4 ms·

Isn't that efficiency highly variable on outside temperature?

pfdietz 4y ago
In practice it's not going to push it over the line, especially with modern heat pumps, which having amazingly good COP even at very low temperature. Gas-fired heat pumps might still be more efficient (or might not), as one is also effectively capturing and using the waste heat from the generation of work from the gas.
- denimnerd42 4y ago
  I don't have an equation on hand to compare costs given a COP but I'm not even seeing any heat pumps that work down to -30F. That's kind of a no go in my area. The record low is -48F
  - pfdietz 4y ago
    Looking at the minimum outside temperature is not what you should do when computing the efficiency of the system. You look at the efficiency suitably averaged over the year. Most of the time you will not be anywhere close to -30 F.
    - denimnerd42 4y ago
      It won't even operate at -30F though. In fact it won't even operate below 0F, so it's just not going to work regardless of the COP above that. As nightly temps are below 0F all the time. So i'll get good savings on my bill when it's daylight and freeze to death at night. Great.
      - pfdietz 4y ago
        At that point you fall back to resistive backup. The overall efficiency averaged over time should still be good. Why are you talking about 0 F now instead of -30 F? Modern heat pumps can work just fine at 0 F. If you find one that doesn't, then don't buy it, get a better one.
        denimnerd42 4y ago
        I'm talking about 0F because you brought up the average. Regardless. Night time temps fluctuate at the bottom of the COP range and then might even need resistive when the heating demand is the absolute greatest. Plus you would need to build an outdoor shelter for the outside unit to handle snow. Then you have to hope the power grid holds up to all these people on resistive power. You can't just look at the happy path of.. well on average it works out great. The system seems to get quite complicated in extreme environments.

In practice it's not going to push it over the line, especially with modern heat pumps, which having amazingly good COP even at very low temperature. Gas-fired heat pumps might still be more efficient (or might not), as one is also effectively capturing and using the waste heat from the generation of work from the gas.

denimnerd42 4y ago
I don't have an equation on hand to compare costs given a COP but I'm not even seeing any heat pumps that work down to -30F. That's kind of a no go in my area. The record low is -48F
- pfdietz 4y ago
  Looking at the minimum outside temperature is not what you should do when computing the efficiency of the system. You look at the efficiency suitably averaged over the year. Most of the time you will not be anywhere close to -30 F.
  - denimnerd42 4y ago
    It won't even operate at -30F though. In fact it won't even operate below 0F, so it's just not going to work regardless of the COP above that. As nightly temps are below 0F all the time. So i'll get good savings on my bill when it's daylight and freeze to death at night. Great.
    - pfdietz 4y ago
      At that point you fall back to resistive backup. The overall efficiency averaged over time should still be good. Why are you talking about 0 F now instead of -30 F? Modern heat pumps can work just fine at 0 F. If you find one that doesn't, then don't buy it, get a better one.
      - denimnerd42 4y ago
        I'm talking about 0F because you brought up the average. Regardless. Night time temps fluctuate at the bottom of the COP range and then might even need resistive when the heating demand is the absolute greatest. Plus you would need to build an outdoor shelter for the outside unit to handle snow. Then you have to hope the power grid holds up to all these people on resistive power. You can't just look at the happy path of.. well on average it works out great. The system seems to get quite complicated in extreme environments.