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Cracked Heat Exchangers

We thought this article from ACCA would be of interest to homeowners. � ACCA is our industry organization, Air Conditioning Contractors of America. � Note: in the article CO2 is carbon dioxide and CO is carbon monoxide.

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Cracked Heat Exchangers

A member asked us:
"What causes heat exchangers to crack? How concerned should I be?"

ACCA's Don Prather responds:

Every year, in the beginning of the furnace startup season, home owners call �ACCA and they all have a similar story: one HVAC contractor has condemned a heat exchanger and turned off a furnace. The homeowner, wanting a second opinion, may call in another contractor, who turns the furnace back on because �he does not �believe the crack was serious enough to need �repair. Now, there is a confused homeowner in the middle wondering if the furnace is safe or not.

When the consumer calls me, I recommend buying a carbon monoxide monitor, and getting a third opinion and a price for repairs. I also recommend using the ACCA Consumer Check List available at www.acca.org/consumer/checklists �to evaluate the contractors. Being a little biased I also recommend they select the third contractor from the ACCA web site as well.

How could a little crack be a problem?
Service technicians / contractors who turn a furnace back on because the crack in the heat exchanger is not serious are not doing the homeowner a favor in the long run. � In fact that contractor is not following American Gas Association guidelines.

The crack itself is only a sign of troubles to come. The cause of the crack needs to be established, and repairs have to be made, or the crack along with the associated danger will grow larger. �

Heat exchangers with cracks do not heal themselves; cracked heat exchangers need to be replaced. �The cavalier act of turning a furnace with a cracked heat exchanger back on without making repairs is in fact putting the technician, the contractor, and the homeowner at risk. Heat exchangers are, simply, fire containment boxes that are inside of the duct system. �They keep the fuel and fire on one side (venting through to the outside) and the supply air for heating on the other. �

Blower fans in most furnaces wait for the heat exchanger to warm up before they come on. �This ensures that the blower will be providing warm air and not a cold draft. �During the warm up period, the flame side of the heat exchanger may have a higher pressure than the supply air side. �

In turn, if a furnace is running rich, CO2 can enter the home, or if it is burning lean, CO can enter. �If the blower should fail to come on, for any reason, this cycle could be repeated, and create elevated levels of CO2 or CO in the living space. Once the blower comes on, the pressure in the heat exchanger will generally be lower than the pressure in the air supply duct system and extra air will be added to the combustion process. �

This extra air can cause the burner to burn lean, creating higher than normal levels of CO. Exhaust systems that are not installed properly may then leak the carbon monoxide into the living space. �Increased levels of CO and CO2 � �are not healthy, and at very high levels they can become fatal. �

Replace or repair?
Many times, with older furnaces, it is more cost effective to replace the entire furnace than it is to pay the labor for heat exchanger replacement. � This is because replacing the heat exchanger requires disassembling the furnace and reassembling it.

Generally, the improved burner efficiency on newer furnaces will allow them to pay for themselves, in reduced fuel usage, over their expected lifetime.

However, just replacing the heat exchanger / furnace may not be enough to ensure long term safety. �

If the heat exchanger/furnace is replaced without finding the cause of the original failure and repairing it, the new heat exchanger/furnace will operate under the same conditions that caused the original heat exchanger to fail. The fix will be temporary and the new furnace/heat exchanger may fail prematurely too.

A technician should do a thorough check of the entire heating system, once a cracked heat exchanger is discovered. This should include reevaluating the size of the furnace to make sure it is compatible with the ducting system. �

Why did the heat exchanger fail?
The root cause of most heat exchanger cracks is fatigue caused by the expansion and contraction of the metal. �Anyone who has bent metal back and forth until it breaks understands metal fatigue; the piece of metal gets weaker at the bend until it cracks. �Then the cracks become larger until it becomes an increasingly larger opening. �

Once the metal is cracked, its original strength is lost and it is only a matter of time until it fails completely. �Manufacturers design heat exchangers to expand and contract without suffering from metal fatigue. �However, their designs depend on proper installation and acceptable operating conditions. �

The most common reasons for heat exchanger failure are described below with some common causes:

Airflow and overheating problems
An important design consideration is the airflow across the heat exchanger. � On the fire side airflow is required to remove combustion gases after they warm the heat exchanger. On the heating airflow side the air must remove the heat and carry it to heated spaces. If either side of the heat exchanger is lacking airflow the heat exchanger will overheat and fail prematurely. �

Supply side airflow problems can be caused by blocked supply ducts or registers, blocked return ducts or grills, dirty filters, or an undersized duct system / oversized furnace combination. Combustion-side airflow problems can be caused by improper fuel mixtures causing soot to build up and restrict airflow, or high combustion temperatures. �

Additionally undersized/blocked exhaust air ducting can cause the heat exchanger to overheat. �Furnaces operating with an overheated heat exchanger will be cycling off on the high temperature safety instead of the thermostatic control. This constant cycling on high temperature safety causes the metal to overheat repeatedly causing expansion and contraction beyond the design limits. That expansion and contraction will in turn eventually cause metal fatigue and a crack or opening.

Moisture Problems
Another common cause of failure is sudden expansion and contraction caused by shocking the metal. �If water is poured into a hot pan it can warp the pan because the place where the water hits cools very fast. �This type of rapid cooling if repeated can cause a crack to form. �A heat exchanger can be damaged by the introduction of moisture. �

Humidifiers, dripping over head pipes, and condensation from the evaporator coil, can all be contributors to this type of failure. �Generally, when water is the cause of a crack, there will also be some rust and a buildup of mineral deposits present near the crack. �

Rust Through
Water if left unchecked can rust through a heat exchanger. Many furnaces have a minimum exhaust temperature/heat exchanger set point to protect them from condensation in the exhaust duct/plenum.

Firing rate
Improper ignition can in extreme cases cause heat exchanger failures. � Too much fuel igniting late can cause a mini-explosion each time a furnace starts. � Or an improperly adjusted flame or pilot could overheat one section of the heat exchanger.

How is a homeowner expected to remember all of that stuff?
The typical home owner obviously is not a furnace or combustion expert. � Furnaces are designed with safety in mind and it is up to the contractor and technician to understand the original equipment manufacturers installation and start up instructions

Don Prather is manager of technical services at ACCA.

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	Serving the Greater Grand Rapids Area