Heat Exchanger (Gas Furnace)

For homeowners

The heat exchanger is the sealed metal compartment where the burner flames burn on the inside and your home’s air flows past on the outside. Heat transfers through the metal walls, but the combustion gases (which contain carbon monoxide, nitrogen oxides, and other things you don’t want to breathe) stay sealed inside until they exit through the flue vent.

Most modern residential furnaces use a clamshell design — a series of formed metal cells, one per burner. The burners fire up into the bottom of each cell, the flame and hot combustion gas travel up through the cell, give up heat to the metal walls, and exit at the top into a manifold that routes them to the flue (on 80% furnaces) or the secondary heat exchanger (on 90%+ condensing furnaces).

The secondary heat exchanger only exists on condensing furnaces. After the primary, flue gas is still around 300°F — the secondary cools it further until water vapor condenses out. The phase change releases enough additional heat to bump efficiency from ~80% up to ~95%. The downside is the condensate is acidic and the secondary heat exchanger must be stainless steel to survive it.

The most important failure mode is heat exchanger cracks. Over years of thermal cycling, the metal can develop cracks at stress points. A cracked heat exchanger leaks combustion gases — including CO — into the airstream that flows around it. The same airstream that goes back to your bedrooms. This is the single most dangerous failure in residential gas equipment. Carbon monoxide detectors in the home are essential, and any furnace with a cracked HX must be shut down immediately.

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For technicians

Primary heat exchanger types.

Clamshell. Most common modern design. Each cell is formed from two stamped sheet-metal halves welded together at the seam. The cells are arranged in parallel with the burners at the bottom and a common manifold at the top. Indoor air flows up between the cells, picking up heat from the metal surfaces. Flue gas flows up through the inside of each cell from burner to manifold. Used in most residential furnaces from 60,000 to 120,000 BTU.

Tubular. Each cell is a continuous bent tube — typically formed into a serpentine path that snakes back and forth inside the cabinet. Indoor air crosses the tubes. Used in some commercial equipment and in modulating residential models.

Serpentine. A hybrid — single sheet-metal cell formed into a long serpentine path. Used in some manufacturers’ designs (Trane, Lennox have variants).

Materials:

• Aluminized steel — most common. Carbon steel sheet with an aluminum-silicon coating. Adequate for non-condensing operation. Subject to corrosion if it sees condensate — which is why aluminized steel is used in primaries only, never secondaries.
• Stainless steel — used in secondary heat exchangers and higher-end primaries. Resists acid corrosion from condensate. Significantly more expensive but lasts decades in condensing service.

Secondary heat exchanger. Only present on condensing (90%+ AFUE) furnaces. Located downstream of the primary, in the indoor air path so the airstream passes the secondary before the primary (counterflow heat exchange — cold incoming return air meets the coolest flue gas first).

Construction is typically a stainless steel coil. Flue gas from the primary enters one end, travels through the serpentine path, and exits to the inducer.

Inside the coil:

• Flue gas enters at ~300°F, still carrying water vapor
• Coil tubes are cooled by indoor air on the outside
• At some point, flue gas temperature drops below its dew point (typically 130–140°F for natural gas combustion products)
• Water vapor condenses to liquid water on the inside walls of the tubes
• The phase change releases the latent heat of vaporization — approximately 970 BTU per pound of water condensed
• That heat passes through the tube walls into the indoor airstream

A typical residential condensing furnace at full firing rate produces roughly 1–2 gallons of water per hour. The condensate is acidic (pH 3–5) and drains through plastic tubing, sometimes through a limestone neutralizer cartridge.

Heat exchanger cracks — the critical failure mode.

Why they happen:

• Thermal cycling fatigue. Every heat call, the metal expands as it heats to 800–1100°F at the flame side. Every shutdown, it contracts. Over thousands of cycles, the metal develops fatigue cracks at high-stress points — typically the inside of bends, near weld seams, near the burner inlet where the temperature transition is sharpest.

• Overheating from low airflow. A furnace with restricted airflow (dirty filter, blower problem, blocked duct) can’t pull heat away from the heat exchanger fast enough. The HX runs hotter than designed, accelerating fatigue. This is why annual filter changes matter — it’s literally true that airflow restriction shortens heat exchanger life.

• Acid corrosion. Oversized furnaces short-cycle frequently. Each shutdown, residual moisture condenses on the cooling metal. The acidic condensate sits on aluminized steel and slowly etches it. Over years, pinhole failures develop.

Detecting heat exchanger cracks.

Visual inspection with the burner compartment open and the furnace running. Look for:

• Visible cracks or holes in the cell walls
• Flame disturbance when the blower starts — if the blower kicks on and the flames suddenly distort, change color, or roll out of the burner, the airstream is pushing through a crack into the flame path
• Discoloration on the heat exchanger surfaces (silver-gray oxide on aluminized steel indicates prolonged overheating)
• Soot deposits in the burner compartment (indicates incomplete combustion, often a consequence of crack-induced flame disturbance)

Camera inspection. A boroscope passed through the burner ports can show the interior surfaces of the cells. Standard practice on furnaces over 15 years old or when CO testing reveals problems.

Combustion analysis. A combustion analyzer measures CO in the flue gas and CO in the indoor airstream simultaneously. Indoor air CO should be near zero. Elevated indoor CO indicates leakage from combustion to airstream — cracks or compromised seals somewhere.

CO monitoring is essential. Every home with gas appliances should have at least one CO detector. Modern best practice is one per bedroom level and one near the furnace. CO is colorless, odorless, and lethal at concentrations above 100 ppm sustained.

What to do with a cracked heat exchanger.

A cracked primary HX is not field-repairable. The standard options:

1. Replace the heat exchanger. Manufacturer warranty often covers the part itself (20-year or lifetime warranties on primary HX are common). Labor to install is typically $800–1,500 depending on access. Not worth doing on furnaces over 10–12 years old when the rest of the system is also aging.

2. Replace the furnace. More common. New 80% or 90% furnaces start around $2,500–4,000 installed. The new heat exchanger has new warranty, the rest of the components are new, and the new furnace is typically more efficient. This is usually the right move.

3. Continue operating — never. A cracked heat exchanger is a CO source. Any service tech who finds one is professionally obligated to shut the furnace down and not relight it until the problem is resolved.

Maintenance practices that extend heat exchanger life.

• Annual filter changes — proper airflow keeps HX from overheating
• Annual inspection — catch problems early
• Combustion adjustment — burners running clean put less stress on the HX
• Correct system sizing — oversized furnaces short-cycle and condense moisture inside the HX
• Annual flue inspection — blocked flues create back-pressure that stresses the HX