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  1. #1
    Join Date
    Aug 2006
    Location
    Glendale, CA
    Posts
    2
    Two years ago I replaced an older ('89) heat pump in a condo that I rent out. At the time, the salesman recommended increasing the tonnage of the unit to 3 tons, but he didn't do a load calc or balance that I'm aware of. I did specifically ask if the duct work was adequate for this increase and he said it is. (If I were to do this again, I'd sure insist on duct sizing and load calculations with hardcopy handed to me, so I could verify the recommendations... live and learn!)
    The tenants at the condo have had problems with the system off and on during these 2 years. Several times now, the system has frozen up on especially hot days, or when they've returned from vacation and tried to bring the air temp down from a high, mid to upper 80s setting, to a normal setting, 77 or so.
    When the AC tech has come out to investigate, he's simply said they need to do a better job of cleaning the air filter, and shouldn't run it for too long at a time. I happen to know for sure that the air filter is being kept pretty clean - they replace it every 30 days like clockwork (better than most people, including myself!)
    Also, last week, when it hit 105 there, the system froze, even though it was a brand new filter.
    Now I'm in a debate with the AC company, where I am stating that I think the return duct was undersized, but they insist it is not, and the problem is dirty filters and poor maintenance.

    Here are some of the facts and figures:
    Rheem RBHC17J furnace/blower
    Rheem 10PJB36 heat pump
    1100 sq. ft condo with vaulted ceilings in half, lots of single pane windows and sun heating. Located i nSaanta Clarita CA, where it is typically 95+ and sometimes 100+ during summer.
    The air return is a 14 x 25 inch grille leading into a 14" round flex duct, with a pretty straight 6 foot run into the plenum.

    So, I have a few questions:

    1) Is the 14" flex duct air return adequate? What additional info is needed to answer this question for sure?

    2) The maintenance manager for the AC company is meeting with me on Thursday to inspect the system. What should I request at that meeting? Static pressure and air flow measurements? Inspect fan settings and coil cleanliness?

  2. #2
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    14 X25 return is undersized,by industry standards.

    14" flex may or may not be large enough,depends on the sizes ,lengths,turns,twists,elbows ,etc.,of the entire duct system.See Manual D ,www.acca.com if you want to know more.


    If your indoor unit is the one I think it is look carefully at page four of this ;

    http://216.122.22.11/FetchDocument.a...0-de9a579f5bb9

    If this is similar to your indoor unit ,note the supply takeoff at the top.The supply duct should be that size for approx. 18" from the unit .If the supply duct/plenum is larger,then that ,or if the "tabs"(not shown) at the outlet are not bent stright up,that could be your problem.

    Failure to do this prevents that model from converting velocity pressure,to static pressure(static pressure is required to move air).

    So if you remember the plenum/supply duct being the same size as the cabinet,that plus possiblely the tabs are the likely problem,or part of it.

    We find these installed with a larger duct every week of the year,very common problem in Florida.

  3. #3
    Join Date
    Aug 2006
    Location
    High Point, NC
    Posts
    155
    The duct off of the air handler should be a minimum of 6" to 12" of the same size as the duct flange on the air handler. 18" would be better. The duct should be wrapped on the outside and not internally lined. If it has internal insulation the air velocity could remove this insulation if it is not pinned & the duct size would need to be increased by the insulation size. If the insulation has come loose it could be blocking the air flow. Outside wrap is better on the first part of this ducting. If the filter is a 14 X 25 it will not do as good of a job filtering the air as a larger size, since the air will be passing thru this filter faster. The Static pressure drop should be checked on the air handler for better answers.

  4. #4
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,122
    Originally posted by iheatncoolinnc
    The duct off of the air handler should be a minimum of 6" to 12" of the same size as the duct flange on the air handler. 18" would be better. The duct should be wrapped on the outside and not internally lined. If it has internal insulation the air velocity could remove this insulation if it is not pinned & the duct size would need to be increased by the insulation size. If the insulation has come loose it could be blocking the air flow. Outside wrap is better on the first part of this ducting. If the filter is a 14 X 25 it will not do as good of a job filtering the air as a larger size, since the air will be passing thru this filter faster. The Static pressure drop should be checked on the air handler for better answers.

    Did anyone here understand this post? maybe I need another cup of coffee but I don't understand anything this person said.
    __________________________________________________ _______________________
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  5. #5
    Join Date
    Aug 2002
    Posts
    35
    Have him verify that the duct flanges are turned up.....afterall some installers ignore that orange sticker on top stating to do so.

  6. #6
    Join Date
    Oct 2004
    Posts
    167
    After reading the previous posts, I understood it. More joe Im thinking. Maybe some espresso

  7. #7
    Join Date
    Aug 2006
    Location
    Glendale, CA
    Posts
    2
    Some more information:
    The system is configured left flow horizontal in the attic. The air return duct comes off the side, not the end, of the plenum, and does about a 6 or 8 foot run with a gradual bend back (away from the inward flow direction) and downward into a vertical ceiling grille. So, the air actually traverses about 120 degrees of bend off of the preferred straight air flow direction (90 degrees due to side mount, plus another 30 degrees for grille position).

    Another thing I'm concerned about is the refrigant line from the outside compressor to the blower. The Rheem manual for my system says it MUST be a 5/16" line which cannot exceed 25 feet of vertical separation. I need to look at the line again, but I remember thinking it looked small, and maybe is a 1/4" line(?) Also, I may be right at, or over the 25 foot limit. The blower is in the attic, which is actually the 3rd floor from ground - ground level is the garage, 2nd floow is the downstair neighbor unit, 3rd floor is my unit, and then the blower is in my attic. So, it might be something like 30 feet up.... I need to check. Certainly, if enough liquid cannot flow, that would account for the exterior unit freezing when the system runs for hours and hours.

  8. #8
    Join Date
    Jul 2003
    Location
    Los Angeles, California
    Posts
    184
    5/16" refrigeration copper is the same size as 1/4" plumbing copper. Refrigeration measures outside diameter.

  9. #9
    Join Date
    Dec 2005
    Location
    California/Nevada
    Posts
    3,729
    the return duct is slightly smaller then it should be ,but it's not the cause.

    ...some kind of mechanical problem.
    i would start by checking the metering device.

  10. #10
    Join Date
    Jul 2006
    Posts
    1,091
    Mrbill? You got your coffee?

    For a 3 ton unit, we install a 24x24 return air grille minimum. Flex duct, 18" minimum.

    My empty thoughts:
    105* outside, the unit will run and run and run and run...

    A 14x25 filter would need to be replaced every couple of days.

    So yes, lack of maintenance; because they didn't INSTALL THE RIGHT SIZE RETURN AIR!!!!!

    Orange sticker.... what orange sticker?

    [Edited by rickboggs on 08-15-2006 at 07:39 AM]

  11. #11
    Join Date
    Aug 2006
    Location
    High Point, NC
    Posts
    155
    If the supply duct is not done properly,low air flow problems will exist. The installation instructions for the air handler would explain how the supply ducting should be installed. I think this is on page 11 of the installation instruction under the heading of ductwork. You would need to read the entire section.

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