5 ms·

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 tempera

by 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.
      - dd36 4y ago
        Might it be because the surrounding air only has so much heat in it? Thus, smaller units, more spread out, have access to more heat?
      - yial 4y ago
        I don’t know why - But your single air exchange heat pump driving a single head unit in the 9,000-12,000 range seems to be incredibly efficient. Once you go to a larger unit, and more heads, the efficiency drops. It’s fascinating to even see the huge increase in switchable units - running your 4/5 ton unit at 4 tons is much more efficient. (Usually)
- beojan 4y ago
  If you look at the numbers that's clearly not the minimum COP. I'm not sure what it is, I think it's the COP when operating at minimum power.
  - xxpor 4y ago
    Look in the right most column for COP at max power
rainsford 4y ago
Cold weather heat pump technology is consistently getting better, but I also think people underestimate how many heating days are above freezing even in cold areas and how much of the US and other places is mild enough in the winter for heat pumps to be fairly efficient nearly every day during the winter (especially if you look at population rather than land area). If you look at a place like Minneapolis, which is normally considered a pretty cold US metro area, you do have a few months where the average temp is below zero. But you also have several more months where the temperature is cold enough to turn on the heat but well above freezing, where a heat pump is dramatically more efficient. When picking a heating system, it makes sense to consider comparative efficiency over the entire year, not just performance during the worst conditions in the middle of winter. Even in a city like Minneapolis, it's not obvious that heat pumps are less competitive over the entire year, even if their performance in January probably isn't fantastic. I also think the practical geographic area extends further than people think. New York City definitely isn't the south, but per some random Googling, it doesn't appear to have any months where the average temperature is below freezing. Even Boston only has 2 months that barely drop below freezing. Most of the US northeast (definitely the mid-atlantic) by population arguably also falls into the range where heat pumps make sense.
- FredPret 4y ago
  But then you need two systems - heatpump for most of the year, and natural gas for the cold days when you would die without it. At least in Canada I’d need to know that my system can handle a week of -30 or -40C in my city. Some places are even colder, for longer at a time.
  - tialaramex 4y ago
    No, electricity is versatile, you can just make electricity into heat on that day when it's too cold for your heat pump. Resistive heating is not exciting > 100% efficiency technology, but it works and if you live somewhere that realistically gets too cold to run the heat pump that's what you add, not a whole gas burning system.
    - 4y ago

by xyzzyz 4y ago

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.
    - dd36 4y ago
      Might it be because the surrounding air only has so much heat in it? Thus, smaller units, more spread out, have access to more heat?
    - yial 4y ago
      I don’t know why - But your single air exchange heat pump driving a single head unit in the 9,000-12,000 range seems to be incredibly efficient. Once you go to a larger unit, and more heads, the efficiency drops. It’s fascinating to even see the huge increase in switchable units - running your 4/5 ton unit at 4 tons is much more efficient. (Usually)
beojan 4y ago
If you look at the numbers that's clearly not the minimum COP. I'm not sure what it is, I think it's the COP when operating at minimum power.
- xxpor 4y ago
  Look in the right most column for COP at max power

rainsford 4y ago

Cold weather heat pump technology is consistently getting better, but I also think people underestimate how many heating days are above freezing even in cold areas and how much of the US and other places is mild enough in the winter for heat pumps to be fairly efficient nearly every day during the winter (especially if you look at population rather than land area). If you look at a place like Minneapolis, which is normally considered a pretty cold US metro area, you do have a few months where the average temp is below zero. But you also have several more months where the temperature is cold enough to turn on the heat but well above freezing, where a heat pump is dramatically more efficient. When picking a heating system, it makes sense to consider comparative efficiency over the entire year, not just performance during the worst conditions in the middle of winter. Even in a city like Minneapolis, it's not obvious that heat pumps are less competitive over the entire year, even if their performance in January probably isn't fantastic. I also think the practical geographic area extends further than people think. New York City definitely isn't the south, but per some random Googling, it doesn't appear to have any months where the average temperature is below freezing. Even Boston only has 2 months that barely drop below freezing. Most of the US northeast (definitely the mid-atlantic) by population arguably also falls into the range where heat pumps make sense.

FredPret 4y ago
But then you need two systems - heatpump for most of the year, and natural gas for the cold days when you would die without it. At least in Canada I’d need to know that my system can handle a week of -30 or -40C in my city. Some places are even colder, for longer at a time.
- tialaramex 4y ago
  No, electricity is versatile, you can just make electricity into heat on that day when it's too cold for your heat pump. Resistive heating is not exciting > 100% efficiency technology, but it works and if you live somewhere that realistically gets too cold to run the heat pump that's what you add, not a whole gas burning system.
  - 4y ago