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  1. #40
    Forced company to do a complete analysis. Manual J said I only had 900 CFM ductwork capacity and heat load needed was about 75K. Three new supplies are being added to the upper level to enable 500 CFM more capacity and new equipment will be replaced. I see there are 2 Rheem two stage units for 75K output. Recommendations?

  2. #41
    Quote Originally Posted by RoBoTeq View Post
    Also, no matter what, that furnace is forced into high fire every 12 minutes of run time. All staged condensing furnaces are forced into high fire with 12 - 15 minutes of run time to prevent damage to the primary heat exchanger.
    The factory default for this furnace on page 47 in figure 44 for jumper P7 is "none" and will never go into high fire unless a two stage thermostat calls for high fire. If the jumper is set to non-factory setting s of 5 min or 12 min and a single stage thermostat is used then the unit will go into high fire after those intervals of run time have elapsed. This is my understanding after reading the documentation and talking to the specialists at the Rheem comfort center.

  3. #42
    Quote Originally Posted by makeithappen View Post
    Forced company to do a complete analysis. Manual J said I only had 900 CFM ductwork capacity and heat load needed was about 75K. Three new supplies are being added to the upper level to enable 500 CFM more capacity and new equipment will be replaced. I see there are 2 Rheem two stage units for 75K output. Recommendations?
    Correction the heat load was not 75K but rather heat calc indicated I need a 75K unit after efficiencies and altitude were considered.

  4. #43
    Join Date
    May 2007
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    DC Metro Area (MD)
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    Adding 3 new supplies but keeping the undersized return?

  5. #44
    Quote Originally Posted by RyanHughes View Post
    Adding 3 new supplies but keeping the undersized return?
    A 12" diameter return was previously added to the 2nd level.

  6. #45
    Join Date
    May 2007
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    Quote Originally Posted by makeithappen View Post
    A 12" diameter return was previously added to the 2nd level.
    And made a lot of noise as you describe (high velocity). Going from 1600 cfm to 1400 cfm, that 12" alone still isn't close to enough. What else do you have for return air?

    What was your cooling load after running the Manual J?

  7. #46
    Quote Originally Posted by RyanHughes View Post
    And made a lot of noise as you describe (high velocity). Going from 1600 cfm to 1400 cfm, that 12" alone still isn't close to enough. What else do you have for return air?

    What was your cooling load after running the Manual J?
    I don't have all the data just discussion. The static pressure was high based on insufficient place to relieve the air. On the main level there is a large return panned between two joists with a 10 x 32 grate on it roughly. Some of the grate is decorative so covers part. And there was a return 8" was added to the back area (also prior)

    The cooling calc don't know BTU's but was a hair over a 3 ton A/C requirement requiring 3.5 ton.

  8. #47
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
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    As for the two 2-stage 95% Rheem furnaces offered, I would stick with the RGRM so that you get the variable speed blower. Especially important if you're keeping the RARL 2-stage air conditioner. Rheem has a 75k RGRM that will handle a 4 ton outdoor unit (1 HP blower).

    After the new unit is installed, I definitely recommend checking ESP to see where you stand on ductwork.

  9. #48
    right now there is a 4 ton 16 seer 2 stage unit installed that will be replaced as well. Kinda hard for me to see it go. But also equipment hight including this efficient coil make the unit too tall to duct under the unit. Of the 2 RGRM 75K units I wrestle if the smaller one should be considered and 3 ton A/C to stay with the 2 stage. (dont think 2 stage comes in 1/2 sizes.) What do you think about the airflow of the smaller 75K unit? I'm paranoid about CFM noise now.

  10. #49
    Join Date
    Jan 2009
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    Keokuk, IA
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    5,520
    If your paranoid abotu noise that you don't want anything oversized. You want 2 stage and the smaller the better. If your ductwork is undersize, your probably not getting much more than 3 ton out of the existing 4 ton unit anyway.

  11. #50
    Join Date
    Nov 2000
    Location
    Eastern PA
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    Quote Originally Posted by makeithappen View Post
    The factory default for this furnace on page 47 in figure 44 for jumper P7 is "none" and will never go into high fire unless a two stage thermostat calls for high fire. If the jumper is set to non-factory setting s of 5 min or 12 min and a single stage thermostat is used then the unit will go into high fire after those intervals of run time have elapsed. This is my understanding after reading the documentation and talking to the specialists at the Rheem comfort center.
    Read the actual sequence of operation. Multistaged furnaces must go into higher firing ranges on startup and periodically during longer cycles to prevent flue gas condensation issues in the primary heat exchangers. Also, operation on lower fire must be less efficient due to having to transfer less of a heat differential through the same mass of material in the heat exchanger.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  12. #51
    Join Date
    Nov 2000
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    Eastern PA
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    Quote Originally Posted by motoguy128 View Post
    If your paranoid abotu noise that you don't want anything oversized. You want 2 stage and the smaller the better. If your ductwork is undersize, your probably not getting much more than 3 ton out of the existing 4 ton unit anyway.
    If you are paranoid about anything pertaining to sizing in our industry, you need to get to some training classes to learn more about we do and why we do it. The only thing we need to fear, is fear itself. Paranoia comes from fear of the unknown. Once we know and understand what it is we are doing, we need not be paranoid.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  13. #52
    Here is where I am at:

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    So the new duct work is added, that is the 3 supplies on the upper level from a 10 in spiral ducts (right). The 12" return (left) was done before. So all of this is hooked up to the existing furnace that will be displaced soon. So i am getting an opportunity to evaluate air flow.

    Temperatures in all three units are more uniform. My oversized RGRM (105) two stage 73K / 105K has the blower throttled down to 1150 CFM which even after the ductwork is still too loud to my liking. The ductwork now supposedly support 1400 CFM. New equipment will be selected next week. I would like gear that for heating runs under 1000 CFM and preferably less (for heating)

    My thoughts - OPTION 1 Get the RGRM-07*YBGS 75KBTU at 1182 CFM (low and high fire speed) and DIP switch the blower to 1000 CFM or 850CFM as long as the heat rise is tolerated.

    OPTION 2 - Get the RGRM-07*MAES 75K BTU at 765CFM low and 1080CFM High (and leave the dip switches as is. Pair it with only a 3 ton 2 stage RARL A/C such that this smaller furnace still can support the A/C blower needs of 1200 CFM.

    Seems option 2 would not be pushing the envelope of my duct work (1400) and I could benefit from quieter operation. My unknowns are, ail the sweet spot of low fire at 765 CFM be sufficient to push air around the room and to the 2nd floor to do the job.

    Any feedback on these two options and the pro's con's of lower CFM for stage one please.

    Thanks everyone for your feedback. Seems I am having to push this particular service provider for these types of considerations.

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