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Does Geothermal with slab on grade construction make sense?

Scott K | Posted in Mechanicals on

We are in Minnesota (Zone 6a). Building new this summer on 12 acres. 2000-2400 sq/ft single level (maybe a bonus above garage), slab on grade, with radiant floor heat. Shooting for a 13-15kw grid tied PV. We are looking at Geothermal and have a good site for it, but I’m not sure it makes sense in our situation. I’m hoping not to run duct work just for AC (since we will have radiant), but I think I would have to ducts with Geo to get the benefits of AC. Would putting the money into more insulation, or solar make more sense in this case?

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Scott,
    For a single-family home, investing in a photovoltaic (PV) array almost always makes more sense from a financial standpoint than investing in a ground-source heat pump (sometimes called a "geothermal" system).

    For more information on this topic, see Are Affordable Ground-Source Heat Pumps On the Horizon?

    My usual advice: invest your money in a great thermal envelope (with low levels of air leakage, high R-values, and excellent windows), and you won't need much space heat. If you follow this advice, you won't have to invest in expensive heating equipment.

    By the way, one of the touted advantages of a GSHP is that you can cool the soil during the winter, and heat up the soil during the summer. By having the heat-transfer process reverse directions twice a year, the system design prevents (in theory) the problem of temperature of the soil ratcheting downward from season to season. But if your system lacks air conditioning, you risk the "ratcheting downward" problem.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Scott,
    If you are interested in the benefits of air conditioning -- your post is a little unclear on this point -- and if you are "hoping not to run duct work just for AC," the obvious solution is to install a couple of ductless minisplits to provide air conditioning. And once you have installed the two ductless minisplits, you no longer need to install a hydronic heating system (because the ductless minisplits can provide both heating and cooling).

    If you eliminate the hydronic heating system, you've saved yourself thousands of dollars -- dollars that you can invest in envelope improvements.

  3. Scott K | | #3

    Thanks Martin. I should have mentioned we are leaning toward hydronic for heat and a mini-split for AC. We definitely want and are going to have AC. So, I just meant that if we installed a GSHP, we would need to run ductwork in order to use the GSHP for air conditioning, which seems silly to have the expense of ductwork, only for the AC. Unless there is a way to use a GSHP for AC without ductwork... We are pretty set on hydronic, as I am worried that we would have some very cold feet most of the time, if our concrete floors are not heated...regardless how well insulated the envelope is.

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Scott,
    I'll repeat my advice: If you plan to install minisplits for AC, why not use them for heating, too? You'll save thousands of dollars.

    You wrote, "I am worried that we would have some very cold feet most of the time, if our concrete floors are not heated." Your worries are groundless. Install a thick enough layer of continuous horizontal insulation under your slab, and an adequate layer of vertical rigid foam insulation at the perimeter of your slab, and your slab will be at room temperature. For more information, see this article: All About Radiant Floors.

    It's only a matter of time before some GBA readers from cold climates post comments here -- readers from Maine, Vermont, Wisconsin, and Minnesota, who will write something like, "I have a slab-on-grade home heated with ductless minisplits. My house is comfortable and my slab is not cold."

  5. User avatar
    Stephen Sheehy | | #5

    I'm in Maine. My slab on grade home has 4" of reclaimed XPS ynder the slab. We heat with ductless minisplits. My concrete floors are not cold.
    The other issue with radiant floor heat is that an efficient envelope means that for much of the time, the heat is off anyway. In my house, solar gain on a sunny day means the minsplits are usually off from mid-morning to early evening.

  6. User avatar GBA Editor
    Martin Holladay | | #6

    My prediction came true! It took about an hour. Thanks, Stephen.

  7. User avatar
    Stephen Sheehy | | #7

    Scott: A 13-15 kw solar array will produce about 16-18,000 kwh/year. Can you use that much? Our house is about 1650 square feet of conditioned space. Our 6.6kw array produces around 8,000kwh/year. There are only two of us. We use about 9500kwh/year. That's with a regular electric resistance water heater and a hot tub located in an unheated room.
    Of course if you have an electric car or a net metering deal that pays you for excess generation, extra generation isn't a problem.

  8. Scott K | | #8

    You are a sage Martin. Well this is why I signed up. I don't claim to have all the answers, but I have a lot of assumptions! I am rather shocked that we can have concrete floors that aren't cold. If we can just do mini split, I would be thrilled. I guess this leads me straight into more specific decisions on the envelope (type of wall construction, how much insulation etc.). We are still very much in research mode on the envelope options.

    Stephen, in short, yes it might be, but we have an excellent state rebate program if I am chosen (lottery based) and net metering. I want to make sure I oversize a little, if I can. I am basing the PV size on our current energy use on our current smaller home which does not have a conditioned outbuilding. We will have a conditioned outbuilding on our new property. Of course our efficiency will be better in these new structures though. Overall, I'm guessing our use will be similar to what we have now.

  9. Brendan Albano | | #9

    In areas where you want an extra degree of coziness that your room temperature concrete might not provide aesthetically, invest some of that money you saved in a really lovely rug!

  10. Dick Russell | | #10

    Here is more testimony on insulated concrete floors not being cold. While my house is not slab on grade, the foundation is set into a hill, and in the basement, half of one end wall and all of the long downhill side are walk-out. Frost walls are insulated down to footings, and there is a 4" foam layer under the slab. Heat is by forced warm air, coming out of registers in the ceiling of the basement level; there is no radiant heat anywhere in the house. Most of the living space downstairs has just thin commercial carpeting over the slab, and the three bedrooms down there have laminated flooring over very thin foam underlayment. None of these surfaces feels at all cold, even without slippers on. The bathroom down there has a tile floor, and on bare feet it does feel somewhat cool, but not uncomfortably so, and that is because of the high thermal conductivity of tile bonded to concrete. I'm happy with the decision to use that thin carpeting in the living space downstairs. That little bit of insulation, taking the place of slippers, is all that is needed between bare feet and insulated concrete. The carpeting also provides considerable sound absorption, eliminating what could have been a problem with echoes off hard surfaces.

  11. User avatar
    Dana Dorsett | | #11

    "So, I just meant that if we installed a GSHP, we would need to run ductwork in order to use the GSHP for air conditioning, which seems silly to have the expense of ductwork, only for the AC."

    Not necessarily. With hydronic GSHP (or hydronic ASHP) you can install chilled-water wall coils. Some look like mini-split heads, but there are also thin-profile floor mounted wall coils that can be used for both heating & cooling.

    Before becoming enamored of one solution over another, run an aggressive room by room Manual-J on the proposed design, and update it as any changes are made to the building envelope, windows, R-values, etc. Only when informed by those numbers is it possible to make rational decisions on the heating and cooling mechanicals, or even the size of the PV array it takes to run it all.

    There is significant design risk with every GSHP installation, since the system designs are fully custom, and there is inherent error in the performance estimates of the ground loop heat exchange. In general it's only worth going that route when you have a large pond rather than wells to drill for the ground loop, or a large heating/cooling load that can rationalize the higher risk and higher upfront expense. Many medium sized houses in zone 6A can be heated more cheaply on a lifecycle basis with cold climate ductless minisplits than GSHP. Yes, ductless COP efficiency falls through the floor when it's -10F outside, but it's also a lot cheaper up front, and can be (if right sized) as good as or more efficient than GSHP during the shoulder seasons.

    In my neighborhood (the cool edge of zone 5 in southern New England) GSHP costs about $9K/ton to install, compared to about $3.5-4K/ton for cold-climate ductless air source. It's common to see projects pencil-out here where fattening up the exterior insulating sheathing and improving R-values elsewhere, knocking 20-25% off the peak & average heating load is a more cost-effective way to reduce the annual power use than going with nominally higher efficiency GSHP. Whether that works out in your location really depends on what the stuff costs in your area.

    Whether it's a good idea to go with GSHP or not even if it's cheaper than higher-R depends a lot on the competence of the designer/installers and how well supported the equipment & system will be going forward. Local & regional support is also an issue with ductless mini-splits. Even where support is pretty good there can be delays on getting repair-parts in. I have a relative in WA whose mini-split was damaged by high voltages that incurred when a truck hit a utility pole in the neighborhood taking out three pole mounted transformers in rapid succession as the primary wires shorted to the 240VAC secondaries. A control board in the ductless was destroyed, but was currently out of stock everywhere in the US, even though there was a batch of those boards in-process being manufactured in Asia. If she's lucky the replacement board will be arriving later this week, about 15-20 days after the event. (Fortunately, she also has a wood stove, and resistance electric space heaters, that work now that they repaired the local grid.) Similar or worse situations may occur with GSHP systems. With ductless systems that scenario alone is worth going for multiple independent systems rather than a Medusa of heads/cassettes all hanging on just one multi-split compressor, to avoid having a single point of failure.

  12. Mel Tillyard | | #12

    I'll just add a contrarian viewpoint. We have geothermal with radiant floors in Upstate NY with a similar climate as yours. We have a floorplan that offers a combination of cathedral and regular 8 foot ceilings. I can't imagine a more quiet and comfortable set up than what we have now. We may have been able to save some money with minisplits but the current system has worked out very well. With geothermal it's all about the experience and ability of your installer. Do your homework and make sure you pick someone that you trust and will stand by the system in the long run.

    Please note that (my understanding is) the tax incentives for geothermal expired at the end of 2016. I'm not sure if the financial benefit is there (like it was for us) if they are not renewed.

  13. Scott K | | #13

    Great advice here. Lots to work out, but I love it. I haven't interviewed and got bids from any plumbing/heating contractors yet, but from my research, sounds like getting a Manual-J from them, will be difficult. Any tips or advise on that? Are there tools for me to run do a Manual-J myself?

  14. User avatar GBA Editor
  15. Steve Knapp CZ 3A Georgia | | #15

    Scott. You also have the option of hiring an independent HVAC designer. I worked with David Butler on my project. (http://optimalbuilding.com/)

  16. Scott K | | #16

    Thanks for those links Martin. I had read the first one but not the others.

    Steve, that looks like an excellent option if my local contractors don't want to do it (and do it well).

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