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  1. #1

    Confused

    HI! I bow before your infinite wisdom. I am purchasing a new furnace and was leaning heavily toward the Rheem modulator, but was told by the heating contractor that my 3.5 ton a/c unit will not be compatible with that model. He recommended the Rheem 90+ single stage. He said that the a/c was oversized for my house. He did not perform a manual J. This is only my second estimate and both contractors said " i see this kind of house all the time, a manual J is not necessary." ? can my 3.5 ton a/c work in happy unison with my future modulator, or should I look for something else.

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    34,040
    3.5 ton is fine with the Mod just so the blower is set for 1400 CFM.

  3. #3
    Join Date
    Nov 2002
    Posts
    230
    The RGFD-06 and 07 size models have a 1200 AC cfm limit. The 09 and above can handle 1400 cfm. What size furnace is he recomending?

  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,040
    That's true, the single stage 07 can have a 4 ton drive where the RGFD07 isn't. Close enough especially if the A/C is nearing the end of its life.

  5. #5

    thanks baldloonie and company

    I had a feeling that loonie would be a little intrested in my situation.I am very thankful for that!The single stage he recommended was a 75000btu, not sure it would have been the same for the Modulator. Why would a Rheem dealer steer me away from the Modulator and towards a single stage? Do they have an excess inventory of those? There are only a few "official" Rheem contractors nearby and the first estimate I received, the contractor asked me right off the bat"what do you need?'" Is'nt he supppose to tell me? Will I be able to find a contractor willing to do a manual J in my area (Cleveland, Oh)? Thanks in advance for any help offered.

  6. #6
    Join Date
    Nov 2000
    Location
    Waco, Texas, USA
    Posts
    6,153
    A manual J is a tool that the installing contractor might need (if he is unsure about the load). It is not something the homeowner needs (unless the homeowner is the installer).

    It would be like buying a cat scan machine and telling a doctor you need a scan performed on you. Maybe you do and maybe you don't but it is the doctors call.

    [Edited by Steve Wiggins on 10-06-2005 at 05:05 PM]
    "And remember my sentimental friend......that a heart is not judged by how much you love, but by how much you are loved by others" - Wizard of Oz.

  7. #7
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,040
    The reason the dealer is steering you away from the Mod is as htrguy said, the 75,000 Mod has a 3 ton blower and you have a 3.5 ton A/C. Well, running a 3.5 ton on that blower is about 350 CFM/ton which isn't that out of line, especially if in a humid climate. I wouldn't be afraid of doing that, especially if you don't need the full 3.5 ton capacity and can go to 3 ton when you replace.

    OR, I wouldn't be afraid of upsizing to the 90K either. 15,000 BTU with the Mod isn't that big of a difference.

    The single stage 75K model can be had with a bigger blower to get more airflow.

    So this dealer did catch the blower issue. I wouldn't avoid the Mod. Either get the 75 and be a bit shy on airflow for cooling or get the 90 which isn't that much bigger. That's why I like Rheem 90s, with 6 sizes it's not that big of a deal going to the next one up or down.

  8. #8
    Join Date
    Apr 2004
    Posts
    7,680
    so why cant the dealer consider the use of the +10% cooling airflow adjustment dip switches? 1200+10% = 1320.

    They are designed with the intention they be customized by the installer to the needs of the customer.

  9. #9
    Join Date
    Nov 2002
    Posts
    230
    From what I can tell Rheem states a "Maximum" cfm for the 06 & 07 as 1200 cfm and 2000 cfm for the 90-120. I would interpret the + 10% to mean additional cfm if the unit is set to less than the maximum.

  10. #10
    I just talked with the Rheem contractor who is still steering me away from the modulator. Another reason he is giving me to avoid the modulator is that the duct work would'nt be appropriate for the Mod. Does the Mod demand very specific duct work? He says a Rheem 90 2 stage will be just fine. Any thoughts?

  11. #11
    Join Date
    Nov 2002
    Posts
    230
    Any issues with your duct work will still be there with the two stage. If your ductwork is too restrictive it needs to be fixed regardless of what unit is there. If it were my house, I would put a mod in over a two stage- no question about it.
    I get the feeling your contractor is leery of putting this in your house - whatever his true reasons are, maybe unfamiliarity, doesn't like the high tech stuff, a previous bad experience?

  12. #12
    Join Date
    Apr 2004
    Posts
    7,680
    htr... no, you can still add 10,

    There are situations where low fire and matching low airflow can be a probelm. My house is one of those that simply would not work very well.

    I have a two story. My furnace is an upflow in the garage. Above the coil is a plenum (duh) and out of the plenum are two take offs for the two trunklines. One makes a 90 degree upright move through the second floor to the attic and feeds the second floor, the other makes a 90 degree downturn, goes into the crawl and feeds the 1st floor. Under 40% airflow, the majority of the air would take the upstairs route and very little would flow through the lower one. I could zone or damper it but what if the firing rate changes?

    I also use a dual fuel system and in the heating a cooling mode, I use the same airflow so my ducts are balanced to make a 2 story system maintain very even temps throughout the house all year. My low fire heating speed is actually the same, and 2nds stage gas is slightly higher. Dont mis understand, its the best furnace in most applications.

    Every home is a custom job and the contractor has to know the limits and have some confidence in his ability.

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