FAQs about furnace heat exchanger inspection, troubleshooting, and leak testing.

Q&A on How to inspect a furnace heat exchanger for leaks

Watch out: gas leaks at a furnace heat exchanger risk fatal carbon monoxide poisoning.

Question: how long should my gas furnace last?

what is the normal life of natural gas furnace? - Marlin 10/30/12

Reply:

Just about the warranty period - manufacturers know their equipment well; warranty periods run from 10 years to "lifetime"

Please see FURNACE HEAT EXCHANGER LIFE where we give details about residential heating furnace life expectancy.

Question: why does the pilot flame keep going out on my hot air furnace?

What would cause standing pilot flame to keep going out . thermocouple was replaced hoy air furnace - Bruce 11/17/12

Reply:

Bruce,

Check for these other causes of loss of the gas pilot flame

• A bad replacement thermocouple or one that was bent, kinked, damaged during installation
• Thermocouple not properly connected at the control, e.g. the sensor connector not fully screwed-in
• Thermocouple sensor tip not properly mounted, secured in the flame path
• Sudden drafts
• Low gas pressure, weak flame
• Inadequate combustion air - very dangerous, can be fatal
• Dirt on the equipment
• Something else we haven't thought of
• See THERMOCOUPLE REPAIR / REPLACEMENT for details

Question: is it possible to temporarily patch a leaky furnace heat exchanger?

I have a rooftop unit and have been told i have a few very small holes in the heat exchanger that can be repaired with an appropriate silicone which should hold for a season or two. True? Thanks.

Reply:

Tom,

You may be lucky to find a service tech who will touch a heat exchanger with a leak. Sure, it's plausible that leaks can be sealed with an appropriate heat-proof sealant (there are high temperature silicones).

The risks that I can think of include:

- not being able to see, find, access all of the leaks to seal them safely

- improper surface cleaning, bad bond, a "sealed" leak remains leaky

- since leaks due to rust are in the form of perforations that are usually surrounded by a larger thin-rusted area of steel, there is a real risk that just touching the leak area may poke a bigger hole, making repair difficult, or that the extent of sealant doesn't cover enough area and the patch is not reliable.

Watch out: Even though your heating unit is on a rooftop (thus outdoors) a heat exchanger leak can still send dangerous flue gases, even fatal carbon monoxide into building air.

So while no one is likely to be dumb enough to bet your life on any kind of promise or guarantee, you can yourself reduce risk by making darn sure that you've got working CO detectors properly installed in proper locations in the building.

See CARBON MONOXIDE - CO and CARBON MONOXIDE WARNINGS: HOME HEATERS

Question: safe silicone sealant for heat exchanger on a furnace?

(Nov 10, 2014) Anonymous said:
i have a silicone holding my heat exchanger tubes on some has come off is there a safe silicone for the air stream inside the furnace

Reply:

Anon

Yes probably. I would check with the unit's manufacturer for what sealants they recommend, particularly as heat and other issues will vary among equipment and even location within the same appliance

(Nov 10, 2014) Anonymous said:

Called american standard furnace they gave me a local repair mans # he said go to the automotive store i want the factory type but that wont even go there

Reply:

I doubt that's what the American Standard furnace manufacturer had in mind. Call someone else.

Question:

(Nov 16, 2014) craig loeschke said:
there is water leaking where the secondary heat exchanger is. Would that mean my heat exchanger is bad or is there a pan that is leaking. the condensate trap is doing its job, water around the base of the secondary heat exchanger is located. hoses are clear not plugged.

Reply:

The water leak from condensate could indeed destroy a heat exchanger. A more expert inspection is needed to assure your system is safe.

Question: Safety of furnace heat exchanger test procedures

Susan said: what is a heat exchanger test? Won't a pressure test suggested by my landlord cause damage to y furnace?

Reply:

Susan a heat exchanger test is a check for potentially carbon monoxide leaks out of a furnace into building air. There are several test approaches including the use of a tracer gas that will not risk damaging the system. I'm not sure what pressure test your tech has in mind. Discuss the system safety and the test risks with the service manager. Let us know what you're told.

Question: Oil burner forced hot air odor problem

(Sept 22, 2015) Steve Schassler said:
Oil burner forced hot air odor problem. In this small 1/2 duplex home severe uncombusted air smell. Severe summer summer months when furnace off. Bad also when furnace is on. During summer closed off all supply and return registers (sealed each one tightly) and oil smell has all but disappeared.

We have a new furnace but during pre-closing inspection, inspector could see burner flames via supply plenum access port due to a corrosion problem allowing combustion gases directly into supply plenum.

Could this be that supply ductwork has been contaminated prior to corrosion problem being fixed? Afraid to allow family to live in this unit. Recommendations?

Reply: warning about dangerous heat exchanger leaks

Steve:

Watch out: there is a Primary life-safety issue: if your home inspector could see the burner flames through the supply air plenum then the furnace is unsafe, should not be used, and needs repair or more likely replacement. It's not just an odor problem there's a risk of setting the building afire or dosing occupants with combustion gases that can be themselves harmful or dangerous.

I can't imagine what "fix" was done to a hole in a heat exchanger. The proper repair is to replace the heat exchanger or replace the furnace. Patch jobs are likely to be un-warranted and unsafe and unreliable.

We need a more accurate diagnosis of what you're smelling - "uncombusted air" I don't know what that means.

Secondary issue: If you are smelling heating oil odors then there has been a spill or there is a leak. Those odors may be transported through the building even when the forced air system is off. Follow the oil tank from fill through tank through piping through burner to look for leak possibilities. A spill of heating oil itself in the ductwork would be odd unless some fool ran oil lines though the ducts.

See ODORS FROM HEATING SYSTEMS

and

see OIL HEAT ODORS & NOISES

Question: is it safe to use a furnace with a small crack in the heat exchanger

(Mar 14, 2016) Anonymous said:
can i operate my furnace if there is a small crack

Reply:

Anon:

What we MUST say for LIFE SAFETY: depending on the heat exchanger crack size, location, and other conditions such as the blower fan and duct properties, it is possible for a heat exchanger crack to kill the building occupants with fatal carbon monoxide.

Also keep in mind that what's a "small crack" (at least in your view - where "small" is not defined), it can suddenly become large as heat exchanger temperatures fluctuate.

The fact is that not every small heat exchanger crack is fatally dangerous:

For example in some positions (as I said it depends on where the crack occurs), when the blower fan runs air is pushed into the heat exchanger rather than allowing dangerous flue gases out into the building's duct system.

When that's the case the only time you can even measure CO at the heat exchanger is during that interval between when the burner has turned-on and before the blower fan starts.

Watch out: But as I warned, small cracks can become big fatal ones. And there may be other cracks, holes, or damage that you've not seen. So no one in their right mind would tell you , not on-site, and not by e-text, that your heating system is safe. It is not.

On 2020-01-24 - by (mod) - concerned about cabon dioxide levels - don't confuse CO with CO2

Jack,

Of course I'm sorry that your wife's health is suffering.

Have you asked her doctor about the probably cause of elevated carbon dioxide - CO2 in her lungs? High CO2 is not necessarily related to a gas or oil fired heating system or heat exchanger at all, though I can imagine cases where it might be.

We discuss CARBON DIOXIDE - CO2 - levels in detail at https://inspectapedia.com/hazmat/Carbon_Dioxide_Hazards.php

At CO ALARM CAUSES - that's Carbon MONOXIDE or CO (not Carbon Dioxide or CO2) - we discus CO in some detail.

I mention CO (Carbon Monoxide) because even though you asked about CO2 (Carbon DIOXIDE), you asked about furnace heat exchanger leaks.

And while there are certainly test instruments of several types that can measure either CO2 or CO down to very low levels, it's useful to start your answer to a possible heat exchanger leak by starting with CO (not CO2).

CO poisoning can quicky be fatal.

Make certain that you have CO detectors properly located, installed, and tested in your home.

While there are instruments that will give a near-immediate measurement of actual CO (or CO2) level in a building, in my experience the actual indoor levels of either of those gases can vary tremendously as building conditions change:

- are windows open or shut

- how long has a furnace with a leaky heat exchanger or some other operating defect been running continuously

- how significant are the leaks or leak sources

- and a myriad of other factors.

SO what installing a CO detector buys us is a "test" that is monitoring building CO levels over time, responding either to a small level of leakage over a protracted period OR a high level of CO leakage over a short period.

Having that equipment in place is more-reliable and more fundamental and IMO more important than ANY one-off sample or measurement that you might make for either gas. And it is a homeowners first step in addressing a concern for a leaky furnace heat exchanger or from flue gas leaks from a chimney, combustion air, or draft problem that is affecting the heating equipment.

AFTER you've made sure that you've got CO detectors properly in place and tested, a visual inspection of the heating system by an expert and perhaps some measurements right at the heater, can often detect evidence of damage or flue gas leaks.

While I like test tools and have used many of them myself, in my experience it is a deadly error to rely on instruments and tests alone.

On 2020-01-24 by Jack Kotyk

Our primary care physician performed a blood gas draw and says my wife has CO2 in her lungs, which is causing deterioration in her health.

I am concerned that her condition may be caused by leaks of products of combustion from our gas appliances, or other sources, which might be too small to be detected by an ordinary household detector. Are my concerns justified, and how can I get tests performed at that level of concentration?

On 2019-12-10 by (mod) - duct cleaning company says I need a new furnace

Gary

It will help me if you can direct my attention to what on the picture is considered to be the defect or failure in the heat exchanger. The first photo was a little uncertain.

Certainly if there is a perforation or crack the heat exchanger would be unsafe.

That's what you're looking for. It's also possible that if there was extreme corrosion at the inspector may consider the system unsafe even if she couldn't see an exact opening, perforation, or crack.

That's a subjective opinion.

Meanwhile in order to assure that you're safe, if you use the system that you need to be sure that you have working carbon monoxide and smoke detectors properly installed in your home.

It certainly sounds strange for someone to leave you for any length of time with no heat.

On 2019-12-10 by Gary

Hello everyone,

I had my furnace inspected yesterday by technicians after they performed duct cleaning in my house.

The inspection was deemed mandatory as part of the duct cleaning process.

Upon boroscope visual inspection, the technicians identified a damaged heat exchanger(see attached pictures). They deemed my furnace “unsafe for my family” and shut off my gas.

They had quotes already prepared for a new furnace and advised it should be replaced right away. No other inspections were done to my knowledge. They turned off my gas and left me in a pickle.

I told them I would like to get a second opinion before making a decision and called a different company. The second technician came in and and visually inspected the heat exchanger by pulling the blower assembly.

He performed various visual checks including checking the heat exchanger with a high powered light, checking the flame for movement, etc. He also used his analyzer to check CO levels by the furnace and my vents and found nothing wrong.

Based on your knowledge, would you condemn a furnace based on those two pictures and nothing else? Would you turn off the gas to a customer without offering anything else than replacing the furnace right away after hours on overtime?

Thank you,

Gary

On 2018-12-03 - by (mod) - Flue gases contain more than one harmful substance

Thanks for sharing your opinion, George.

I agree that flue exhaust contains multiple gases that, depending on concentration can be harmful.

The most-widely-accepted and used test approach for heat exchangers does not merit the cost nor benefit from the information gained by testing for multiple different gases.

Better still, as this article explains, happily, as the gases appear in a mix, we dont have to test for all of them individually to check for unsafe flue gas leaks.

Testing for CO, for example, is the focus of home safety detectors worldwide (though for production cost savings reasons even those may respond to more than CO)

I'd welcome further specific article content suggestions, critique, research citations, etc.

On 2018-12-03 by George

One dimensional thinking in this article. Flue gases also contain SO & Nox that can be harmful.

On 2017-09-30 by Anonymous

Rachel,

Douglas DeWerth, who is discussed in this article, did the seminal work on exactly the question. You ask, re phrased as

What is the allowable heat exchanger combustion gas leak rate, recognizing the limits of construction.

The answer depends on the heaters input BTU rsting and other factors.

At the References and Citations section at end of this article you will find thst research in full detail.

For a summary of thst data for non-engineers

See our article HEAT EXCHANGER LEAK ALLOWED

On 2017-09-30 by Rachel

I am hoping to buy a new RV furnace the smallest one I can find is a 12000 BTU but I would really like to know where to find the specs on those kinds of furnaces. The specifications of how much propane can actually leak from the Firebox into the heat exchange and still call it safe.

If you have any idea where I could find those specifications I would really appreciate it. I find it I am extremely reactive to even small amounts of propane. I know for a fact I do not want to have propane residue of any kind in my tiny little home. Thank you so much and I hope you're having a wonderful day! :-)

On 2017-03-18 by Anonymous - someone deliberately sabotaged my heating system

Carla,

Use the on-page search box at the top or bottom of this page to search is inspectApedia.com for

ODOR SOUCE DETECTION

For advice

On 2017-03-18 by carla

I believe someone deliberately sabotaged my heating system as a smell is in the home that is sweet and very drying it almost feels like a powder on the lips

On 2017-02-10 - by (mod) -

Peter,

You can use the page bottom CONTACT link to send me some sharp photos and details if you're willing. That would help others.

On 2017-02-10 by Peter Deptula

Ive had 3 pulse setified companies tell me I cant have a hole in my heat exchanger 2 refused to come out. I covinced the 3rd to test for leaks. He did not, raised the fan speed from 3 to 4 and it just diluted the smell.

3 nightmire days later, I cut a hole in the case and found a hole I could stick my finger into. 3am I went to Mjiers and got muffler patch. These tech are Horable

...

Continue reading at HEAT EXCHANGER LEAK TEST - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

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• CARBON MONOXIDE WARNINGS: HOME HEATERS
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• COMBUSTION GASES & PARTICLE HAZARDS
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• ODORS FROM HEATING SYSTEMS
• SAFETY RECALLS CHIMNEYS VENTS HEATERS
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• SMOKE PENCIL / SMOKE GUN SOURCES

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