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So what is your procedure of checking heat exchangers on 90% furnaces?

Discussion in 'Residential' started by ct_hvac_tech, Feb 23, 2012.

  1. ct_hvac_tech

    ct_hvac_tech I specialize in kicking ass.

    Messages:
    1,112
    Location:
    Connecticut
    As most of you know, I don't really deal with a whole lot of gas, as our company primarily deals with oil.

    so, I never really learned what the proper way is of checking these. With the clamshell ones I can use my see-snake. But how do you check the tubular ones?
  2. Lurk

    Lurk New Member

    Messages:
    3,760
    there is no agreed upon way of checking them.

    here is the order of importance to the customer:

    1. multiple CO detectors in home
    2. proper cleaning and combustion analysis
    3. heat exchanger inspection

    that "inspection" can be done with any or a combination of methods.

    this has been a sore spot for me for as long as i've been in the trade.

    especially considering in commercial, you can take out ten screws and visually inspect the heat exchangers.

    it's absolutely stupid that you can't do that from the back of ALL these resi furnaces.

    i've been known to cut the back right off with turbo-shears if in doubt.

    but these 90%ers run with such powerful vent motors, that a serious heat exchanger defect usually will create very noticeable issues with the flames and the pressure switch and show up in the combustion analysis as well.

    my two cents. however, i don't know it all. lol

    one more thing: i think the manufacturers want the customer to buy a new "upgraded" furnace every six years anyway, so not time for heat exchangers to corrode away, lol.
  3. ct_hvac_tech

    ct_hvac_tech I specialize in kicking ass.

    Messages:
    1,112
    Location:
    Connecticut
    How do you go about combustion analysis?

    With oil burners, we have an efficiency test kit that takes several tests like CO2, etc.

    Do they make something like this for gas furnaces? And if so, how does one get to know what the proper readings are?
  4. otto

    otto Bakery Chef

    Messages:
    18,656
    Location:
    in a van down by the river
  5. otto

    otto Bakery Chef

    Messages:
    18,656
    Location:
    in a van down by the river
    There's a lot of info out there, just google "heat exchanger combustion analysis" or some other form of the same.
  6. Snoring Beagle

    Snoring Beagle Need Little - Want Less - Love More

    Messages:
    10,325
    Oil

    I have used a magnahelic to see pressure changes caused by the the big blower fan, especially with the intake and exhaust ports plugged with rags.

    I remember some old timer that would put some mint oil in the pot and then turn the fan switch on to see if they could smell it in the living area.

    Then there was the smoke bomb test.

    But the magnahelic would see a pressure increase even with the burner running. It would go up when the main blower would come on.
  7. CincyHVAC

    CincyHVAC Hoptimus Prime, Master of Beer-Fu

    Messages:
    45,946
    Location:
    Ohio's Southern Shoreline
    Proper readings for NG (using a combustion analyzer):

    No more than 400 ppm CO in the flue at start up, and then the CO #'s should drop to @ 100 ppm and stay steady. Any time CO is above 100 ppm or unstable, there is a problem. If the CO concentration keeps rising above 400 ppm, chances are there is a problem with the HX, and requires visual confirmation. Could be a crack, could be a plugged up secondary, who knows.....
    O2 should be between 6% and 9% and stable.
    Flue temp on an 80% furnace should be 170* PLUS the supply air temperature.
    Flue temp on a 90% furnace should be about 100*-130* (or pretty close to supply air temp).

    You want to establish a trend through the entire cycle: start up, run, and shut down.

    The above readings are just barely scratching the surface of understanding combustion diagnostics
  8. jeepman1

    jeepman1 chief idiot herder -1

    Messages:
    152
    Location:
    iowa
    This is it.... when I first learned combustion analysis I was so excited. Just to have the guess work taken away.
  9. Lurk

    Lurk New Member

    Messages:
    3,760
    as the others have said, but just to add another point.

    the answer is: there is no good (or easy) way to completely inspect a heat exchanger.

    i have never been a fan of the telescopes.

    i see just as much with an inspection mirror and a bright led flashlight than i ever have with those.

    i think the other part of the equation is learning "where" different heat-x's tend to fail.

    the moral of the story is (for me), the heat exchanger gets a cursory inspection at best.

    what i am really focusing on is:

    - the installation (safety of venting)
    - presence of quality CO detectors in the home
    - visual flame condition, disturbances, yellow flames, presence of soot
    - difference in appearance from one heat exchanger cell to the next (indication of problem)
    - tripping pressure switches
    - flame rollout
    - failed combusion analysis

    at that point, you KNOW there is an issue that warrants you either:

    - scoping the hell out of it
    - pulling it
    - or just quoting a new one

    it's process of elimination, which i hate.

    but combusion analysis, and use of a magnehelic gage will eliminate a lot of guesswork.

    you want to know the total CO content of the exhaust? walk outside and stick it in the exhaust pipe
  10. ct_hvac_tech

    ct_hvac_tech I specialize in kicking ass.

    Messages:
    1,112
    Location:
    Connecticut
    Wow lots of good advice here.

    As for the CO Analyzer. is there a good one to get? and where can I get one and how much do they cost? I'm prolly going to be dealing with a lot more gas equipment when I move on so good things to know.

    With oil units, the procedure is to drill a hole in the flue pipe so we have a test hole to put our instruments into. Is this also done with gas units?
  11. Chuck

    Chuck Old New Member

    Messages:
    3,424
    Location:
    NW WI
    One thing to keep in mind, cracked HX are almost never a source of CO poisonings. The only ones I have heard of are the Lennox Pulse which has positive pressure pulses which will "puff" exhaust out through a crack into the airstream. Almost all others have negative pressure in the HX. Does this mean you can just leave a cracked furnace in operation? NO! It needs to be repaired/replaced.

    No single method will find the cracks in all furnaces. So use all the methods at your disposal. A good high resolution camera and a combustion analyzer are 2 good ones. And when using a camera it is best to inspect the air side of the HX rather than the fire side, especially on furnaces with inshot burners.

    As Lurk said, the important thing is to know where to look. Training by Shamrock Industries' Tom O'Connor is great, and you get a large book with photos documenting hundreds of different models and where they are most likely to crack. Also Greg Hunsicker (spelling?) with Furnace Safety Consultants puts on a good class, he has his own book of failure pics. I have both books and they are valuable for training as well as showing the homeowner that this is a documented problem, we aren't just trying to rip them off.
  12. Chuck

    Chuck Old New Member

    Messages:
    3,424
    Location:
    NW WI
    The Testo 327 is a good product for around a grand. Bacharach models are fine, but they have a slow pump and slower response time. Don't bother with the models that require a hand pump to operate. A continuous sample throughout the combustion process from light off through shut down is very important.

    I always drill into the venting to take samples on oil or gas appliances. It takes too long to run outside every adjustment you make to see where you are at. Plus many vents terminate on the roof.
  13. ct_hvac_tech

    ct_hvac_tech I specialize in kicking ass.

    Messages:
    1,112
    Location:
    Connecticut
    With that said, what is your opinion on patching the hole you drilled in the vent pipe?

    Around here, (at least with oil anyway), we don't bother patching or plugging the hole as it is a hole of less than 1/4 of an inch. Common place here, but I'd bet money in other areas people would think we are nuts lol
  14. Bender

    Bender Promoted to Regional Turd Polisher

    Messages:
    22,744
    Location:
    Mamby Pamby Land
    Check out these videos.....this guy knows his shit.......:D

    If you're looking at a cased coil on top of your furnace, you have easy VISUAL access.....:thumb:





    Yes, these are 80% furnaces, but the only way to get access to the primary is through the top of a 90%......
  15. Lurk

    Lurk New Member

    Messages:
    3,760
    pulling the blower motor IF NECESSARY gives good access too.

    another furnace that, if i remember correctly, runs at a highly positive pressure (forced draft as opposed to induced) was the Coleman/Evcon downflow trailer furnaces. and they had heat-x problems. this can be found by removing the burner (four screws) and using an inspection mirror with flashlight. a bad heat-x there will get ya.

    i'm practical about it. not every furnace i touches gets a thorough heat-x inspection (cursory, at best). the reason being, i can't add an hour to each service call i run, and the customer isn't going to want to pay for that either.

    to be honest, i just look at a furnace and tell people "this is old, i can't guarantee the heat-x is sound simply due to age/corrosion/condition/whatever" and put it in writing that they need a new unit. i just don't have an hour to dick with checking every heat-x i come across. CYA and COMMON SENSE goes a long way, as does PAPERWORK.

    i've seen plenty of people drill into the side of a 90 percenter's ventpipe, they even make little plugs you can forcefully push in that are removable for future checking. just don't do it on the bottom or you will have a leak.
  16. ct_hvac_tech

    ct_hvac_tech I specialize in kicking ass.

    Messages:
    1,112
    Location:
    Connecticut
    hahaha dipyy was here! :p
  17. Snoring Beagle

    Snoring Beagle Need Little - Want Less - Love More

    Messages:
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    A fuel oil furnace is a power burner. You can pressurize one of those very easy. In fact as the heat exchanger builds up soot it's common to see the air shutter getting opened more and more to get a clean burn. The thing with a fuel oil furnace is it will stink the joint up.
  18. Snoring Beagle

    Snoring Beagle Need Little - Want Less - Love More

    Messages:
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    Dam it! I read the OP wrong this ain't about oil!


    Nevermind.............:alcy::rolleyes:
  19. Bender

    Bender Promoted to Regional Turd Polisher

    Messages:
    22,744
    Location:
    Mamby Pamby Land
    :D
  20. tuna

    tuna New Member

    Messages:
    2,176
    That is a good point. Has a cracked heat exchanger on an induced draft furnace ever caused a CO poisoning? If so Id like to hear the details. Id also like to hear about a cracked hx in a gas pack causing CO poisoning if its ever happened.

    IMHO way to much attention is given to furnace hx's. Why? Because its a way to sell furnaces. Find a little crack, bingo, you need a new furnace or you all may die. In reality the real danger is in the flue piping. Thats what needs to be looked at close.

    When Im old and cant crawl under my house, the guy looking at my furnace will not have a camera, soapy stuff the spray on the hx, or a combustion analyzer. He'll be checking out the flue pipe. Ill be looking in the access door shouting instruction. :lachen:

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