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  1. #1
    Join Date
    May 2008
    Posts
    5

    Arrow Advice Please: Dual Fuel vs. GeoThermal

    I'm building a new house and I've narrowed my HVAC choices to a Trane dual-fuel system or a Waterfurnace HP. However, I can't seem to choose between the two, so I'm looking for some advice. Here are the particulars:

    House - 2100 sf ranch, low e vinyl windows, blown cellulose (ceilings R50), St. Louis Climate (hot, humid summers, avg. winter temps in 30s); elec - $0.10/kwh; gas - $1.11/therm

    Dual Fuel - Trane 3 ton XL16i HP (4TWX6036B1000A); 80,000 BTU XV80 gas furnace (2-stage, variable speed); Coil (RXC036S3HPD).

    Geothermal - Waterfurnace Envision NSV042 with variable speed blower and single speed compressor with R410A refrigerant and hot water assist.; 10 k.w. aux heat.; vertical loops.

    Both companies have good reputations. Payback on Geothermal was estimated (by the Geo bidder) to be about 8 years. What would YOU do?

    Thanks in advance for any help!

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,874
    I'd ask the geo guy to redo that pay back calc including the well drilling this time.
    Your gas rate isn't that high.
    I'd also ask why its a 1/2 ton larger then what the Trane guy is putting in, and why only 10 KW of strip. Will that be enough to heat the house if the HP failed when it was at OD design temp.

    I'd ask both what their load calcs came out to.
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  3. #3
    Join Date
    May 2008
    Posts
    5
    I'll ask about the load calculations and the 10 k.w. aux. strip. I remember the Geothermal guy mentioned that undersizing the Envinsion was a bad thing to do. I thought this contradicted other things I had read on this forum. I don't recall his reasoning for oversizing the unit, but it seemed a bit odd to me.

  4. #4
    Join Date
    Mar 2008
    Location
    Shelby Twp MI
    Posts
    820
    I'd see if you can get a field loop instead of vertical loop, unless you don't have a big enough yard for it. Typically you'll find a bigger geo therm rating concerning tonnage. 8 years on return investment????? Should be closer to 4-5 years max.
    Jim

  5. #5
    Join Date
    Mar 2003
    Location
    PA/DE area
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    1,535
    Ask guy that bid water Furnace for GIZMO print out,the GEO sizing maybe right it comes out based on a heat/cool ratio and you selected based on the hours of back up that it would be needed. We put in 5 ton with 15KW,because it it is on it is REAL COLD or it is an EMERGENCY!!
    It's NOT the BRAND,it's the company that installs it!!!!!

  6. #6
    Join Date
    Dec 2007
    Location
    oregon
    Posts
    885
    Go geo, Of course, I sell both waterfurnace and Trane! Remember, when your desuper heater is on it will take some of the available btu's in the heating mode for water heating. The 10 kw sound normal to me, but I am in a much milder climate. Without seeing the load calcs on your home its hard to say about the back up heat. If you were sizing the strips for total load at winter design temps then 15 sounds more advisable, but they will rarely be on with the geo unit. What was the btu's required at winter design temps from your load calcs? The trane guy may be low and counting on the gas to supply the rest in the heating mode?
    I agree the comment on horizontal loops. Geo will be the most cost effective operating costs in the long haul. The eers will be about twice that of your air source system.
    Genius = The guy who can do anything...except make a living!

  7. #7
    Join Date
    Nov 2007
    Posts
    86
    It’s not just the purchase price, it’s the total life cycle cost.

    The geothermal unit may well last twice as long as the conventional HVAC system. So when you do a total life cycle cost comparison analysis, you will probably be buying the conventional system twice over the life of a geothermal system; once at today’s prices and a second time at a much higher future price.

    Geothermal has the highest owner satisfaction rating of any heating & cooling system available today. There is also the premium you will receive on increased resale value of your home because of the integration of green technologies. That alone is a factor that will continue to be of greater importance over time. How much will a therm cost in 10 ~ 15 years?

    SR

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    The trane guy may be low and counting on the gas to supply the rest in the heating mode?

    On dual fuel you size the HP to the cooling load. And use the gas aux when the OD is below balance point.
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  9. #9
    Join Date
    May 2008
    Posts
    5
    Thanks for the advice.
    Concerning the possibility of a field loop, we just don't have the room, so we are going to have to go with vertical wells. What's the typical spacing of these wells? The geo guy said he would only need about a 10 ft x 10ft area for the wells. I was assuming they would have to be spaced farther apart.

    I don't have any load calculations, but I'm going to try and get them today. Does the geo guy's statement about "rounding up" on the tonnage make sense? Again, he said the worst thing you could do with geo is undersize the equipment. However, I thought I read elsewhere that it is better to undersize the unit to help with humidity control. Am I dreaming, or does this sound right?

  10. #10
    Join Date
    Dec 2007
    Location
    oregon
    Posts
    885
    Buy a two stage unit for the best humidity control. If your hp is under sized, the risk is freezing your loop field. I would space the vertical wells at least ten feet apart.
    Get refferals from your geo guy and call them. Satistfied customers don't lie. Get at least three from each bidder. If possible make sure or request that the refferals are atleast three years old so you can get a good feel for the system integrity and the customer service of the contracter who installed it.
    Genius = The guy who can do anything...except make a living!

  11. #11
    Join Date
    Dec 2007
    Location
    oregon
    Posts
    885
    Quote Originally Posted by beenthere View Post
    The trane guy may be low and counting on the gas to supply the rest in the heating mode?

    On dual fuel you size the HP to the cooling load. And use the gas aux when the OD is below balance point.
    Good point, But I would still want to see the load calcs. The question was retorical, and the hybrid guy should have explained that to the OP.
    Genius = The guy who can do anything...except make a living!

  12. #12
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,874
    So would I. That 3.5 ton geo, even with a VS blower may not remove enough moisture.
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  13. #13
    Join Date
    Nov 2007
    Posts
    86
    [QUOTE=geodude;1852081]I would space the vertical wells at least ten feet apart.


    For residential installations minimum borehole spacing should be 15 feet.

    SR

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