Is geothermal a reasonable option for me?
Last Post 16 Nov 2023 10:41 PM by newbostonconst. 1 Replies.
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Dan HeidelUser is Offline
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14 Nov 2023 11:30 PM
Here's the quick summary of the build I'm in the planning stages for:

Large ICF house, approx 8400SF.  It'll be a mix of personal residence and rental house with several rental rooms and lots of workshop and project space.  I'm looking to hit at least passive house levels of ACH and code compliant insulation but not much beyond that.  (R-20 walls, R-49 roof)  I haven't decided on the exact HVAC topology yet but am considering one of those water/air output heat pumps that let you do both forced air and in-floor/fan coil water mediated heating/cooling.  House design isn't far enough along to have a proper HVAC analysis yet but I'm assuming that it will need approx 6-7 tons of capacity.  Build be be in metro Detroit.

I'm going to be doing as much of the build DIY as possible including rough excavation.  Since it's relatively low cost for me to add some trenching to the house excavation, I'm considering adding a ground-exchange loop system.  The build is on a ~1/2 acre plot, so there's enough room for a slinky horizontal install but not enough for straight line horizontal unless I go with some sort of crazy 6-pipe per trench nonsense.  Going by basic rule of thumb loop length calculations, there should be enough room to place at least 9-10 tons worth of loop in the space I have.

From reading on here, it sounds like I'm right on the edge of what makes sense economically to go with geothermal vs air exchange.  I also have a couple of other considerations to add to the analysis.  

First, DTE power is notoriously unreliable and often has blackouts during periods of high and low temperature.  I'm going to be installing a solar panel array on the house with (down the road a few years) a modest battery bank to help weather those outages.  Geothermal's higher COP, especially when the external air has a large delta to the interior temp, is appealing since it would mean that I could keep the HVAC working with a higher duty cycle during those events.  I'm particularly worried about really cold winter storms where the heating demand and downed trees cause DTE power to regularly fold like a sloppy house of cards and some people are in the dark for a week or more.  Having the lower air exchange COP and the COP ~1 electric supplemental heat seems like it would burn through any stored energy so fast that it would be a struggle to keep the HVAC running at any meaningful amount.

Second, I kind of hate all the noise generated by the outside air exchange units.  It's not a deal-breaker by any means but the idea of a silent underground water loop vs a big, noisy fan is a selling point for me.

What are people's thoughts?  Is the additional work and cost worthwhile in this case?  Are the added failure modes of the ground exchange loops and geothermal heat pump something to avoid?  Will I see enough power demand delta between air exchange and ground exchange to make a significant difference?
newbostonconstUser is Online
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16 Nov 2023 10:41 PM
I am on the north side of ann arbor....have had geo at two houses. Nature Gas is over 20% cheaper to run then Geo in just fuel costs and not counting the expensive equipment. For solar go steep with the roof for snow to self clean the panels after a snow storm and a south facing roof is a must. Make sure your electrician puts the right size panel for the 120% rule and you might want a sub panel with all your important stuff going to that panel so it can be used for solar backup later.

A half acre is 20,000 sure you are going to have room for a geo field, need to get with a designer to do heat load calc and see. In my mind geo isn't worth it...I just use it for ac and plan soon to run it off my excess summer solar.
"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
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