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Heat Pump vs. Geothermal Heat Pump

Discussion in 'Energy, Environment, and Policy' started by user212_nr, May 4, 2020.

  1. user212_nr

    user212_nr Member

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    I'm sure everyone is familiar with Elon's ramblings about creating a Tesla Home HVAC that seem to get more serious every day. After learning about heat pumps, which was a new concept for me, I also learned about "geothermal heat pumps". Obviously the heat pump is more efficient when transfers to 55 deg earth vs 100/32 degree air.

    So I'm confused why is there so much talk about improving the efficiency of heat pumps themselves, while at the same time ignoring "geothermal heat pumps" which are more efficient no matter how much the heat pumps improve.

    It just seems to me like people don't know what geothermal is - I didn't know myself until recently. I thought you needed a volcano for geothermal anything. Sure, you need to dig or drill a hole, but I think that that is overrated. Big deal, we have machines that can drill holes. New homes can have geothermal installed even more cheaply, and there are new buildings made every day, but I bet that few have geothermal.
     
  2. arnolddeleon

    arnolddeleon Supporting Member

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  3. nwdiver

    nwdiver Well-Known Member

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    Cost; An Air Source heat pump might use ~20kWh/day and cost $10k. A Ground Source heat pump might use 7kWh/day and cost $20k. Do you invest $10k in a ground source heat pump to reduce consumption by 13kWh/day or in 4kW of Solar PV to increase production by ~20kWh/day?

    Economics matters. Outside a very narrow set of circumstances ground source heat pumps just don't pencil especially now that air source heat pumps are increasingly able to operate below -5F.
     
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  4. Brando

    Brando Active Member

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    Another view/perspective of air vs ground geothermal products.

    Considering Elon's back to basics thinking.
    - cost of elements used to build product
    - cost of assembling elements to build product

    At first blush, seems to me, less elements needed for air system (less work too)
    (no pipes & no drilling)

    Careful not to discount location & climate of that location.
    In many places a simple basement acts as heat sink in summer (cooler)
    and a heat source in winter (warmer). This works really well in much of the Western side of the US (I have no direct experience in other parts of US). Is a basement considered a vertically integrated solution for smaller structures? just rambling thoughts

    yes, economics matter

    suggest you try searching YouTube with something like:
    DIY home geothermal system
     
  5. Saghost

    Saghost Well-Known Member

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    As a retrofit to an existing house, I doubt the continuing energy savings of a ground source heat pump will ever repay the initial investment cost.

    For new construction, though, the cost delta can be much smaller, especially if the house is being built with a concrete basement or slab foundation. Make the concrete a little thicker, put tubing through it and a foamed insulation above that, and you should be good.
     
  6. Zythryn

    Zythryn M3 Silver, M3 Midnight Silver

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    The efficiency varies depending upon the soil conditions, as does the cost.
    You also need to have the room for the loop field, or the added cost for vertical wells.

    Don't get me wrong, I love geothermal, use it extensively in my house.
    As others have stated though, cost is an issue and geography can be another.

    Geo is much easier to do at the planning phase of the structure, and more difficult as a renovation.
     
  7. iPlug

    iPlug Member

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    Hard to argue with individual system owner cost issues as noted. For most cases, cost opportunity favors air sourced.

    However, should point out that lower power demand from ground source is gentler on the grid, so some consideration should be given there.

    Consider a bunch of heat pumps running at peak demand on a hot summer evening right after sunset or cold winter morning before sunrise. In this scenario more solar could cover net annual energy use, but is tougher to balance grid supply/demand.
     
  8. user212_nr

    user212_nr Member

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    Probably that is true of any HVAC system except when replacing something really old and inefficient. If you can go far enough into the future (20-40 years) and assume low interest rates, then in some cases its maybe break even.


    Lots of room for a reduction in cost of drilling or heat exchange. I'm not a geo engineer, but I have that kind of a feeling. Especially in regard to favorable landscapes.
     
  9. Saghost

    Saghost Well-Known Member

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    Compared to an inefficient oil or electric baseboard system or the like, I'm sure you can make a geothermal heat pump retrofit pencil out. But I'm pretty sure they would save more money with a modern air source heat pump installation. There isn't that much extra savings to be had.
     
  10. mongo

    mongo Well-Known Member

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    Are you saying use the slab as the heat sink/ source? That ends up developing the max temperature differential right at the house boundary. Instead, put the insulation under the slab and use the heat pump for radiant floor heating. Insulating the foundation perimeter also helps a lot.

    Geothermal horizontal loops use 500-600 feet or pipe per ton.Vertical is 125-120 feet per ton.
     
  11. user212_nr

    user212_nr Member

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    I mean that replacing a furnace with an air-source heat pump (for example) is likely not to be cost efficient, if you already have installed a furnace. The savings are a small percent vs the upfront. Or maybe not.
     
  12. dgpcolorado

    dgpcolorado high altitude member

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    That's likely true for my house. My gas furnace is twenty-one years old and has worked flawlessly over that time. The cost of the natural gas to run it for a winter, in my four seasons climate, is about $300. The cost to replace it with a heat pump would be enormous and the savings in gas versus electricity would be minimal, I would guess. When the old furnace dies I'll take a look at the numbers because it would be nice to have a heat pump, versus another gas furnace, but I'll be surprised if the heat pump is affordable. I also expect that it would require major surgery on my concrete house, something that I would rather avoid.
     
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  13. mongo

    mongo Well-Known Member

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    When I looked into it, the best heat pumps were the same $/BTU as natural gas furnaces (0.12 a kWh, $1 per CCF). Much better than propane though.
     
  14. user212_nr

    user212_nr Member

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    It depends on the region. Heat pumps are great down south. Point being made was if you have 20% savings on your electric/gas bill that doesn't justify a 10k replacement of your HVAC system. In addition, they only need one unit and can avoid natural gas installation altogether. Doesn't make any difference if you already have a gas furnace.

    So it makes sense when discussing air-source vs geothermal heat pumps, to talk mostly about the new installations.
     
  15. mongo

    mongo Well-Known Member

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    Right, even with new installation (existing system was DOA), heating costs were going to be the same. So there was no ROI for the added cost. Plus capacity was lower.
     
  16. jackbowers

    jackbowers Jack Bowers

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    I actually got quotes for both options two years ago (my 5000 sq ft Reno home is at the 6000' level and I wanted to replace two problematic gas/AC units). The quote for a closed geo-loop system was about $110k ($50k for vertical drilling costs), versus $44k for a system based on two Mitsubishi Hyper-Heat units that work down to -15 F at full efficiency. I went with the latter and am happy with it. Mitsubishi gets a lot of efficiency gain by running their fans at low speeds (both outside compressor and interior air handlers), which makes for a very quiet system all around. But the reduction in air movement means they don't tightly control the temperature in the house. For us that hasn't been a big problem as we can run our gas fireplaces during cold winter days, but I did need to add a small mini-split AC unit in the master bedroom to keep it cooler in the summer months.

    The heat pumps use a lot of electricity in the winter, but by adding Powerwall batteries to my existing solar panel set-up I was able to go to time-of-use, so I'm only paying 6-7 cents per kWh for it, making the cost comparable to gas heat (the Reno grid runs mainly on cheap geothermal power in the winter). So it's unlikely I would have ever recovered the extra cost of a geo-loop system.

    Should also mention that 20 years ago I did put in a small geo-loop system for the master bedroom wing at my previous residence in Rocklin (even back then it was about twice the cost of a regular heat pump). I liked that setup too - it used very little power and cranked out ice-cold air on those 110 F days when I was paying 40 cents per kWh on tier 5. But it was more expensive than gas heat in the winter.
     
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  17. SageBrush

    SageBrush 2020: Drain the Sewer

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    #17 SageBrush, May 14, 2020
    Last edited: May 14, 2020
    My 2800 s.f. home in the high desert of Albuquerque in 2019 consumed 538 Therms for home and water heating and it cost $483. NG is only around 25 cents a therm but I end up paying about 3.5x that amount after connection fees and delivery, etc fees are tacked on. I have an efficient condensing furnace but I figure I am lucky if 70% of the heat makes into the living space. A therm is ~ 30 kWh, so 21 kWh of heat make it into my home's living space at a cost of 90 cents or about 4.3 cents a kWh of heat.

    So a big part of the money calculation is related to NG losses in my ducting. Moreover, there is more ability to direct heating and cooling to the spaces we are in if I install mini-splits.

    Mini-splits really come into their own when paired with home PV and a use profile optimized for them. I am confident that I can heat my water when the sun is shining and the air is warm; home heating is trickier but I hope to time shift most of it to the early afternoon. I'm confident of managing a COP of 3 year round for all heating from heat pumps, and maybe as high as 4 if I also improve my home's envelope to reduce energy losses from drafts. I currently pay 10 - 12 cents a kWh for electricity from my utility, whereas a DIY system would work out to 2 cents a kWh. At a COP of e.g 3.5 I pay 2/3.5 = 0.57 cents a kWh for heat.

    My main point here is that money savings are amplified by having an all electric, integrated system.
     
  18. Evbwcaer

    Evbwcaer Member

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    What this scenario potentially fails to account for is 1) You could get a mini-split or ducted air source heat pump that would supplement your furnace, not replace it. Because you already have a furnace, you could get a cheaper ASHP. You would not be buying a new blower motor, for example. 2) An ASHP is also an air conditioner. So if your AC dies or is inefficient, you have new options for savings.....its not just about heat. If your AC fails and you get a mini split to replace it, you now have more efficient cooling and heating...and you have your furnace for supplemental or back-up heat.

    Regarding the costs of Geo/ground source...at some point you'd think developers building new communities would drill common wells. That would really drive the costs considerably. One reason Geo is really good is that in the summer it rejects the heat from in your house into the ground, and then puts it back into your house in the winter. Air source just dumps the heat and it blows away.
     
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  19. Feathermerchan

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    Many Geothermal heat pumps also make hot water. Waste heat in the summer and offset electric/gas in the winter. That can add a lot of savings. Plus they are charged at the factory so much less chance of improper charge or leaks.
     
  20. dgpcolorado

    dgpcolorado high altitude member

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    #20 dgpcolorado, May 15, 2020
    Last edited: May 15, 2020
    My situation is a bit uncommon, especially by TMC standards, since I live in the mountains and don't use AC. (I've lived in six places in three states and not one ever had AC. My hope is to keep that trend going!)

    In summer I just open the windows at night and shut things up during the day. I designed my house to be "sun-tempered" by orienting the long side facing due south and sizing the windows for heat gain, using a formula from NREL. [Unfortunately the builder put in sun-blocking "low E" windows; when I moved in I had to change all the south windows to plain glass, with a huge gain in SHGC (solar heat gain coefficient). I left the low E windows on the east, west, and north sides because they save more energy through reduced heat loss than any solar gain.]

    So, the ability of a heat pump to do cooling is of no value to me.

    That solar gain helps a lot with heating on sunny days from late fall to early spring. In summer, the sun doesn't come in the south windows at all because it is too high in the sky. That is part of the sun-tempered design.

    Other reasons my heating bill is so low, despite snowy winters and high altitude cold, is that my house is relatively small, at 1550 square feet, and I keep the thermostat at 62°F mornings and evenings and 52° at night and just dress appropriately for the season. That saves a lot on heating cost.

    I will say, however, that I've never heard the term "mini-split" and don't know what it refers to.
     
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