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    Hi Siliang,<br>
    I think you are looking at this the wrong way from reading your
    email exchange.  I don't understand why you would consider putting
    in a high temp high pressure steam boiler with a heat exchanger
    between it and the working loop, ie the water loop.  Effectiveness
    and cost dictate you put in a low temp, low pressure boiler directly
    into the water loop.  No heat loss to the ground or to the heat
    exchanger.<br>
    The real reason you don't use that set up is you can't control the
    fluid quality on the ground loop side.  There is going to be grit
    and scaling in the ground loop and it is going to foul your heat
    exchanger.  The heat exchanger will need to be shut down regularly
    for cleaning.  You would be exposing, what I assume is your backup
    boiler, to the same crud coming out of the ground and would be
    taking it off line at the same time for cleaning.  It can't serve as
    a backup boiler.  The water loop is a closed system and you can
    control the fluid quality in that loop.  The water boiler would be
    much less expensive and will provide heat when you have to take the
    ground loop heat exchanger off-line for cleaning.<br>
    I would also revisit your thinking on the size of the small boiler. 
    In an ideal world you would take the heat exchanger off line in the
    low heating season and have a lower demand on the backup boiler.  It
    could also allow you to extend the cleaning of the heat exchanger to
    a better time, maybe.  Real world may require cleaning of the heat
    exchanger once in the peak heating season and your water boiler
    would be too small.  Your low cost heat source will lose a lot of
    advantages and efficiency if your water boiler has to do too much
    topping up of the heat as the ground water exchanger fouls.  Maybe a
    second heat exchanger in parallel might be a cheaper way.  Be
    careful with that idea too.  Depending on how much grit there is in
    the ground system the other thing you have to deal with is erosion
    of your piping system.  2 heat exchangers will do you no good if you
    blow out an elbow on the pipe supplying them.<br>
    You may be able to model the fouling factor on a time based
    schedule.  That would still be a guess unless you have data from
    other local installations but it could give you useful numbers for
    evaluating the economics of the system.  eQuest is a tool and gives
    idealized results.  Your system will run in the real world.  The
    real skill is in manipulating this tool to deal with all the what
    if's and figuring out all the what if's that could apply.  There
    will be a 1000 perfect days but your reputation is based on the 1
    bad day.<br>
    I would suggest the smallest size for your water boiler would be to
    supply enough heat to maintain the building at say 50F on the
    coldest day of the year.  The occupants would still not be able to
    work but you wouldn't freeze the building.  That would be a very bad
    day.<br>
    Bruce Easterbrook P.Eng.<br>
    Abode Engineering <br>
    <br>
    <div class="moz-cite-prefix">On 8/8/2016 8:58 AM, Siliang Lu via
      Equest-users wrote:<br>
    </div>
    <blockquote
cite="mid:CAGb_anKFzuDOSWGNvY+S-YXwcTAs4+=HKmiPzh8OkpOOb-NvPg@mail.gmail.com"
      type="cite">
      <div dir="ltr">
        <div>
          <div>
            <div>
              <div>
                <div>Hi Sharad,<br>
                </div>
                 Thanks for your reply. <br>
                 My design is ambient geothermal heat pump system. The
                water loop temperature ranges between 55-85F and the
                ground temperature is not so high as 360-370F. What I am
                wondering is that if the boiler setpoint could be much
                higher than ground heat exchanger setpoint if the size
                of boiler is much smaller than that of GHX? Thanks a
                lot!<br>
                <br>
              </div>
            </div>
            Have a great day!<br>
            <br>
          </div>
          All the best,<br>
        </div>
        Siliang<br>
      </div>
      <div class="gmail_extra"><br clear="all">
        <div>
          <div class="gmail_signature" data-smartmail="gmail_signature">
            <div dir="ltr">Siliang</div>
          </div>
        </div>
        <br>
        <div class="gmail_quote">On Mon, Aug 8, 2016 at 5:46 AM, Sharad
          Kumar via Equest-users <span dir="ltr"><<a
              moz-do-not-send="true"
              href="mailto:equest-users@lists.onebuilding.org"
              target="_blank">equest-users@lists.onebuilding.org</a>></span>
          wrote:<br>
          <blockquote class="gmail_quote" style="margin:0 0 0
            .8ex;border-left:1px #ccc solid;padding-left:1ex">
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              <div class="gmail_extra">
                <div class="gmail_quote">
                  <div><font color="#073763">Hi Siliang,</font></div>
                  <div><font color="#073763"><br>
                    </font></div>
                  <div><font color="#073763">The Water loop set-point
                      that you have got as may be 70 °F is the water
                      temperature that is 21.11 °C.</font></div>
                  <div><font color="#073763"><br>
                    </font></div>
                  <div><font color="#073763">It is obvious to take
                      set-point of the boiler below the Ground source HX
                      as then it will operate.</font></div>
                  <div><font color="#073763"><br>
                    </font></div>
                  <div><font color="#073763">The ground source steam
                      temperature may shoot up-to 360-370 °F.</font></div>
                  <div><font color="#073763"><br>
                    </font></div>
                  <div><font color="#073763">So may be taking steam
                      boiler or steam plus hybrid boiler that is
                      electricity and keeping the set-point from 180 to
                      230 °F can be a take.</font></div>
                  <div><font color="#073763"><br>
                    </font></div>
                  <div><span style="color:rgb(7,55,99)">One can keep the
                      set-point of the ground water heating loop as more
                      than 230 °F to 370 °F may be 360 °F.</span><br>
                  </div>
                  <div><font color="#073763"><br>
                    </font></div>
                  <div><font color="#073763">Then the functioning will
                      be okay.</font></div>
                  <div><font color="#073763"><br>
                    </font></div>
                  <div><font color="#073763">Generally the boiler
                      set-point of electrical heater is 180 °F for steam
                      water to 230 °F which is nearly 82 °C (180 °F ).</font></div>
                  <div><font color="#0000ff"><b><br>
                      </b></font></div>
                  <div><font color="#0000ff"><b>Thanks</b></font><span
                      style="color:rgb(7,55,99)">,</span><br>
                  </div>
                  <div><font color="#073763">Sharad.</font></div>
                  <div><font color="#073763"><br>
                    </font></div>
                  <div><br>
                  </div>
                  <div>---------- Forwarded message ----------<br>
                  </div>
                </div>
                <div>
                  <div class="h5">From: Siliang Lu via Equest-users <<a
                      moz-do-not-send="true"
                      href="mailto:equest-users@lists.onebuilding.org"
                      target="_blank">equest-users@lists.onebuildin<wbr>g.org</a>><br>
                    To: "<a moz-do-not-send="true"
                      href="mailto:equest-users@lists.onebuilding.org"
                      target="_blank">equest-users@lists.onebui<wbr>lding.org</a>"
                    <<a moz-do-not-send="true"
                      href="mailto:equest-users@lists.onebuilding.org"
                      target="_blank">equest-users@lists.onebuildin<wbr>g.org</a>><br>
                    Cc: <br>
                    Date: Sat, 6 Aug 2016 16:14:27 -0400<br>
                    Subject: [Equest-users] Boiler temperature for
                    hybrid geothermal heat pump system<br>
                    Hi eQuesters,<br>
                    <br>
                     I have a problem regarding hybrid geothermal heat
                    pump system built in  a heating dominated building.
                    From ASHRAE Handbook, it suggests that boiler be
                    installed separately from geothermal heat exchanger
                    loop due to high temperature of boiler.<br>
                    <br>
                    In addition, one report says that even if there are
                    some cases that boiler has to be installed into GHX
                    loop, the setpoint shall be 5-10F lower than GHX
                    setpoint( e.g. 45F). That is because if the boiler
                    is installed in the GHX loop, it is likely to dump
                    heat back into the ground if boiler setpoint is
                    higher than GHX setpoint.<br>
                    <br>
                     However, heat exchange is related to both
                    temperature and flow rate. With smaller size of
                    boiler, can we allow to install boiler with higher
                    setpoint higher than GHX setpoint into the GHX loop?
                    Thanks!<br>
                    <br>
                    All the best,<br>
                    Siliang<br>
                  </div>
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