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  1. #1
    Join Date
    Dec 2003
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    25

    Question

    The compressor fried on my 1998 Rheem 10 Seer 5-ton unit and I am in the bidding process for a replacement. I am looking for a few objective second opinions.

    Our Rheem 10 seer 5-ton installation was never right. It seemed the furnace & AC seems to cycle too often, did not seem to cool the house well and we dislike the noise and disruption of the furnace blowing at full speed when the AC is on.

    Is smaller and slower better? A local contractor recommended that we replace with a 4-ton unit. The logic being the line set is too small for a 5-ton, the ducting seems to not support the 5-ton and a 4-ton would run longer doing a better job of cooling the house. Does it this seem right and like a good idea?

    If I should downsize to a 4 ton.
    - Can we use the existing 5-ton Rheem coil and adjust/replace the flow rate?
    - We would much perfer a slower, less noisy blower speed and to run the AC longer. Can the ComfortMaker RJ2 with a General 90 50A50 and a 2000cfm blower with 4 speeds be made to run a lower speed setting (rather than high) and the AC tonage & coil sized to that lower cfm setting?


    Thanks Much,

    Bill

  2. #2
    Join Date
    Nov 2001
    Posts
    1,874
    Only some one who is there to see your layout, and does the load calc.

    Can say if you need a 4 vs a 5 ton,

    And only someone who sees your house can tell if the duct will handle either 1.

    way to many variables to get a good answere.
    load calc. And a duct design program would need to be used.
    If you try to fail, and succeed.
    Which have you done ?



  3. #3
    Join Date
    Nov 2004
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    2,927
    depending on where you live...you will need so many cfm's per ton.An average 350 cfm per ton.
    350x4 ton=1400cfm.

    Your ducting must handle the volume of airflow at a velocity that you find acceptable for noise levels.

    If you were disatisfied with the contractor,then learn a lesson there.It would be wise to your homework on duct sizing before having someone put in a new unit.

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,750
    Click the bulls eye in the upper right corner, and you can do your own load calc, and see what size a/c, and furnace you should have.

    I think it will also give you duct sizes.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  5. #5
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,633
    "It seemed the furnace & AC seems to cycle too often,"
    That is a sign of an oversized air conditioner but no guarantee that it is.

    "did not seem to cool the house well"
    In what way? Was it too hot? Too humid? Too uneven?

    "and we dislike the noise and disruption of the furnace blowing at full speed when the AC is on."
    That's a product of duct design, furnace location, furnace selection, etc. Obviously a smaller AC isn't going to change that a whole lot. You could slow the fan down and make it a bit quieter. But I doubt it will make a lot of difference.

    "Is smaller and slower better?"
    Yes, if smaller and slower also happens to be the correct size. Bigger and faster is better too when bigger and faster is what the load calc says you need.

    "A local contractor recommended that we replace with a 4-ton unit. The logic being the line set is too small for a 5-ton,"
    What size is the lineset? What are the pipe's outer dimensions?

    "the ducting seems to not support the 5-ton"
    If your tech had measured or offered to measure (even for a price) the airflow then "seems" wouldn't have been the word. You should to know for certain, not guess.

    "and a 4-ton would run longer doing a better job of cooling the house. Does it this seem right and like a good idea?"
    Maybe.

    "If I should downsize to a 4 ton.
    - Can we use the existing 5-ton Rheem coil and adjust/replace the flow rate?
    "
    Yes. But then your cooling coil would be oversized. That's usually good in dry climates. But that's generally bad in humid climates. Oversized coils increase heat removal at the expense of humidity removal. If you’re in a humid climate and if a 4 ton AC is the correct size then a 4 ton coil should most likely be put in as well. But we still don’t even know if a 4 ton unit is actually the correct size. You need a proper heat load calc. At least tell us what part of the country you’re in, how humid it is and what your square footage is.

    "- We would much perfer a slower, less noisy blower speed and to run the AC longer. ""
    Noise and performance should be separated in your mind. The two are not in opposition to each other if you design the system right. But if you're not going to spend the dough to do that then I'd suggest picking performance over noise.

    "Can the ComfortMaker RJ2 with a General 90 50A50 and a 2000cfm blower with 4 speeds be made to run a lower speed setting (rather than high) and the AC tonage & coil sized to that lower cfm setting?"
    A so called blower setting sets the airflow to nothing in particular. In other words the airflow is a product of blower speed tap, blower rating, duct size, duct installation and so on. You and your tech will NOT know what the airflow is unless it's actually measured. As absurd as this sounds few techs measure airflow. It takes one or more very expensive tools to properly measure delivered airflow. Airflow through the furnace can be calculated with a fairly cheap tool. But that doesn't give you delivered airflow. And amazingly few techs do that either!

    The short take is this: What you've presented gives us a very incomplete picture of what’s going on. And despite the fact that he’s already made solid recommendations, I’ve got ten bucks that says your technician is playing it by ear too. IF that’s the case then don’t feel too bad about it. You’re just experiencing the typical HVAC experience.

  6. #6
    Join Date
    Apr 2004
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    7,680
    Well said Irascible.

    Havent seen you in a while. WB

  7. #7
    Join Date
    Dec 2003
    Posts
    25
    Originally posted by beenthere
    Click the bulls eye in the upper right corner, and you can do your own load calc, and see what size a/c, and furnace you should have.

    I think it will also give you duct sizes.

    Great tip. I have downloaded the demo and watched the movie. I will break out the tape measure and pad.

    Thanks much,

    Bill

    [Edited by billyray on 09-07-2005 at 11:00 PM]

  8. #8
    Join Date
    Dec 2003
    Posts
    25
    Originally posted by jacob perkins
    depending on where you live...you will need so many cfm's per ton.An average 350 cfm per ton.
    350x4 ton=1400cfm.



    Thanks much.. 350 cfm per AC/cooling ton is a great tip. Oh ya.. I live in Southern California 10 miles from the coast. Never over 100, never too cold. Low humidity.

    Originally posted by jacob perkins
    If you were disatisfied with the contractor,then learn a lesson there.
    I hired the same guy the builder used to install HVAC to reduce my risk.. did not help this time. The design (his design) was poor to begin with and he did not seem to know how to get it right on service trips.

    I am not up for being too hard on the guy because it wont do any good and the guy seems to be struggling to make life work. Live and let live.

    Bill


    [Edited by billyray on 09-07-2005 at 10:55 PM]

  9. #9
    Join Date
    Dec 2003
    Posts
    25
    Wow, great questions. I appreciate the time you took to post them.


    Originally posted by Irascible

    "did not seem to cool the house well"
    In what way? Was it too hot? Too humid? Too uneven?
    Answer: Too uneven. I have two story. 40% of my glass faces southeast, 40% of my glass faces southwest. Cold air would blow strong in the north & north east rooms close to the furnace. Without closing off half the vents in the house very little air flow would reach the big rooms with lots of glass and southern exposure.

    It does not take a rocket scientest to know that something is wrong when you have a the 8" duct pouring out 5 feet from the furnace into a small cold corner with no windows and then see the 6" duct to the furthest, hotest part of the house. My master bedroom could be at 95F and at same time the northeast downstairs corner will feel like a walk in freezer. In addition to a new unit I also expect to make adjustments to the existing ducting.


    Answer: Too hot. The tsat is in the northeast corner on the first floor .. the coolest part of the house. Even when I close all the registers in that part of the house it is still the coolest. I have to set the tstat to 72F to get the upstairs southwest facing MBR down to 82-84F. I have been wondering if relocating the tsat upstairs would be a violation of the UBC or otherwise unadvisable.

    Originally posted by Irascible

    "Is smaller and slower better?"
    Yes, if smaller and slower also happens to be the correct size. Bigger and faster is better too when bigger and faster is what the load calc says you need.

    "the ducting seems to not support the 5-ton"
    If your tech had measured or offered to measure (even for a price) the airflow then "seems" wouldn't have been the word. You should to know for certain, not guess.
    Ok, those are both good points.



    Originally posted by Irascible

    "A local contractor recommended that we replace with a 4-ton unit. The logic being the line set is too small for a 5-ton,"
    What size is the lineset? What are the pipe's outer dimensions?

    My condenser is 50 feet of line length away and 4 feet physically lower than the coil. Liquid line is 3/8", Vapor line is 7/8" OD. A table in the existing unit's install manual implies that a 1 and 1/8" OD vapor line would have been the right size for the existing 5-ton unit that fried.

    Thanks for any insight and wisdom,

    Bill



    [Edited by billyray on 09-08-2005 at 12:10 AM]

  10. #10
    Join Date
    Dec 2003
    Posts
    25
    Originally posted by Irascible


    "If I should downsize to a 4 ton.
    - Can we use the existing 5-ton Rheem coil and adjust/replace the flow rate?
    "
    Yes. But then your cooling coil would be oversized. That's usually good in dry climates. But that's generally bad in humid climates. Oversized coils increase heat removal at the expense of humidity removal. If you’re in a humid climate and if a 4 ton AC is the correct size then a 4 ton coil should most likely be put in as well. But we still don’t even know if a 4 ton unit is actually the correct size.

    You need a proper heat load calc. At least tell us what part of the country you’re in, how humid it is and what your square footage is.

    I am 10 miles from the Pacific in San Diego county. Humidity is low. Not as low as Arizona but nothing like OK, Houston or Alabama. Two story, 2670 sq ft. The big rooms with lots of glass have either a south eastern or southwestern exposure. There is some shade on the Southeastern side and non on the Southwestern side. The house was built in 1995 but is not tight. There is a 1/8" gap the full width of the 5' front door. The house has cheap dual pane metal framed windows with open weep holes big enough to shove a nickle through. The windows have tinting film which seems to trap the heat right at the windows. R30 bat insulation in the ceiling, R11 in the walls. Spanish tile roof.

    Thanks much,
    Bill

    [Edited by billyray on 09-08-2005 at 12:19 AM]

  11. #11
    Join Date
    Dec 2003
    Posts
    25
    Originally posted by jacob perkins
    depending on where you live...you will need so many cfm's per ton.An average 350 cfm per ton.
    350x4 ton=1400cfm.
    Ok. I have been following up on this tip.

    A table in the furnace manual says that when the blower is set to:
    - High with and external static pressure of .6 the blower rates out at 410cfm for a 5-ton load.
    - Medium-High with and external static pressure of .5 the blower rates out at 425cfm for a 4-ton load.
    - Medium-low is assigned to "Heat" so it is not an option for cooling.
    - Low with and external static pressure of .4 the blower rates out at 389cfm for a 3.5-ton load.

    So... looks like my best blower option is 4 tons and 3.5 will work. Now to see what the tonage the house demands.

    I am running the blower right now set to Medium-High. It's only a 17.8% drop in cfm but the air noise is down. However, even from the far end of the house the furnace hum still looms large. I will need to figure out how to dampen noise from the cold air return.


    Thanks again,
    Bill

  12. #12
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,633
    Thanks Doc.

    Bill:

    Since you're in a dry climate you want as much air as you can get. Personally I shoot for 500 CFM per ton. That's likely an unrealistic goal unless you replace all the ducts. But 400 is an absolute minimum IMO. Whatever you end up with, it'll be an issue of getting big enough ducts.

    The unevenness problem is also an issue of getting the ducts right. In a two story home it's virtually impossible to get even comfort without installing a zone system. Read the following and be depressed.
    http://hphaa.com/services/installati...m#Zone_Systems

    Ideally the suction line would be 1 1/8 inch. But 7/8 inch IS approved most of the time with the understanding that there will be a small loss of capacity.
    http://hphaa.com/services/installati...rigerant_lines

    You live in a dry enough climate that oversizing is NOT your issue. It's ducts, period - end of story. They need to be upgraded and zoned.

    Yeah, I know. We can't possibly know for sure unless we're there. And that's true. You could be a schizoid drunk that lives in a shopping cart and browses the internet at the local library until some lady with her hair in a bun chases you out. But assuming that's not the case; and assuming your details are accurate and you haven't left any material fact out (like the possibility that you already have a zone system); then ducts are your issue. Make them bigger and zone them. If a lot of them are sitting behind sheet rock then you need to break sheet rock or forget about it. The ducts MUST be upgraded. Am I being emphatic enough?

  13. #13
    Join Date
    Dec 2003
    Posts
    25
    Thanks much Irascible,


    Originally posted by Irascible
    Thanks Doc.
    You could be a schizoid drunk that lives in a shopping cart and browses the internet at the local library until some lady with her hair in a bun chases you out.
    Oh no, I have been found out! You must have seen me. But the last few days I tricked the old bitty into think I am one of her own by wearing my hair in a bun. Now I can use thier Internet connect all afternoon no problem.


    Ok, I get it. Ducts! Ducts! Ducts! Zone! Zone! Zone! And fixing my return shortage by adding an upstairs cold air return might not be a bad idea either.

    So if your willing I want to come back to the to replace or not coil question. Given my cliamte if I contract for a 4-ton (or even 3.5) 10 Seer replacement outside should I just reuse my existing 5-ton 10.2 Seer Rheem coil? On bid suggested adding a TXV to the existing coil.

    What if I replace with a 12-13 Seer unit instead? Improve and reuse or chuck the current coil?

    Given my climate even if I purchase a new high eff coil should I upsize .5 to 1-ton over the new outside high eff condensor?


    Thanks much to all who share wisdom here.

    Bill







    [Edited by billyray on 09-08-2005 at 03:16 PM]

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