What is the cause of heat exchanger cracks?

Overheating causes nearly all premature heat exchanger cracks. When a furnace cannot get enough airflow, the heat exchanger overheats and suffers excess stress from expansion and contraction. Over time, the heat stress causes cracks near weak areas such as bends or welds. The most common cause of an overheated heat exchanger is as simple as a dirty air filter. A clogged air filter restricts airflow through the furnace, overheating the heat exchanger, and eventually resulting in stress cracks.

An over-sized furnace can also cause overheating and heat exchanger cracks. We frequently see too much furnace with too little ductwork or too little house. A furnace with under-sized ductwork will lack proper airflow and suffer a similar fate as that of a clogged filter.

The situation of a large furnace on a small home will take a bit more explanation. Natural gas combustion produces  water as one of its byproducts. When a furnace first lights, the flame impinges on the cold heat exchanger and water vapor from the flame actually condenses on the inside of the heat exchanger. After just about ten minutes of run time, the heat exchanger reaches operating temperature and the condensation evaporates. An over-sized furnace heats the home so quickly that the furnace shuts off after only a few minutes, so the heat exchanger stays wet and rusts from the inside out. The frequent cycling of an over-sized furnace also increases the expansion – contraction heat stress on the heat exchanger.

Proper diagnosis of a failed or cracked heat exchanger.

Diagnosis of a heat exchanger crack typically starts with a no-heat service call. A cracked heat exchanger allows air from the furnace blower to interfere with the flame causing it to flutter or even roll out. This trips a safety switch and shuts down the furnace. Beware of an unethical technician who finds a crack with a camera on a furnace that seems to be running just fine. While big cracks start as small ones, some technicians will search for anything that looks like a crack in order to sell a new furnace and earn a commission. We’ve even seen some technicians draw a line on the heat exchanger with a pencil, show the homeowner the line on a fiber optic camera, and convince them they are looking at a dangerous crack. In 2009, the AHRI published a guideline for inspecting heat exchangers which states: “Any crack or hole that is big enough to affect combustion will be easily visible to the naked eye. Do not use water, cameras or smoking agents to check for leaks. Furnace heat exchangers joints are not hermetically sealed, so a small amount of leakage is normal.”

In most cases, a true crack will disrupt the flame or set off a carbon monoxide detector in the home. If you suspect a false diagnosis, call us for a free second opinion.

Typical cure and cost

Unfortunately, heat exchangers cannot be repaired. When a heat exchanger cracks or rusts through it must be replaced. Because the heat exchanger is at the center of the furnace, nearly the whole furnace must be disassembled. Even if the parts are covered under warranty, the labor and freight will start around $750. Without a warranty, a heat exchanger replacement could run as much as $3,000 (more than the cost of a new entry-level furnace).

If you would like more info on cracked heat exchangers or if you need to schedule a free second opinion, feel free to call our office at 905-939-2350 or contact us through this website.