testing a secondary heat exchanger

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Old 03-09-10, 03:58 PM
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testing a secondary heat exchanger

I'm a student working on a 90+ gas furnace. Our instructor told us to find out how to test a secondary heat exhanger for cracks. I can't find anything on the internet on how to do it. If anyone has any experience with it that could point me in the right direction, it would be a great help.
Thanks in advance,
Oona
 
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Old 03-09-10, 04:19 PM
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Look for water dripping down into the fan compartment for openers --- on upflow furnaces, anyway.

Pull out the fan and check the secondary heat exchanger (as much as is visible) for cracks, holes, corrosion or deterioration, again on upflow furnaces.

Test the warm air produced by the furnace for carbon monoxide. If present, you'd have to track down the actual cause of that.
 
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Old 03-09-10, 04:37 PM
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Thank you for the fast reply. But I think he wants us to physically test it. He mentioned something about the pressure switches. I can't remember right off hand if he said to bypass them or use them. It's a new Heil furnace. It's not cracked or anything. He just wants us to learn how to do it.
 
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Old 03-09-10, 06:34 PM
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I found this:

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I'd also add that if the crack(s) are bad enough you would think it make the pressure switch open, since the force of the circulation blower could overcome the weaker inducer blower, and cause circulation air to go into and UP the exchangers, diminishing the draw in the inducer fan that is pulling the vacuum.

The teacher only asked about cracks, and not anything about if dirty on the outside, or clogged on the inside?
 

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Old 03-09-10, 07:26 PM
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I think I'm messing this up. We are working in a shop at a vocational school. This furnace is only a few years old. We go through instructions, finding, identification, demonstrations, etc. to learn.
 
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Old 03-09-10, 07:55 PM
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Quite a few repair "technicians" and contractors detect the presence of a cracked heat exchanger by checking to see how old the furnace is. If it's more than five years old, they condemn the heat exchanger, turn off the gas and suggest replacement of the furnace or the imminent death of the entire family.

This method is the easiest and often produces the largest bonuses and commissions for the repair "technician.


Pretty much any other method is difficult and involves ambiguities.

I used to work as a repairman for a gas utility. We often turned on furnaces that might be in any kind of condition many times per day. The company had an interesting standard for this kind of safety issue:

1: was there evidence of flame interference? If so, the furnace was shut down and red tagged if the condition couldn't be corrected. Finding a cracked heat exchanger wasn't necessary

2: Was there more than 100 PPM carbon monoxide in the undiluted flue products of the equipment? If so, the equipment was shut off and red tagged if the condition could not be corrected.

This was interesting because it involved simple, objective observations. And if these conditions didn't exist, it was unlikely anyone was going to be harmed, even if exhaustive analysis might disclose that a crack or hole in the heat exchanger did exist.


So you could try running that line of analysis past your HVAC instructor. I'd be interested in what he might reply to that.

You could also try running the first line of analysis I gave above --- that seemed to be very popular too!

And I'd be interested to hear what he says the "right" answer might be. I think there are a variety of reasonable answers myself.
 
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Old 03-10-10, 08:48 AM
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Originally Posted by hvacwoman View Post
I think I'm messing this up.
You mean like you think maybe we weren't understanding what you are trying to do......or what the instructor wants out of you? If so, everythign both me and SP has said, still applies.

Or do you mean you are messing up how to solve the problem?
 
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Old 03-10-10, 08:52 AM
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Originally Posted by SeattlePioneer View Post
1: was there evidence of flame interference? If so, the furnace was shut down and red tagged if the condition couldn't be corrected. Finding a cracked heat exchanger wasn't necessary

2: Was there more than 100 PPM carbon monoxide in the undiluted flue products of the equipment? If so, the equipment was shut off and red tagged if the condition could not be corrected.

This was interesting because it involved simple, objective observations. And if these conditions didn't exist, it was unlikely anyone was going to be harmed, even if exhaustive analysis might disclose that a crack or hole in the heat exchanger did exist.


So you could try running that line of analysis past your HVAC instructor. I'd be interested in what he might reply to that.

You could also try running the first line of analysis I gave above --- that seemed to be very popular too!

And I'd be interested to hear what he says the "right" answer might be. I think there are a variety of reasonable answers myself.
Don't forget that with the analysis, some of what you say also applies to primary heat exchanger cracks. There is some reason why the instructor wants her to separate the primary issue from the secondary issue. There must be a specific answer he is looking for, that is solely relevant with the secondary.

Our furnace man has replaced 2 heat exchangers on Heils. I am going to call him up and ask him what for him was the tip off.
 
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Old 03-10-10, 06:55 PM
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Testing Secondary

The only sure fire way I know of (other than seeing a crack or water leak) is to remove & pressurize it. Then check for leaks with a sniffer type leak detector, submerge in water & look for bubbles, or wet it with a soap solution & look for bubbles. A change in the composition of the flue gas when the circulating fan comes on will tell you there is a leak, but not where.
 
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Old 03-10-10, 07:28 PM
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SP happened to mention this -so did Grady -and so did the supply house HVAC pro I ran this question by - and said that is there is a pretty good chance condensate water will leak out the secondary onto the blower/blower compartment.

Other than that the dept. pro mentioned odd behavior or color change of the flame after the blower came on that are affected in every primary exchanger cell, and not just one cell, if the primary were the cause.

This person also said that with some secondaries there were plastic connectors that were used that were prone to getting brittle.

Then again, the OP question could be taken a couple different ways. Maybe analyzing symptoms first/concluding secondary is bad, has nothing to do with the instructors request. Maybe there is no challenge to prove if the secondary is even bad, as opposed to simply knowing how to "test" for such cracks. If THAT is the case, then what Grady suggests would apply.

............................................................................................

Unfortunately for us here, the winter season is fast disappearing. I'll hate to see the postings here coming to an end, for the next quite a few number of months.
 
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Old 03-10-10, 11:39 PM
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<<Unfortunately for us here, the winter season is fast disappearing. I'll hate to see the postings here coming to an end, for the next quite a few number of months.>>



As a furnace repairman (no AC), I'm used to having my services be chopped liver during the summer.


But when I was self employed, I doted on having the summer free. I could be up in the islands with my fett on the transom
of my boat and schedule work for when I planned to be back in town, for the odd person who wanted their furnace service in August.
 
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Old 09-06-14, 08:15 AM
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remove and tape over the intake, where the pipe goes in. Disconnect the exhaust pipe and tape over it. Put a magnehelic gauge on your pressure port on side of burner box..... burner box must have cover on it. Your heat exchanger should now be sealed up and no air can escape. Now start your blower motor and watch the gauge to see if air is entering the heat exchanger.
Another test is 100 ppm out the exhaust when unit is running.

Also, before you start thinking that it is ok no matter what the exhaust ppm is. Don't be fooled. I have personally saw a unit putting out 1911 ppm out the exhaust. If the heat exchanger breaches, and puts CO into the living space. I believe 1200 will cause death within 1 to 3 minutes. That is part of the reason it's a problem.
 
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Old 09-06-14, 10:55 AM
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Homer.... welcome to the forums and thanks for the additional information. The original poster has not back back since the posted her original question from school. At this point we hope she's out servicing in the field.
 
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