Geothermal Sizing Does Not Make Sense
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  1. #1
    Join Date
    May 2008
    Location
    Grand Haven, MI
    Posts
    14

    Geothermal Sizing Does Not Make Sense

    I have been reviewing three different quotes on a geothermal system, but I'm not sure that any of them make sense for my house.

    We currently have a 3 ton A/C unit (3 1/2 ton coil) and a 110,000 btuh input propane furnace (80% efficient). The A/C seems sized appropriately as it just keeps up on our warmest days. I suspect the furnace is oversized.

    All three contractors took plans for our house and did load calculations. There were slight variations, but the numbers were approximately 56,000 btuh for heat and 36,000 btuh for cooling. We live in western Michigan. Our supply and return plenums are 10" x 24".

    All three contractors suggested two stage units. Two of them recommended 4 ton units and one recommended 5 ton. The most common unit suggested was the ClimateMaster Tranquility 27 (or equivalent rebranded). When I look at the specs on this unit, I don't see how it fits well in our home. From the ClimateMaster manual (closed loop numbers):

    Model:049
    Full: 50,000 btuh cooling, 37,500 btuh heating
    Part: 39,600 btuh cooling, 31,2000 btuh heating

    Model: 060
    Full: 64,800 btuh cooling, 48,000 btuh heating
    Part: 49,800 btuh cooling, 37,500 btuh heating

    Two contractors are suggesting the 4 ton unit. This seems consistent with our duct sizes, but I don't see how 37,500 btuh is close enough to our heat loss to avoid lots of time on the backup electric heat during the middle of winter. I suspect the cooling will be very reasonable at this size, but never use second stage.

    The third contractor suggested a 5 ton unit. I feel more comfortable with this from a heating perspective. However, I am concerned about the air flow through our ducts. I thought that 10" x 24" was appropriate for 1400cfm. I see second stage air flows of 1750, 2050, and even 2280 cfm in the ClimateMaster manual (depending on setting). Also, looking at the cooling numbers above, it seems like we will be dramatically oversized for cooling even on the first stage which concerns me for dehumidification.

    Anyway, sorry for the long post, but am I missing something here? It seems like this discussion really illustrates the importance of two stage units in geothermal. However, the difference between first and second stage with Copeland scroll compressors does not seem to handle my particular situation very well (not just ClimateMaster, but I looked at other brands as well). It seems like there needs to be a more dramatic difference for the 5 ton solution to work, but even then my airflow concern remains.

  2. #2
    Join Date
    Apr 2005
    Location
    NE PA
    Posts
    698
    It is common for dealers to quote geo units similarly to conventional heat pumps. This is sizing based on cooling loads. This generally leaves the unit undersized for heating by quite a bit, requiring some additional backup. There are a number of us who size more based on the heating load, rather than cooling. The dual capacity units make this much more practical, as the lower capacity can cool and not be oversized when on low capacity.

    I would be tempted to go with the the smaller unit. You may have a shortfall in heating on the coldest of days, but the ClimateMaster 8 KW would cover the difference. The larger unit is a bit oversized for cooling at low capacity, but could still do a descent job. It would of course be a better fit in the winter. I think either would work, but I would opt for the 049.

    paul

  3. #3
    Join Date
    May 2008
    Location
    Grand Haven, MI
    Posts
    14
    Thanks tecman. I noticed a common theme of sizing based on cooling. I could easily see that working in the south, but not so much in Michigan. You mentioned the ClimateMaster 8 KW. Is that only supposed to provide supplemental heat or should it be sized to cover the whole house in the event of a compressor failure? Two guys quoted 10 KW auxiliary heat and one guy quoted 15 KW. Also, one of the guys mentioned something about the electric backup coming on incrementally (i.e. only as much electric as you need). I could see where this would be desirable - let the geothermal do as much as it can and only rely on the electric to do just what it needs to do. However, I don't see how this could work with the thermostat. I would expect the third stage to be either on or off.

  4. #4
    Join Date
    Apr 2005
    Location
    NE PA
    Posts
    698
    The 8kw I mentioned would be for "makeup" heat. For emerg. backup you would need 15 kw for a total backup, but 10 kw would likely be suitable. Perhaps a bit cooler on emerg. but it would do. With the 10 kw, the Climatemaster controller will stage the heat, 5 kw and if you don't satisfy the demand after a time, the second 5 kw would kick in.

    paul

  5. #5
    Join Date
    May 2008
    Location
    Grand Haven, MI
    Posts
    14
    Thanks!

  6. #6
    Join Date
    Jun 2006
    Location
    Lititz, PA (Lit' itz) not ( Li' titz)
    Posts
    122
    I ran your spec in geodeisgner( from climatemaster) the 5 ton TTV gives a balance point of 1 degree the 4 ton gives balance of 14 degrees I put in Grand Rapids as the city hopefully it is close. The 5 ton give 96% on first stage heat with 1 KW AUX heat needed and 14 KW full back up.. the 4 ton is 85% first stage heat with 4 KW AUX Heat needed 14 KW full back up

    iF YOUR ELECTRIC RATES ARE 10 CENTS A kw THE 4 TON WILL COST AN AVERAGE 1269 PER YEAR AND THE 5 TON IS 1289



    SO FOR 20 BUCKS A MONTH I'D PUT IN THE FOUR TON WITH MAYBE A 5 TON GROUND LOOP. NOT SURE OF YOUR AREA. MORE GROUND LOOP WON'T HURT AT ALL EXCEPT THE POCKETBOOK

  7. #7
    Join Date
    May 2008
    Location
    Grand Haven, MI
    Posts
    14
    wagnerhvac: Thanks - that is very reassuring. I am going 4 ton so that the cooling performance will be better.

  8. #8
    Join Date
    Jun 2006
    Location
    Lititz, PA (Lit' itz) not ( Li' titz)
    Posts
    122
    Both Sizes Will Run 100 On First Stage Cooling

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