8 ms·

>Today, a Heat Pump can be like 200% efficient, so you 100W of chemical energy turns into 40W of electricity, then turns into 80W of heating from the heat pump.

• ericpauley 4y ago
  This appears to be the minimum for geothermal. The vast majority of heat pump installs would be air source, and while these can often get 3+ it’s highly dependent on climate and local grid efficiency. It also depends on local prices. In Madison, WI our implied (not actual) electric grid efficiency based on prices is under 20%.
  • xxpor 4y ago
    The energy.gov page has more details, it's 3.3 for air source at 47 deg F and a reasonable size: https://www.energy.gov/eere/femp/incorporate-minimum-efficiency-requirements-heating-and-cooling-products-federal#airsource https://www.energy.gov/eere/femp/incorporate-minimum-efficie...
• maxerickson 4y ago
  Not for air source. Source: your link. Air source heat pumps are a lot easier, so they are the majority of installs. I expect the national code doesn't give a minimum COP for air source because the performance changes with air temperature. The bulk of my heating cost occurs when it is 10-20 ⁰F outside, where someone located in a warmer spot might have most of their heating at 35-45 or whatever.
  • xxpor 4y ago
    I definitely read it too quickly, but clicking through to the energy.gov page does clarify that the COP is measured at 47 deg F (which is warm)
• xyzzyz 4y ago
  You will not get the COP of 3 during most of the winter in the interior US with an air source heat pump. Their efficiency go down significantly once the temperature is below freezing, due to required thawing cycles, and because the bigger delta between inside and outside temperatures reduces pumping efficiency in general (efficiency is best when outside is not much colder than inside). Air source heat pumps are really good tech overall, especially because they also double up as AC in summer. However, in winter, in a head to head comparison, they only handily win in areas with mild climate, like eg. all west, or much of southern US. In the northwest or midwest, they are unlikely to beat efficient gas furnaces.
  • xxpor 4y ago
    Here's a heat pump that has a minimum COP of 2.93 at 5 F, and 9.42 (!!!!) at 47 F: https://ashp.neep.org/#!/product/56885/7/25000///0 https://ashp.neep.org/#!/product/56885/7/25000///0
    • yial 4y ago
      Your point stands, but your 1,500 sqft home needs about 68,000 btus. That heatpump would be for about a 300-400 sqft room.
      • morsch 4y ago
        Seems like a bigger one would only be more efficient.
        • rgmerk 4y ago
          Surprisingly, this doesn’t seem to be true for most of the split system ductless heat pumps (aka “reverse cycle air conditioners), I’ve seen. The smallest ones are almost the most efficient ones in the range. I’m not sure why this is, and whether it generalises to house-size ducted systems, but it’s pretty striking nonetheless because it’s the exact opposite of most heat related systems I can think of.
• zrail 4y ago
  Geothermal is kind of amazing. We have a geothermal system that can get to 5.1 COP. We used 650 kWh for heat in December, which translates to 22 therms. The average household in my area uses roughly 110 therms per month for winter space heating.
  • twothamendment 4y ago
    Yup, I have a ground source heat pump, sometimes called geothermal - it is amazing! My neighbor has a slightly larger house, but close. Their bill one month was almost $500 and mine was $110. It didn't cost that much more to install and there isn't any equipment outside in the weather.
    • daydream 4y ago
      My understand was geothermal installation is extremely costly due to the deep bores you need to dig. Or do you have enough land to go out instead of down? Can you or the GP share more about the economics of your installations?
      • twothamendment 4y ago
        I've build 3 houses. For the first I had to get a bid for 3 bore holes, that was too much for me. I opted to insulate like crazy and use natural gas. I didn't bother with a bid the second time because the situation was the same. Air source was really coming along at this point (at least in the US), I should have pushed harder to get it. My current home has enough land to go horizonal. There are 4 trenches, 6 feet deep, 3 feet wide and 300 feet long. The pipe loops down and back on each trench. Excavation was about $1000 (full day for a man and a machine) and another $1000 to bury them and a day of labor to lay the plastic pipe and weld it together. Equipment was about $8,000 more than my other options (no natural gas here), but after tax breaks it was about $3000 more. I think it was worth it, even if there weren't tax breaks. I really had to push hard to get this. I had to use a general contractor on this house and he swore it wouldn't work. I made the calls and found someone to do it. The system runs often when the highs are -20f, but I've never flipped on the backup electric resistance heat. During the summer the A/C is so cheap it is practically free. It doesn't take much compared to heating. Cooling might have to get the house 30 degrees less than the outside air, but heating has to raise the house nearly 100 degrees f warmer than the outside air. Ground source is always my first recommendation, but air source should be more popular than it is. A funny note about the builder who thought ground source wouldn't work well, he also tried to talk me out of a tankless hot water heater because the well water would be too cold in the winter! I guess he didn't understand that the ground down far enough is a pretty steady temperature.
• pandaman 4y ago
  The US Code vs the Carnot's theorem, who would win? The answer might surprise you!