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Geothermal with 80 gal tank

Mandy LeazenbyMandy Leazenby Posts: 9
edited April 2017 in HERS ratings
Anyone know how to model the water heater portion of a geothermal system? Not much experience with geothermal. Thanks for any help!


  • David ButlerDavid Butler Posts: 3,889
    The equipment manufacture will have a modeling tool that includes desuperheater performance by temperature bins. For example, ClimateMaster's tool is called Geo Designer. Whether or not this is useful depends on what you're trying to accomplish. Are you doing life-cycle cost analysis or a HERS rating? Or something else?
  • Thanks, David! This is for a HERS rating. I can figure out the specs on the heating and cooling parts but not sure how to deal with the water heater portion.
  • David ButlerDavid Butler Posts: 3,889
    edited April 2017
    Excellent question. I'm moving this discussion to the HERS ratings category.

    First, it's important to specify whether you're talking about a desuperheater or a water-to-water heat pump. A desuperheater cannot supply 100% of DHW since it's a byproduct of the heating/cooling compression cycles, thus the setup typically involves a secondary storage tank on the input side of the primary water heater.

    A water-to-water heat pump produces hot water directly and thus can be dedicated to DHW, or more likely, serve double-duty for hydronic spacing conditioning and DHW.

    Note that the impact of a desuperheater on overall DHW efficiency is not covered in the RESNET standard. This means it's up to the software developer as to whether and how it's captured in the HERS rating.

    Please clarify which type of system and which software you're using and I can provide further guidance.
  • Thanks, David! Yeah, it's a desuperheater and I'm using REM/Rate.
  • David ButlerDavid Butler Posts: 3,889
    edited April 2017
    OK, then.You need to set up your primary water heater as you normally would, then open the equipment properties form for the geo heat pump. There should be a checkbox for 'desuperheater'. I don't have latest version of REM/Rate, so let me know if you don't find this option.

    I just ran a quick test on my own home. With primary DWH set to electric and one of the standard geo heat pumps from the library, my HERS score was 63. When I select desuperheater option, it drops to 59. If your primary DHW is natural gas, I would expect a smaller drop, but I didn't test that.