HVAC · Lesson 1 of 13Beginner7 min read

The Big Picture

Heat always flows from warmer things to colder things, all by itself. In winter it leaks out of your house; in summer it leaks in. A heating and cooling system pushes back: it adds heat or carries it away, swaps stale air for fresh, and keeps the air from getting too damp. This lesson gives you the map, and every later lesson zooms in on one part of it.

Heat moves from hot to cold

Physicists define heat as energy that moves because of a temperature difference, and it moves on its own in one direction only: from the warmer object to the cooler one. Touch a cold window and heat leaves your hand. Set a hot pan on the counter and heat leaves the pan.

A house is in the same situation all year. On a winter night, heat flows out through the roof, walls, windows and floor, and warm air slips out through cracks while cold air leaks in. On a summer afternoon the flow reverses: hot outdoor air and sunshine push heat inside. Left alone, the house would end up close to the outdoor temperature.

A heating system adds heat as fast as the house loses it. A cooling system has the harder job: it must carry heat from the cool inside of the house to the hot outdoors, against the direction heat flows on its own. That takes work, which is why an air conditioner needs a compressor and a good share of electricity. Lesson 4 shows how it’s done.

Heat always flows from warmer to colder. The equipment replaces what the house loses in winter and removes what it gains in summer. Source: OpenStax College Physics 2e, 14.1.

Four jobs: heat, cool, ventilate, dry

HVAC stands for heating, ventilation and air conditioning. In a home, that adds up to four jobs:

Heating. A furnace burns gas or oil to heat air, a boiler heats water, a heat pump moves heat in from outdoors, or electric resistance turns electricity straight into heat.
Cooling. A central air conditioner or a heat pump pumps heat from inside to outside.
Ventilation. Exhaust fans and fresh-air systems swap stale indoor air for outdoor air. Most heating and cooling systems don’t bring in fresh air on their own (Lesson 8).
Humidity control. An air conditioner pulls water out of the air as it cools. EPA suggests keeping indoor humidity below 60 percent, ideally between 30 and 50 percent, to keep mold in check.

Two more pieces tie it together. Ducts or pipes carry the heating and cooling to each room, and controls, starting with the thermostat, decide when everything runs.

A map of the systems

Here are the systems drawn in one house. Point at a system (or tap its name below) to see what it does, and click it to open its lesson. Switch views to compare a gas furnace with central air against a heat pump.

Point at a part of the house.

The house is drawn with a basement so every part shows. Many homes put the furnace or air handler in a closet, attic, crawlspace or garage instead, and many run their ducts through the attic.

The systems at a glance
Part	What it does	Signs of trouble
Furnace	Burns natural gas, propane or oil (or uses electric elements) to heat air that the blower sends through the ducts.	No heat, short cycling, odd smells, a CO alarm.
Air conditioner	An outdoor unit and an indoor coil pump heat out of the house and pull water out of the air.	Warm air from the vents, ice on the lines, water around the indoor unit.
Heat pump	The same loop as an air conditioner, run in either direction, so it heats in winter and cools in summer.	Weak heat in cold weather, an outdoor unit that stays iced, high bills.
Refrigerant lines	Two copper lines carry refrigerant between the outdoor unit and the indoor coil.	Oily spots or ice on the lines; any refrigerant work needs an EPA-certified technician.
Ducts and filter	Carry air from the blower to every room and back through a filter.	Weak airflow, rooms too hot or cold, dust, high bills.
Ventilation	Bath and kitchen fans, and sometimes a fresh-air system, swap stale air for fresh.	Foggy mirrors that won’t clear, musty smells, mold.
Controls	The thermostat and the low-voltage wiring that tell the equipment when to run.	Nothing runs, the system never shuts off, rooms miss the set temperature.
Insulation and air sealing	Slow the flow of heat and air through the house, so the equipment has less to do.	Drafts, cold walls, high bills, ice dams.

The house is part of the system

Heating and cooling use nearly half of the energy in a typical home, according to ENERGY STAR. How much of that energy is wasted depends as much on the house as on the equipment:

Sealing air leaks can save 10 to 20 percent on heating and cooling bills, says the Department of Energy.
In a typical house, about 20 to 30 percent of the air moving through the ducts is lost through leaks, holes and poorly connected ducts (ENERGY STAR).
A system that’s too big for the house cycles on and off too often, which wastes energy, wears out parts and, in summer, leaves the air clammy (Lesson 10).

So when a room is always cold, the answer is often a leaky duct, a missing patch of insulation or a blocked return grille, not a bigger furnace.

Common setups

Furnace and central air. A gas furnace heats. For cooling, an outdoor condensing unit is connected by refrigerant lines to a coil that sits on top of the furnace, and the furnace’s blower moves the air for both.
Heat pump. One outdoor unit does both jobs, paired with an indoor air handler that usually has electric heating elements for backup.
Boiler and radiators. Hot water or steam heats radiators, baseboards or floors. Cooling, if any, comes from a separate system such as ductless mini-splits.
Ductless mini-splits. An outdoor unit feeds one or more wall-mounted indoor units with no ducts at all.
Geothermal heat pump. Pipes buried in the ground or set in a pond trade heat with the earth, which stays at a fairly steady temperature year-round.

NoteManuals, codes and licensed pros

This course explains how HVAC systems work in general. Your equipment’s installation and owner’s manuals give the exact settings, filter sizes and procedures. Local codes, enforced by the authority having jurisdiction (usually your city or county building department), decide what needs a permit and an inspection. Refrigerant, gas and electrical work inside the equipment belongs with licensed or EPA-certified professionals.

Check yourself

Tap an answer to check it, or open “Show the answer.”

Which way does heat flow on its own?
Show the answer
From warmer things to colder things. Heat moves on its own from warmer to colder. Moving it the other way, from a cool house to the hot outdoors, takes work.
What does HVAC stand for?
Show the answer
Heating, ventilation and air conditioning. Heating, ventilation and air conditioning. Humidity control and air distribution come along with them.
Why does an air conditioner need a compressor and plenty of electricity?
Show the answer
To move heat from the cool inside to the hot outside, against its natural direction. Heat won’t flow from cool to hot by itself. The compressor supplies the work that pumps it “uphill.”
About how much of the air moving through a typical home’s ducts is lost to leaks, according to ENERGY STAR?
Show the answer
20 to 30 percent. About 20 to 30 percent, through leaks, holes and poorly connected ducts.

Go deeper

ENERGY STAR: Heat and Cool Efficiently ENERGY STAR’s guide to heating and cooling efficiently.
U.S. Department of Energy: Energy Saver Guide: Tips on Saving Money and Energy at Home (2022) The Department of Energy’s free guide to saving energy at home.
Science Lab: Entropy and the Second Law Why heat flows from hot to cold and never back on its own.
OpenStax College Physics 2e, 14.1 Heat The physics of heat, from an open textbook.

Sources

I finished this lessonSaved on this device only.

A free course from Teddy Rook Book. It explains how HVAC systems work in general and follows U.S. sources; your equipment’s manuals and the local code are the final word, along with your gas utility for anything involving gas.