How to determine if you have a damaged heat exchanger in your furnace.
Periodic furnace maintenance is an important function for any homeowner to insure that their heating, ventilation, and air conditioning system (HVAC) is in proper working order. While failure to the air conditioning components can be annoying and uncomfortable, failure to the heating components can be extremely dangerous and potentially life threatening. Regular maintenance such as filter changes can reduce the potential hazards dramatically, but it is also important to check what’s going on behind the filter.
Dirt and rust can accumulate inside the air handling unit which can reduce the efficiency of the furnace and shorten the life of the components. A visual inspection and cleaning should be performed twice a year in the spring and fall.
While this article focuses on gas-fired furnaces, the process is similar for an oil-fired furnace.
If you find any rust, water damage, soot deposits, or melted wiring insulation, shut off the power to the unit and call a licensed HVAC service technician to inspect and repair the unit.
Air Handler Components
Heat exchangers are thin-walled tubes or shells that allow hot combustion gases to remain separated from the circulating air inside the home. The thin metal walls of the tubing or shells along for rapid heat transfer and increased efficiency. Heat exchangers are designed to exchange heat, not air.
Newer heat exchangers in high efficiency furnaces are made of aluminized steel or stainless steel, but older heat exchangers were made of plain steel. Heat exchangers should last 15 years or more, but uneven maintenance and negligence can shorten the life of the heat exchanger as well as other system components.
Heat exchanger failure typically occurs due to overheating which results in cracking. Once a hairline crack starts to form it will usually get larger as the burner is cycled on and off. The greatest danger with a cracked heat exchanger is flame roll out, not carbon monoxide poisoning. Most furnaces use an induced draft blower that pulls the hot gases through the heat exchanger and fire box. This motor is placed after the firebox and pulls the air through the heat exchanger exiting into the vent through the roof.
Hairline Cracking on Heat Exchanger Shell
The induced draft motor comes on a few seconds before burner ignites to create negative pressure inside the heat exchange. While there is a vacuum inside the heat exchanger, the main blower motor comes on and creates a positive pressure on the outer surface of the heat exchanger as it forces the air through the unit and to the rooms in the house.
When a crack develops in a heat exchanger it will usually cause the blower motor to push air into the heat exchanger chamber and distort the burner flame. If the cracks become large enough, the air rushing through the flames will push the flames right out of the firebox and out the front of the furnace. This is the location of the gas valve, gas controls and most homes now have a thin flexible gas line called corrugated stainless steel tubing, CSST. The gas tubing feeding the gas valve does not have any safety controls to shut off the flow of gas in the event of an emergency. If that this flexible gas line melts or burns open, the furnace and surrounding area can become engulfed in flame.
Large crack on tube of Heat Exchanger
If you discover a problem with your heat exchanger, have an HVAC technician come out to inspect the unit. If the air handler is less than 10 years old, you can probably replace the heat exchanger. On older units, it may not be possible to find a suitable replacement so the entire furnace will need to be replaced.
Corrosion on outside of Heat Exchanger Tubes
It is important to change the filters regularly to maintain airflow across the heat exchanger. Restricting return air is another possible cause for overheating. Make sure that large furniture is not blocking return air registers.
Finally, make sure that the blower is clean and that the belts are in good condition. Loose or worn belts will prevent the motor from transferring enough power to the blower. Dirt accumulation on the blower will also reduce fan efficiency.
Vacuum cleaner with brush attachment
½” Plastic Tubing
Brushes - wire, soft and stiff bristle
There are two ways to inspect your heat exchanger for damage.
Removing the Burner
Removing the burner will give you direct access to inside the heat exchanger. Use a flashlight and inspection mirror, inspect the walls for cracks or burn-through. Inspecting immediately after the furnace has run will give you the most visible results, but use caution as it may be hot.
There is usually a screw on either side of the burner assembly. After the screws are removed the burner can slide out to inspect inside the firebox. Look for any excessive rust deposits, soot on the front of the firebox, or streaking inside the heat exchanger.
Evidence of flame rollout in front of the firebox
Visual Flame Inspection
Watching the flame of the burner while it is running can give you clues to problems with the heat exchanger. Note: Most furnaces will have a fan interlock switch which will prevent the unit from firing while the access panel is removed. You need to locate this switch and hold it in to allow the unit to start.
Set the thermostat to heating mode and raise the temperature to energize the furnace. If your unit has a flame sight glass you can watch the burner flames. Inspect the burner flame before and after the main blower comes on. The flames should be fairly even and almost entirely blue. If the flames start to move erratically or flatten immediately when the blower kicks on it is probably a sign that blower air is rushing in through a hole or crack.
Cleaning inside the Unit
If you do not see and problems with the burner flame or heat exchanger you should clean the area before putting everything back together. Any dirt accumulation on the outside of the heat exchanger will reduce the amount of heat transferred to the air, cause the metal to overheat, and cause the unit to run for longer periods of time, wasting energy and can damage the heat exchanger more.
1. Shut off the power to your furnace by shutting off the breaker or the red emergency switch. Shut off the gas or oil and remove the front access panel from your furnace.
2. Vacuum the interior area. Use caution so as not to disconnect any wiring. You can use an soft bristle brush to carefully remove any dirt while vacuuming. You can tape a piece of ½” plastic tubing to the end of the vacuum hose to clean inside the firebox and alongside the heat exchanger.
3. Clean the blower motor and fan assembly. The blower usually slides inside a track located behind the filter. There may be a clip or screws to keep the fan in place. Remove build-up from all surfaces of the assembly with a toothbrush, especially the fins on the blower.
4. Reinstall the blower assembly and install new filters. Replace the access panels and turn the power back on to the unit.
5. Allow the furnace to run for several minutes and check the airflow at the registers and make note of any unusual odors. Cleaning the blower will have disturbed dirt inside the unit that you may smell. If there is any lasting odor or burning smell, shut off the unit and contact a professional.
You can often contact your local gas or oil supplier to have them come out and perform a safety check of your furnace.
Caution: Never store any chemicals near your furnace or gas-fired air handling unit. Seemingly safe chemicals can become corrosive when drawn into the combustion chamber and heated. This corrosion will shorten the life of the heat exchanger and can cause premature failure.