersThe blower, supply and return ducts, registers and grilles, static pressure, filters and MERV ratings, duct leaks and sealing, balancing and zoning. Intermediate9 min✓ Done
8. 8 🪟 Ventilation and Indoor AirWhy homes need fresh air, exhaust fans, HRVs and ERVs, humidity targets and mold, and carbon monoxide from fuel-burning appliances. Beginner6 min✓ Done
9. 9 🎛️ Controls and ThermostatsThe 24-volt control circuit and the letters on the wires, on-off cycles, programmable and smart thermostats, setback savings, and where a thermostat belongs. Intermediate9 min✓ Done
10. 10 📐 Efficiency, Sizing and the HouseHeat loss through walls and ceilings, R-values, why oversized equipment short-cycles and leaves the air clammy, load calculations, and every efficiency rating side by side. Advanced8 min✓ Done
11. 11 🔍 Troubleshooting by SymptomA finder for the most common problems, and a calm order for thinking them through: safety, settings, power, airflow, then the equipment. Beginner5 min✓ Done
12. 12 🧰 Maintenance and Doing Your Own WorkWhat owners can do (filters, the outdoor unit, the thermostat, breakers, vents and the condensate drain), the yearly professional checkup, what the law reserves for pros, and the skill ladder. Beginner6 min✓ Done
13. 13 👷 Careers in HVACWhat HVACR technicians do, pay and job outlook from the Bureau of Labor Statistics, training programs and apprenticeships, EPA certification and state licenses. Beginner4 min✓ Done

Troubleshooting finder

Something wrong with the heat or the air conditioning? Pick a symptom, answer one question, and see the likely causes, what you can safely check, and when to stop and call. Lesson 11 explains how to think it through.

The skill ladder

Build skill one rung at a time, with the manual in hand and the power off. Lesson 12 explains what the law and safety reserve for pros.

1. 1

   First jobs

   Checks and simple care. No panels off, no tools beyond a flashlight.

   • Change the furnace or air-handler filter
   • Test smoke and CO alarms
   • Reset a tripped breaker, once
   • Set, schedule and change batteries in the thermostat
   • Keep registers, returns and the outdoor unit clear
   • Clear snow and leaves from outdoor vents and the outdoor unit
   • Find the model plate and keep the manuals together
2. 2

   Next steps

   The manual in hand, the power off, and a helper the first time.

   • Replace a low-voltage (24-volt) thermostat, following its instructions
   • Check that the condensate drain flows, as the manual describes
   • Measure humidity and run a dehumidifier
   • Clean bath-fan grilles and register covers
   • Bleed air from hot-water radiators, if your boiler manual allows
   • Install a window air conditioner, following its instructions
3. 3

   Intermediate jobs

   Research, the right protective gear, and a mentor. Mistakes can damage equipment or the house.

   • Seal accessible duct joints with mastic or metal tape
   • Insulate ducts in an attic or crawlspace
   • Air-seal and add attic insulation
   • Hose the dust off an outdoor coil, power off, as the manual describes
   • Straighten bent coil fins with a fin comb
4. 4

   Leave these to licensed or certified pros

   The law, your safety or your warranty reserves them.

   • Anything that touches refrigerant: gauges, charging, leak repair, recovery (EPA Section 608)
   • Gas piping, gas valves, burners, ignition and venting
   • Electrical work inside furnaces, air handlers and outdoor units
   • Combustion, CO and heat-exchanger testing
   • Installing or replacing furnaces, boilers, air conditioners and heat pumps (permits may apply)
   • Line-voltage thermostats and new circuits
   • Duct design, leakage testing and balancing
   • Geothermal loops and wells

Careers

Heating, air conditioning and refrigeration mechanics and installers, or HVACR technicians, install, maintain and repair the systems that keep buildings comfortable and their air clean. Here are the U.S. Bureau of Labor Statistics figures from its Occupational Outlook Handbook (page last modified August 27, 2026):

Most technicians complete a 6-month to 2-year program at a technical school or community college; others learn through apprenticeships that usually last several years. EPA requires nearly all technicians to be certified to handle refrigerants, and some states and localities require a license. Lesson 13 has the details.

Sources

Each lesson lists the sources for its own facts. Here they all are in one place.