Furnace Heat Exchanger Evaluation and Carbon Monoxide Response

Telling a family their furnace is unsafe is one of the hardest calls in this trade. Get it wrong one way and you leave a hazard in the home. Get it wrong the other way and you sell a furnace nobody needed, and lose the customer's trust. A sound decision rests on evidence, not on one test or one photo. The same care applies when you walk into a home with a carbon monoxide (CO) alarm sounding. This work belongs to qualified, licensed technicians, under your state's rules, your gas utility's rules and your employer's process.

What You Will Be Able to Do

• State the key CO exposure limits, and what they do and do not protect.
• Respond safely to a CO alarm or a CO symptom call.
• Evaluate a heat exchanger with several kinds of evidence.
• Tell a real crack from harmless marks.
• Shut down, tag and document a furnace correctly.

What Carbon Monoxide Does

CO is a colorless, odorless gas from incomplete burning. It takes the place of oxygen in the blood. Early symptoms are headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion, which is why it can be mistaken for the flu. High exposure can cause loss of consciousness and death. People who are asleep may die without feeling symptoms. Babies, older adults, pregnant people and people with heart or lung disease are at higher risk.

Know these figures, and know what each one is for.

The worker limits are for healthy adults on a work shift, not for a family living in the air around the clock. A listed home alarm is built to avoid false alarms, so it will not sound right away at low levels. A quiet alarm does not prove the air is clean. Your own CO monitor with a low-level display tells you more.

Responding to a CO Call

Your safety comes first, and so does the safety of the people in the home.

Warning: If an alarm is sounding or anyone has symptoms, get everyone out into fresh air and call 911 or the fire department before anything else. Do not enter a home with a sounding alarm without your own working CO monitor, and do not stay if your monitor climbs. Anyone with symptoms needs medical care, so tell them to get it.

1. Turn on your personal CO monitor before you enter, and watch it as you move.
2. Ask questions outside: who has symptoms, when the alarm sounded, and what was running.
3. If the fire department or utility has cleared the home, ask what they found.
4. Work through every fuel-burning source, not only the furnace. Water heaters, boilers, gas ranges, fireplaces, generators and an attached garage are all suspects.
5. Test each one, and find the source. Do not assume it was a false alarm because the reading is low now, because the home may already have been aired out.
6. Replace or add CO alarms as your company policy and local rules require.

Why No Single Test Is Enough

A crack does not always raise CO, and high CO does not always mean a crack. On many furnaces the blower pushes air at a higher pressure than the flue side, so air leaks into the flame path instead of flue gas leaking out. So no single test proves or clears a heat exchanger. Use several, and follow the maker's procedure.

Reading What You See

Heat exchangers often fail where metal flexes most: at bends, crimped seams, welds and the top of a cell. Each heating cycle expands the metal and shrinks it again.

Often a real problem:

• A line that goes through the metal, with light or air passing through.
• A crack that opens wider when the metal is hot.
• A hole, or a seam that has separated.
• Rust that has gone all the way through.
• A breached secondary heat exchanger on a condensing furnace.

Often not a defect:

• Stamped lines or formed folds from the factory.
• Surface scale and light rust that can be wiped away.
• Discoloration from normal heat.

A photo that shows a dark line does not prove a crack. Compare the mark with the same spot on the next cell. A hairline mark needs a closer look before you call it anything.

Maker criteria differ, and some state or utility programs have their own definitions. Use them when they exist. If you are unsure, say so, and get a second qualified opinion from your supervisor or the manufacturer's representative. Your job is to report what the evidence shows.

Find Out Why It Failed

A cracked heat exchanger is usually the result of heat stress or corrosion.

If the furnace is replaced and these faults remain, the new heat exchanger is under the same stress. Note the cause on the report.

Making the Decision

Shutting down a furnace

1. Close the gas valve to the unit, and turn off the power.
2. Tag it, using your company's or the utility's tag. Some utilities must be called, and some can lock the meter or valve.
3. Tell the customer in plain words what you found and what it means. Show them the evidence.
4. Give the notice in writing, and get a signature if your company requires it.
5. Check that the home has working CO alarms.
6. Talk about heat options, such as a repair, a warranty claim or a replacement. If it is cold, tell them how to stay safe in the meantime, and warn them not to use ovens or outdoor heaters indoors.

State law and your employer may say what you can and cannot do when a customer refuses the shutdown. Know that policy before you need it. Never leave a home without a written record that you warned them.

Documentation That Protects Everyone

• Model and serial number, and the age of the furnace.
• Photos with location marked, including the same spot on a good cell.
• Combustion readings, before and at blower start.
• Static pressure, rise and input.
• Ambient and supply air CO readings.
• What you told the customer, and when.
• Warranty status. Heat exchangers often have long warranties, and the maker may ask for the evidence.

Honesty Protects the Trade

Do not condemn a furnace to sell a new one, and do not pass one to keep a customer happy. Sales targets have no place in a safety decision. Showing a customer a stamped seam and calling it a crack is dishonest, may be fraud, and can cost you your license and your job. Leaving a real breach in service because the customer does not want the news can cost a life.

Worked Example

A customer has headaches on winter mornings. Their CO alarm has not sounded. Your monitor reads 6 ppm in the hallway on arrival.

1. You check the water heater and range first. Both test clean.
2. The furnace reads 62 ppm air-free at steady state. When the blower starts, CO jumps to 340 ppm and oxygen rises.
3. A camera shows a split at a bend in cell 3, and the same spot on cell 2 looks intact.
4. Static pressure is high and the rise is 15°F over the plate, because of a restricted return.

The breach is real, and it has several kinds of evidence behind it. You shut the furnace down, close the gas valve, tag it, give the written notice and advise the family to see a doctor about the headaches. Your report also names the return restriction, so a new furnace does not suffer the same stress.

Common Mistakes

Check Your Understanding

You arrive for a no-heat call and your CO monitor climbs as you walk in. What do you do? Get out, get everyone out and call 911 or the fire department.

A dark mark on a heat exchanger wipes away, and it does not go through the metal. What does that suggest? Surface scale or discoloration, which is often not a defect. Record it.

CO in the supply air is zero. Does that prove the heat exchanger is sound? No. The blower pressure can keep flue gas out of the airstream even with a crack.

A cracked heat exchanger is being replaced, and pool chlorine is stored next to the furnace. Why mention it? Corrosive air can attack the new heat exchanger too, so the cause goes on the report.

The fire department aired out a home, and your monitor now reads 0 ppm. Is the call over? No. Test every fuel-burning source and find what caused the alarm.

Key Takeaways

• If an alarm sounds or people have symptoms, get out and call for help first.
• Worker limits and alarm times are not safety limits for people at home.
• Test every fuel-burning source, not only the furnace.
• No single test proves or clears a heat exchanger.
• Shut down, close the gas, tag it, and tell the customer in writing.
• Evidence decides, and sales never do.