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  1. #27
    Join Date
    May 2004
    Location
    Malden Mass
    Posts
    376
    Post Likes
    [QUOTE=jdere;25631243]I was wandering when someone was going to jump in and ask for the static pressure readings. I had a similar issue 5 years ago on an oversized furnace.
    I believe the original installer found a deal on a furnace, even though it was oversized by 20k btus or more.[/QUOy E]

    Regarding my furnace problem 5 years ago, two previous tech's had diagnosed that the return was undersized. I go to the house and do static pressure check on the supply and return ducts and I found the supply plenum to be around 1,35" water column and return duct about .4".
    I couldn't add any more supply ducts or easily increase the existing ducts so
    I ended installing a 6" bypass between supply and return duct with a manual damper and got the static pressures to equalize somewhat.

  2. #28
    Join Date
    Sep 2016
    Location
    Saint Augustine, Florida
    Posts
    52
    Post Likes
    JC , Perform a static pressure check at your unit with and without the filter. The static meters these days are easy to use and accurate enough to diagnose the majority of ducting issues. Also remember to check the blower wheel itself for buildup. We all know how to many of us neglect our filter changes.

  3. #29
    Join Date
    Dec 2015
    Location
    Virginia
    Posts
    29
    Post Likes
    Quote Originally Posted by JCBirney View Post
    Hello guys. I've got an issue with a Thermal Zone 80% gas furnace. Model - TZ81UH104TA . Unit has been installed and working since 2013 without any issues. Now within past week it began cycling on the high limit. Acceptable temp rise range for this furnace is 30-60 degrees. I am getting 70+ degrees of rise. With the high side being checked on supply trunk 2 ft from plenum the highest reading I have gotten is 145 degrees. Fan is set to medium high which is factory setting on this unit. All supplies are open and have good airflow. However, there are NO returns on first floor. This house used to have boiler heat. Only returns are on 2nd floor in bedrooms. I also did a load calc out of curiosity and the furnace is oversized by 10-12 thousand btus. I also found it will run within normal temp rise limits if filter is removed from return and filter door is left open. Even with the oversize furnace I believe I can get this thing to settle in and work with more return air, but before I jump in and go that route I wanted to get another point of view. The thing that makes me scratch my head here frankly is the fact that this unit has run without issue until this year. Being oversized for house coupled to serious lack of return airflow one would think this very issue would have reared it's head far sooner than 5 years down the line.

    Check your flue pipe for restrictions, I have run into the same thing on a carrier furnace once and it drove me crazy until I figured it out

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