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  1. #14
    Join Date
    Mar 2008
    Location
    Central Maryland
    Posts
    246
    Quote Originally Posted by gsxr View Post
    what is so hard about installing a mod compared to any other 90+ furnace?
    maybe a few more tstat wires and a supply sensor?
    Yes on both counts. My installer had to run a cable with more wires up to the thermostat, even though I had a semi-modern ChronoTherm III. He was also unfamiliar with Rheem's latest modulating thermostat, but I got him to find it and install it anyway.

    The wiring is somewhat more complex, I can attest, after spending an hour staring at it making sure he wired it as a mod and not a quasi-mod furnace controlled like a single or double stage.

    I get the feeling that 90% of HVAC work consists of customers asking "can you do it cheaper?" and **** long-term costs of buying an 80 versus a 90+ mod.

    Not me.

    Since most HVAC units are purchased by the people who DON'T pay the energy bills (building contractors and rental owners), why would any installer be familiar with the good stuff?

    I also get the impression that linear increases in wiring complexity result in logarithmic increases in installation mistakes.

    -HF

  2. #15
    Join Date
    Apr 2004
    Posts
    7,680
    It's got 1 more terminal and it's marked with a V at both ends. If your installer messes that up, he needs to find a new line of work.

  3. #16
    Join Date
    Feb 2006
    Posts
    96
    Doc do I just need to wire in a humidistat to run the On Demand Dehumidification?

  4. #17
    Join Date
    Apr 2004
    Posts
    7,680
    No, there is more to it than that, it depends on your application. Do you have a two stage cooling unit?

  5. #18
    Join Date
    Feb 2006
    Posts
    96
    no single stage

  6. #19
    Join Date
    Apr 2004
    Posts
    7,680
    Then the ODD jumper has to be removed and yes. Just remember to use a close on humidity rise. (humidistat)

  7. #20
    Join Date
    Feb 2006
    Posts
    96
    ok thanks
    I really enjoyed my first winter with my MOD
    no complants especially with my heating bill and electric bill being so low

  8. #21
    Join Date
    Apr 2004
    Posts
    7,680
    Is there a Y2 on the thermostat? Yes, there is, it's a mod stat. (I would have your contractor connect that to the Y2 (YH) on the furnace and pull the ODD jumper rather than waste my time with a humidistat). You'll make your single stage cooling act like a 2 stage, lowering your sensible capcity and increasing your latent work when 1st stage cooling is called for. If second stage calls, it will bring the fan to full speed and bring the sensible up to handle the temperature you feel.

    That's not in the book, but it's good advice (and probably should have been done at installation if they sat through a class that discusses ODD).

  9. #22
    Join Date
    Feb 2006
    Posts
    96
    thanks again doc

  10. #23
    Join Date
    Feb 2007
    Posts
    999
    Quote Originally Posted by docholiday View Post
    Is there a Y2 on the thermostat? Yes, there is, it's a mod stat. (I would have your contractor connect that to the Y2 (YH) on the furnace and pull the ODD jumper rather than waste my time with a humidistat). You'll make your single stage cooling act like a 2 stage, lowering your sensible capcity and increasing your latent work when 1st stage cooling is called for. If second stage calls, it will bring the fan to full speed and bring the sensible up to handle the temperature you feel.

    That's not in the book, but it's good advice (and probably should have been done at installation if they sat through a class that discusses ODD).

    Hey Doc, that's really good information (not in the installation manual), if I ever get to have one installed. I was more concerned with humidity control (ODD), and never really got into 2 stage a/c's. While we average around 1000 cooling degree days, having longer running cycles would help during milder humid weather.

    Would you know what the sensible capacity of an RANL-042-JEZ with coil RCFLHM4821AC would be in 'first stage'?
    Full capacity is 30,350 BTUs sensible.

    Guess I'd need 7 wires for the stat. You're always good for some useful information.

    AM

  11. #24
    Join Date
    Apr 2004
    Posts
    7,680
    There is no published information, but it will not make it a two stage unit. The sensible capacity could probably be figured out once it's installed but I would imagine it's something like 92%-100% but that's enough to stretch your run times and increase the latent work.

    Fact is, the furnace has it, the stat has it, why not do it at the cost of one wire?

  12. #25
    Join Date
    Feb 2007
    Posts
    999
    Quote Originally Posted by docholiday View Post
    There is no published information, but it will not make it a two stage unit. The sensible capacity could probably be figured out once it's installed but I would imagine it's something like 92%-100% but that's enough to stretch your run times and increase the latent work.

    Fact is, the furnace has it, the stat has it, why not do it at the cost of one wire?
    My mentioned coil has excess latent capacity as it was selected based on its (derated) sensible capacity. Operating as you've described above, the driving force would be temperature, rather than humidity.

    Based on your estimate (won't hold you to it), I wonder if using a humidistat would achieve better (humidity) results by forcing low speed blower for as long as it takes to reach a level of say 45 - 50%.

    AM

  13. #26
    Join Date
    Apr 2004
    Posts
    7,680
    The control would be based on actual humidity if you chose to do it that way, assuming your humidistat is located in the right place and it were accurate. I just calls em as I sees em. The unit set up like I mentioned will not make your house too dry by any stretch but you will have no problem achieving a 45% rh home either.

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