Cracked Heat Exchanger: How to Tell, and the Cost

Watch the burners when the blower kicks in

The furnace lights, and the blower waits about half a minute before it starts: 25 seconds on Carrier’s 58SC0A, 30 on Goodman’s GMSS96 and Lennox’s SL280UHV. That gap is what makes this work.

Turn the thermostat up five degrees and stand where you can see the burner flames. On many furnaces there is a small sight window, and on others the burners are visible once the lower door is off with the power off.

Before the blower starts, the flames should be steady, mostly blue, and pointing the same way.

At the moment the blower starts, watch closely. Flames that suddenly lean hard, lift off the burner, turn yellow and lazy, or roll out towards you are telling you that blower air is getting somewhere it should not be.

What it does not prove. A steady flame is good news and not a clean bill of health, because plenty of cracks only open up once the metal is hot and expanded. Put the thermostat back where it was when you have seen enough.

Look for the marks a failing exchanger leaves

Soot. Black powdery deposits around the burners or inside the cabinet. Clean combustion leaves none.

Rust flakes in the burner area, or rust where no water should be. On a standard furnace with a single metal flue up a chimney, the exchanger should stay dry. Flaking rust below it is worth a proper look.

An alarm that only sounds during heating. Especially one that goes off a few minutes into a cycle and settles once the furnace stops.

People feeling ill on a pattern. Better at work, worse at home, worse in the morning, better in spring.

What is not evidence. A furnace being old. A slightly high gas bill. A cracked looking mark on the outside of the cabinet, which is not the exchanger at all. And a smell, because the things people describe here are usually something else: what each furnace smell actually means.

Three questions, asked politely, before any quote

This is the part nobody tells homeowners. A furnace being condemned is a decision you are allowed to see the working for.

“Can you show me the crack?” Many technicians now carry a camera on a flexible cable. A crack is photographed, not felt. Ask to see the picture and where on the exchanger it is.

“What is the carbon monoxide reading, and where did you take it?” A combustion analyser gives a number in parts per million. A reading taken in the supply air, with the blower running, is the measurement that matters, and a clean furnace reads at or near zero there.

“Is this a known failure point on this model?” Some models have one, and a technician who knows it will say so plainly.

Then get a second opinion, from somebody not selling you the replacement. If the first answer was right, the second one agrees in twenty minutes. If it was not, you have just saved the price of a furnace.

Do not be talked out of the cold. A furnace switched off in January is a genuine problem, and that pressure is exactly why this gets decided badly. Space heaters and one cold night are cheaper than a wrong decision.

Work out whether to fix it or to replace the furnace

The part is usually under warranty. Most makers cover the heat exchanger for twenty years or for life, to the original owner, and some of that carries over. So the metal itself often costs nothing.

The labour is where the money goes. The furnace comes apart almost completely to change one, which is most of a day.

Which means the sum is simple. A new heat exchanger in a furnace that is eighteen years old buys you the same furnace, eighteen years old, with everything else in it the same age. The same money is a large part of a new one.

On a younger furnace with the warranty alive, repair usually wins. Over fifteen years old, ENERGY STAR says it may be time to talk to a contractor about a replacement.

And find out why it cracked. Most do it from years of overheating, which comes from restricted airflow: a filter left in too long, closed registers, undersized ducts. Fit a new furnace behind the same restriction and it will do the same thing: what a tripping limit switch was trying to tell you.