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How to check a Heat Exchanger for a leak

Leaks in heat exchangers are a sales mans best friend. Well at least a furnace salesman, first words out of their mouth will be “YOU HAVE TO INSTALL A NEW FURNACE”. You see you will feel backed into a corner and have (safety wise) nowhere to turn. So we will discuss everything you need to know about heat exchanger cracks here and now. Remember you have probably had a crack in the furnace for quite some time, so dont jump to fast. Not to say that they cannot be dangerous, just saying take your time to make an intelligent decision.

What are Heat Exchanger cracks?

Over time the metal in a heat exchanger has a lot of heat expansion and contraction stress. The stress sometimes makes a small crack in the metal, at other times the crack is rather large. I have even seen the seams on the folded metal leak water when we put some into a heat exchanger that was pulled out of an older furnace. You could even have the crack get larger when the furnace is running due to expansion, at least thats a theory I have heard but never seen. You would think the crack would get larger without heat, basic expansion with heat, contraction without theory….?

Whats the Danger?

So what? Big deal cracks or holes? The concern is that a cracked heat exchanger could allow exhaust gas from the furnace to fill your home with carbon monoxide, which could kill you in large quantities.

Fire!
There is a fire in your heat exchanger as well. I have seen cases where the fire is blown out from a crack in the heat exchanger, or where the fire has extra air and burns very very erratically. On one furnace I even seen the flames coming out of the heat exchanger and burning the wires and controls that in effect control the gas and furnace. So there can be a real danger. But a quick visual inspection of the furnace while the flame is on and the blower moving air will tell you if the flame is erratic or not.

Carbon Monoxide ?
The silent killer. For carbon monoxide to get into your home the furnace must be producing high levels of carbon monoxide (meaning its way out of adjustment/ or something is effecting the way the furnace is mixing the air and the fuel) then the exhaust gas must be mixing with the household air (through a hole or crack in the heat exchanger). For a good example of a hazardous heat exchanger, check out the photo below showing a large rust hole in the heat exchanger of this high-efficiency furnace that was only ten years old.

Large Hole in Heat Exchanger

www.furnacebook.com

So what to do if your expecting a crack, or you have evidence that there is a crack in the heat exchanger.

So what’s the big deal with cracks or holes? The concern is that a cracked heat exchanger could allow exhaust gas from the furnace to contaminate the household air with carbon monoxide. In order for this to happen, the furnace must be producing high levels of carbon monoxide AND the exhaust gas must be mixing with the household air. For a good example of a hazardous heat exchanger, check out the photo below showing a large rust hole in the heat exchanger of this high-efficiency furnace that was only ten years old.

Cracks, on the other hand, I’m not so sure about. With the majority of the cracked heat exchangers that I’ve seen, I’ve always been curious how the exhaust gas from the furnace could possibly leak out of those tiny cracks enough to contaminate the househouse air. Of course, what I’m curious about doesn’t matter… but sometimes my curiosity gets the best of me, and I have to find out for myself.

What Excellent HVAC Contractors Do Many years ago, I attended a seminar put on by a very reputable HVAC firm, where the speaker talked about what was involved with a furnace ‘certification’, which was the type of inspection they would do when a Home Inspector suspected a problem. The guy leading the class talked about using mirrors, borescopes, smoke bombs, leak seek tests, and basically dismantling a furnace to get a good look at the heat exchanger to check for cracks. He assured us that if there was a crack to be found, they were happy to go out of their way to find it, and that’s what a furnace certification was all about.

What Other HVAC Contractors Do To gather information for this blog, I contacted 40 different HVAC contractors (23 responded). The price for a furnace certification varied between $135 and $219, and almost every contractor said that a furnace certification consists of an Orsat test. That’s it. An Orsat test measures CO² and 0² in order to determine the efficiency of an appliance, and that’s about it. It won’t have anything to do with a crack. After making all of these phone calls and sending all of these emails, I don’t think I’ll ever recommend another furnace certification. The efficiency of a furnace doesn’t have anything to do with a cracked heat exchanger.


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