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  1. #1
    Join Date
    Jun 2004
    Location
    Eugene, Oregon
    Posts
    1,018

    Using manometer to diagnose furnace problems.

    A while back someone posted some good information in a thread on how to use a manometer to diagnose problems with a furnace heat-exchanger, pressure switch, or inducer motor. I'm hoping that person can share that information again.
    The poster explained how certain pressure readings at various locations in the combustion system could indicate problems with restricted intake, restricted exhaust, cracked primary hx, cracked secondary hx, bad inducer, or bad pressure switch.
    For example, how can you tell if a bad inducer motor is preventing the pressure switch from closing or if it's a leaking secondary hx?
    "The bitterness of poor quality remains long after the sweetness of low price is forgotten". --Benjamin Franklin
    "Don't argue with an idiot, they will drag you down to their level and beat you with experience". --Mark Twain
    http://www.campbellmechanical.com

  2. #2
    Quote Originally Posted by air1 View Post
    A while back someone posted some good information in a thread on how to use a manometer to diagnose problems with a furnace heat-exchanger, pressure switch, or inducer motor. I'm hoping that person can share that information again.
    The poster explained how certain pressure readings at various locations in the combustion system could indicate problems with restricted intake, restricted exhaust, cracked primary hx, cracked secondary hx, bad inducer, or bad pressure switch.
    For example, how can you tell if a bad inducer motor is preventing the pressure switch from closing or if it's a leaking secondary hx?
    Check press swt rating against inducer press.

    Still learning combustion science so I am not sure on the remainder of your question but a CA seems to be in order for the rest of it.

  3. #3
    Join Date
    Nov 2011
    Location
    St. Louis
    Posts
    3,224
    While teeing an electronic manometer inline with the pressure switch is IMO absolutely imperative to resolving pressure switch, inducer and venting issues; I don't believe it could reliably used to diagnose HE issues - with the exception of tipping off a plugged secondary in a 90%. It would have to be a pretty big breach in the HE for the inducer not to be able to overcome it. All the cracks I've found to date did not effect the operation of the furnace at all and were a complete surprise to the HO.

    I've read here where some like to tee in and then turn on the blower by itself to see if it effects the pressure...but I don't put any stock in this.

  4. #4
    Join Date
    Mar 2010
    Location
    New Haven, Indiana
    Posts
    37
    I like to use my manometer and combustion analyzer (? spelling) to get all the results you were talking about.

  5. #5
    Join Date
    Mar 2013
    Location
    Joplin,Missouri
    Posts
    310
    I use mine a lot for pressure switch issues and to measure static pressure in ducts but I do not use it to diagnose heat exchanger issues

  6. #6
    Join Date
    Jul 2007
    Location
    Tulsa, OK
    Posts
    975
    I have just blatantly lobbed the tubing into each chamber of atmospheric cells and have found it either negative or positive different than the others and then got busy looking etc. It has proven 100% every time I've tried it.

  7. #7
    Join Date
    Jun 2004
    Location
    Eugene, Oregon
    Posts
    1,018
    Thanks for the replies everyone. I know I read a post a while back from someone who provided good information of what pressure readings to take and what certain readings might indicate. I've tried searching but I can't seem to find it. I'm pretty sure it was a thread about a leaking secondary hx.
    "The bitterness of poor quality remains long after the sweetness of low price is forgotten". --Benjamin Franklin
    "Don't argue with an idiot, they will drag you down to their level and beat you with experience". --Mark Twain
    http://www.campbellmechanical.com

  8. #8
    Join Date
    May 2007
    Location
    Central NJ Area
    Posts
    788
    You can check pressure drop across a heat exchanger to detect restrictions in secondary heat exchangers. I know in some manufacturers tech lit it will give you recommended pressure drop. If it is exceeds "x" amount then usually their is an issue with the heat exchanger, vent piping or related issues. I have definitely ususd my Testo 510 on checking secondary heat exchanger after getting excessive CO readings when i suspected secondary HX restriction issues. I do trust my combustion analyzer much more. Take NCI class with Jim Davis and you will not be disappointed in the least. It probably the best class i've ever taken.. Has anyone else taken any classes comparable to Jim Davis's CO class? If so please let me know!!

  9. #9
    Join Date
    Jan 2013
    Location
    Dallas, TX
    Posts
    99
    Quote Originally Posted by air1 View Post
    Thanks for the replies everyone. I know I read a post a while back from someone who provided good information of what pressure readings to take and what certain readings might indicate. I've tried searching but I can't seem to find it. I'm pretty sure it was a thread about a leaking secondary hx.
    Temporarily cap off your flu orifice (remove vent and seal with tape) to eliminate downdraft, hook your meter to the induced draft PS port and turn on the furnace fan only. If your reading changes, it can indicate a fault in the HX. While not conclusive, it is an option for preliminary testing.

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