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Thread: Recomendation for Rheem Case Coil in a oil to NG conversion

  1. #1
    DIY Junior Member knarfknarf's Avatar
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    Default Recomendation for Rheem Case Coil in a oil to NG conversion

    Hello All,

    I am hoping for some help. I am doing an oil to gas conversion in a 1500 sqft bungalow here in Ottawa Canada. I am converting this house into a duplex. The bungalow is 48' x 26' with 8' ceilings. Both units will be three bedrooms. It will be a shared heating system with two zones and I will be doing the install work myself and having a gas fitter to do the hook up.

    The furnace I have purchased is a Rheem RGFD-07EMCKS furnace and I need to purchase a heat pump and a casecoil.
    I am interested in this heat pump RUUD UPQL030JEZ HEAT PUMP 2.5 TON 15 SEER 410 but not sure which case coil would go with this setup. Here are my choices

    1) RCFLHM3824CC RHEEM CASE COIL 2.5-3.0T 410A MULTI POSITION GREEN or
    2) RCQD-4821AS RHEEM RUUD CASECOIL 16/18 SEER 48BTU 21"

    I am not hooking anything up... I have removed all the old equipment and I am installing the furnace to the existing duct work. I am simply asking which combination is the best….
    Everything else will be installed by a licensed HVAC installer. All I am trying to do is lower an incredibly expensive install quote.

    Any help or suggestions for a setup using this furance would be greatly appreciated.

    Thanks

    Frank
    Last edited by knarfknarf; 04-10-2013 at 10:20 PM.

  2. #2
    In the trades Dana's Avatar
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    I'm not sure how this "buy first, decide what you need later" thing is supposed to work.

    All heating systems start with a room-by-room, zone by zone heat load calculation, and from there you can decide what's appropriate.

    Multi-zoned systems using ducted air delivery come with some inherent inefficiencies. If you're splitting the space up into two 750' units the heat loads of any single unit will be a tiny fraction of the output of the Rheem, and there is no way this place need 2.5 tons of cooling.

    Start with the gross heat load calc. Assuming it's a rectangle with 2x4 walls and 2x12 attic & floor joists you have about 1500' of gross wall surface. Assuming a 15% window & door/floor ratio you're looking at about 225' of that as window and door area. For yuks I'm going to assume you have R40 in the attic (code-min for new construction is now R50.

    The 99% outside design temp for Ottawa is -22C/-8F and lets assume a 20C/68F interior temp, for a 42C/76F delta-T at design condition.

    The U-factor for insulated 2x4s is about 0.1 BTU/degree-F foot, and you have about (1500-225=) 1275' of wall area. At a delta-T of 76F and U-factor of 0.1 your heat loss from walls is about (0.1 x 76F x 1275=) 9690 BTU/hr.

    Assuming you have low performance double-panes and 2" solid doors you're looking at about U0.5 for the windows & doors. So the window & door losses are (0.5 x 76F x 225=) 8550 BTU/hr

    Assuming a 10% framing fraction on the attic floor joists and R38-R40 insulation the U-factor on the ceiling will run in the neighborhood of 0.03 BTU/ degree-foot, so the ceiling losses are around (0.03 x 76F x 1500' =) 3420 BTU/hr. If it's only crappy R19s twice that, so let's call it 2 x 3420 =6840 BTU/hr.

    Add it all up and it's still only 9690 + 8550 + 6840 = 25,030 BTU/hr for the whole house.

    Give it a hefty 30% allowance for floor losses and air leakage and you're still at only 33,000 BTU/hr, only about half the output of that furnace, and that's for the whole place. And that's before deducting for interior heat sources like hot-bodies and the 24/7 plug loads like refrigerators, etc. Odds are pretty good that the true whole house load is in the 30K range, which would put it at about 15,000 BTU/hr per apartment for heating loads. Cooling loads are likely to be under 1-ton per apartment (unless one has all the windows facing west with no trees or buildings blocking the afternoon sun.)

    Seriously, use a spreadsheet with room-by-room wall/window/door/ceiling dimensions to come up with room by room subtotals, and then the apartment by apartment subtotals. If your walls & windows are different from described tell me what they are and we can come up with more appropriate U-factors. With a reasonable heat load calc to start with it's easier to figure out what the appropriate solutions are, but it's probably not going to be the equipment you have in hand. (Can you return or exchange it?)

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