HVAC · Lesson 3 of 13Intermediate12 min read

Furnaces and Boilers

A furnace heats air; a boiler heats water. A modern gas furnace runs the same careful start-up routine every time it lights, checking its own safety devices before it lets any gas flow.

How a furnace heats a house

A forced-air furnace has two airstreams that must never mix. Inside the heat exchanger, burning gas makes hot combustion gases, which flow through the exchanger’s passages and out the vent pipe. Outside the heat exchanger, the blower pushes house air across its hot metal walls and up into the supply ducts. Cooler air returns from the rooms through return grilles and the filter, and the loop repeats.

The wall of the heat exchanger is all that separates the flame’s exhaust from the air you breathe. That’s why the Department of Energy notes that a leak in the heat exchanger can let combustion gases into the house, and why a yearly professional inspection includes it.

A typical sequence for an induced-draft gas furnace with a hot-surface igniter. Timing, parts and order vary by model; the furnace’s installation manual and the wiring diagram inside its door describe yours. Pick a fault to see how a safety stops the sequence.

The start-up sequence

Each time the thermostat calls for heat, a typical modern gas furnace goes through the same steps, and its control board won’t move to the next step until the last one has proved itself:

Call for heat. The thermostat connects its R and W wires, sending a 24-volt signal to the furnace’s control board (Lesson 9).
Inducer starts. A small draft inducer fan starts pulling air through the heat exchanger and pushing it out the vent, clearing out any leftover gases.
Pressure switch proves the draft. A switch connected by a small hose senses the suction the inducer makes. It closes only if air is really moving through the vent.
Ignition. A hot-surface igniter glows bright orange, or a spark igniter starts sparking.
Gas valve opens. Gas flows to the burners and lights.
Flame sensor proves the flame. A thin metal rod in the flame carries a tiny electric current that only a flame can complete. If the board doesn’t see that current within a few seconds, it shuts the gas valve.
Blower starts. After a short delay, so the heat exchanger can warm up, the blower starts sending warm air to the rooms.
Shutdown. When the room reaches the set temperature, the thermostat opens the W circuit. The gas valve closes, the inducer runs a little longer to clear the flue, and the blower runs on to pull the leftover heat out of the heat exchanger.

Safety switches: prove it, or no gas

The pattern is the same throughout: no proof, no gas. The pressure switch must prove the vent is drawing before ignition. The flame sensor must prove a flame within seconds or the gas shuts off. A high-limit switch on the heat exchanger opens if it overheats, which usually means the blower can’t move enough air, for example because the filter is clogged; the burners shut off and the blower keeps running to cool things down. Many furnaces also have flame rollout switches near the burners that shut the gas off if flames spill out of the burner area.

When a safety trips over and over, it’s doing its job, telling you something is wrong. Many control boards blink a status light in a pattern that the furnace’s manual explains.

Gas furnace parts and what they do
Part	What it does	Signs of trouble
Burners and gas valve	The valve meters gas to the burners; the burners mix it with air and burn it.	Yellow, lazy flames or soot mean it needs a technician.
Igniter	A hot-surface igniter glows to light the gas; some furnaces use a spark.	Clicks or glows but no flame; a cracked igniter.
Flame sensor	Proves a flame is present by passing a tiny current through it.	Burners light, then go out after a few seconds.
Draft inducer	A small fan that moves combustion gases through the heat exchanger and out the vent.	Humming or rattling; no ignition.
Pressure switch	Proves the inducer is moving air through the vent before ignition.	Inducer runs but the furnace never lights.
Heat exchanger	Metal passages that carry the hot combustion gases and pass their heat to the house air.	Cracks can leak combustion gases; inspected by a pro.
High-limit switch	Shuts the burners off if the heat exchanger overheats.	Heat that starts and stops; often a clogged filter.
Blower	Pushes house air across the heat exchanger and through the ducts.	Weak airflow, squealing or grinding.
Control board	Runs the sequence and watches every safety.	A blinking fault code; nothing happens on a call for heat.
Vent pipe (flue)	Carries combustion gases outdoors: metal for most standard furnaces, plastic for condensing ones.	Rust, gaps, disconnected sections, a smell of exhaust.
Filter	Catches dust before the air reaches the blower and heat exchanger.	Dirty or clogged; change it on schedule.

Efficiency: AFUE

A furnace’s efficiency is rated by its annual fuel utilization efficiency, or AFUE: the share of the fuel’s energy that becomes heat in the house over a typical year. A 90 percent AFUE furnace turns 90 percent of the fuel’s energy into heat for the home; the rest is lost up the vent and elsewhere. The Federal Trade Commission requires new furnaces and boilers to show their AFUE.

The Department of Energy describes three generations:

Older, low-efficiency systems with a natural draft and a continuous pilot light: 56 to 70 percent AFUE.
Mid-efficiency systems with an exhaust fan and electronic ignition: 80 to 83 percent.
High-efficiency condensing systems with a second heat exchanger and sealed combustion: 90 to 98.5 percent.

A condensing furnace cools its exhaust so much in the second heat exchanger that the water vapor from combustion condenses, giving back its latent heat (Lesson 2). The condensate is acidic, so it needs a drain, and the cool exhaust usually leaves through plastic pipe. With sealed combustion, the burner takes its air from outdoors through a pipe instead of using heated house air, which also lowers the risk of combustion gases entering the house.

Under the federal rule in 10 CFR 430.32, most new gas furnaces made since November 19, 2015 must reach at least 80 percent AFUE, and the same rule sets 95 percent for those made on or after December 18, 2028. Standards can change, so check the Department of Energy for the current ones.

AFUE = annual heat delivered to the home ÷ annual fuel energy used
Example: a 95% AFUE furnace burning gas worth $1,000 a year puts about $950 of it into heat for the house.

A condensing furnace pulls extra heat out of its exhaust in a second heat exchanger, so the exhaust is cool enough for plastic pipe and its water vapor condenses into an acidic water that must drain away. Simplified. Source: U.S. Department of Energy, Energy Saver: Furnaces and Boilers.

Electric furnaces

An electric furnace heats air with resistance elements, like a giant toaster. It has no flame and no vent, so nearly all of its energy becomes heat: the Department of Energy puts electric furnaces at about 95 to 100 percent AFUE. But electricity usually costs more than gas or oil for the same heat, so DOE suggests that people heating with electricity consider a heat pump instead (Lesson 6).

Boilers and radiant heat

A boiler heats water instead of air. Hot-water systems pump the water through pipes to baseboard radiators, cast-iron radiators, tubing in the floor (radiant floor heat) or a coil in an air handler. Steam boilers send steam through pipes to radiators, where it condenses and runs back as water; they run hotter and are less efficient than hot-water systems.

A hot-water system adds a circulating pump, an expansion tank that makes room for the water as it heats and expands, and a pressure-relief valve. DOE lists the pro’s yearly checks: testing the pressure-relief valve and high-limit control on hot-water systems, and draining sediment and testing the low-water cutoff on steam boilers. Under 10 CFR 430.32, gas-fired hot-water boilers made since January 15, 2021 must reach at least 84 percent AFUE, and gas-fired steam boilers 82 percent.

A hot-water (hydronic) heating loop. Steam systems use radiators and return condensed water instead of a pump loop. Source: U.S. Department of Energy, Energy Saver: Furnaces and Boilers.

You canWhat you can safely do

Check and change the filter on schedule (Change a Furnace Air Filter). A clogged filter is a top cause of limit-switch trips.
Keep anything that can burn at least 3 feet away from heating equipment, as the U.S. Fire Administration advises for all heat sources.
Keep the furnace’s outdoor vent and air-intake pipes clear of snow and debris (USFA).
Install CO alarms on every level of the home and outside sleeping areas, and test them monthly (Test Smoke and CO Alarms).
Have the furnace or boiler professionally inspected and serviced every year, as CPSC, CDC and the U.S. Fire Administration all advise.

StopWhen to stop

If your CO alarm sounds, move everyone outside to fresh air right away and call 911 or the fire department; don’t go back in until responders say it’s safe (CPSC). Don’t run the furnace again until it has been serviced.
If you smell gas (a rotten-egg odor), follow your gas utility’s emergency instructions right away. A gas smell is an emergency for the utility, not a repair job.
Never jumper, bypass or disable a safety switch, and don’t open the burner compartment or touch the gas valve. Ignition, gas pressure and venting work belong with a qualified technician.
Yellow, wavering flames instead of steady blue ones mean the gas isn’t burning cleanly and an adjustment may be needed (DOE). Soot around the furnace is another sign. Have it checked.

Check yourself

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

What does the furnace’s pressure switch prove before ignition?
Show the answer
That the draft inducer is really moving air through the vent. It senses the suction the inducer makes, proving the vent path is open before any gas flows.
The burners light, but the flame sensor doesn’t detect the flame. What happens?
Show the answer
The control board shuts the gas valve within seconds. No proof, no gas: without a flame signal, the board closes the gas valve, and it may try again before locking out.
A furnace is rated 95 percent AFUE. What does that mean?
Show the answer
About 95 percent of the fuel’s energy becomes heat in the house over a typical year. AFUE is the share of the fuel’s energy that becomes useful heat over a year.
Why does a condensing furnace need a drain?
Show the answer
Its second heat exchanger condenses the water vapor in the exhaust. Cooling the exhaust condenses its water vapor, which recovers latent heat but leaves acidic water that must drain away.
What does a boiler heat?
Show the answer
Water, or water into steam. A boiler heats water, which carries the heat to radiators, baseboards or floors.

Go deeper

U.S. Department of Energy, Energy Saver: Furnaces and Boilers DOE on furnaces, boilers, AFUE and replacement.
ENERGY STAR: Furnaces What ENERGY STAR certification means for furnaces.
U.S. Consumer Product Safety Commission: Carbon Monoxide Questions and Answers CPSC’s questions and answers on carbon monoxide.
Being an Adult 101: Change a Furnace Air Filter Step-by-step filter change.

Sources

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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.