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What problems occur with heat exchangers?

by Manage My Life Last activity date:
April 26th, 2007

What problems occur with heat exchangers in mid and high efficiency furnaces?

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Common heat exchanger conditions include

rust

cracked

clogged

overheating

Rust

is caused by condensation, leakage from humidifiers and leakage from air conditioners. This is the same as for heat exchangers on conventional furnaces. The only difference is that the heat exchangers in high-efficiency furnaces

expect

to see condensation. How well they resist rusting when exposed to that condensation is a function of how they are made and what the materials are. Secondary and tertiary heat exchangers are more likely to suffer rust (corrosion) problems.

Cracks

in heat exchangers can develop just like in conventional furnaces. Cracks are more likely to develop on the primary heat exchanger where the temperatures are greatest and where flame impingement may be a problem on the heat exchanger.

Clogged

heat exchangers are commonly caused by condensate and soot accumulations. The exhaust flue passages in high-efficiency furnace heat exchangers can be as narrow as 1/2-inch and often become clogged. Cleaning them can be an expensive job.

Overheating

is the result of excessive temperature rise usually caused by inadequate airflow across the heat exchanger. This is common with retrofit furnaces with limited duct size. It can also be a problem on new installations if care is not taken to set up fan speeds correctly and ensure free flow of air across the heat exchangers. The presence of air conditioner coils slows airflow and excessive temperature rise is more likely in systems with air conditioners.

The implications of heat exchanger problems are similar to those in conventional furnaces. A no heat condition is possible. More important, there can be life safety issues if the exhaust products leak into the house air.

If you can get a look at the burner, look for evidence of rust, flakes or sheets of aluminum coming off aluminized-steel heat exchangers, or sand-like deposits from coated heat exchangers.

In some cases, you can look at the second or third heat exchanger from the fan compartment. On some furnaces it's relatively easy to pull the fan out and get a good look.

If you see evidence of moisture on the outside of heat exchangers, it usually means that condensate is leaking through a hole or a crack. This very likely means a failed heat exchanger.

If there is a crack in the heat exchanger, the pressure differential switch may not be satisfied and the furnace may be shut down for safety. While you won't be able to verify this, you should allow for the possibility that unexpected shutdowns can be the result of a heat exchanger failure.

It's hard to detect clogged heat exchangers. However, if the clear plastic condensate tubing shows dirt or soot on the tube wall, you may assume that the heat exchanger also has soot on it.

One operational test we encourage on high-efficiency furnaces is to check the temperature rise across the heat exchanger once the unit is at steady state. Make sure the temperature rise does not exceed that stated on the data plate, a condition which can void the manufacturer's warranty and dramatically shorten the life expectancy of the heat exchangers. Causes of this include dirty filters, blowers or ducts, undersized ducts, blower motor on slow speed, etc.

Many mid- and high-efficiency furnaces have

heat switches (spillage switches)

to shut off the furnace on high temperature at the front of the burner if there is a flame rollout or spillage condition. When you run the furnace, make sure it runs long enough to reach steady state. If the furnace shuts off because of this high temperature sensor, you should be able to feel heat spilling back out through the front of the burner opening before it shuts off if this is not a direct vent furnace.

Some of these heat switches are automatically reset. Some of them have a push button reset that must be manually operated before the system will work again. Furnaces that shut out on this basis are in distress. It's likely that the heat exchanger is partially clogged.
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by Manage My Life
April 26th, 2007
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