5 ms·

> it mostly just comes down to the delta in temperature you're trying to achieve Not for cooling; cooling also requires condensing excess water vapor out of t

by pdonis 5y ago

> it mostly just comes down to the delta in temperature you're trying to achieve Not for cooling; cooling also requires condensing excess water vapor out of the air (since the dewpoint of the cooled air is much lower than the dewpoint of the air before you started cooling it). Most of the energy expenditure for A/C is actually condensing water vapor, not cooling air. Air has a very low heat capacity compared to water (particularly water during the phase change from vapor to liquid). Off the top of my head I think costs about 6 times as much energy to condense water vapor as to cool the air for typical A/C conditions; if that number is roughly correct, then for the temperature differences you give, heating actually uses about 2/7 (about 30%) of the energy of cooling (if we let the energy required to cool the air be 1, then the energy required to heat the air is 2, but the energy required to condense water vapor is 6, so the total cooling energy is 7.)

crazygringo 5y ago
Do you have a reputable source for that? I've never heard that before, it seems to defy intuition in a couple of ways, and it defies my own electricity bill. First, water vapor in air is usually less than 2% of air by mass, and contrary to what you say there's absolutely no phase change involved -- water vapor isn't steam, it's far far below boiling. And AC usually doesn't condense more than about 25% of humidity, so it's really a tiny, tiny amount of water being cooled relative to air being cooled. So I'm missing what could possible be so energy-intensive about cooling water vapor, or why condensation would be relevant to energy usage at all? (Cool condensation still helps "chill" the room.) And second, even if that were the case, wouldn't it be the identical energy loss when heating? A heat pump in the winter is literally just air-conditioning the outside, and produces similar condensation in the unit outdoors. In fact, my heat pump runs in reverse for ~15 min once or twice a day on really cold days (suddenly it blows cold air inside and warm air outside) in order to melt the accumulated ice from condensation, to continue operating safely. So I don't see at all how how cooling somehow uses 3.5x more energy than heating. My personal electric bill with my heat pump is also about 2x as expensive in the coldest winter months (heating from 25°F to 70°F = +45°F) as in the warmest summer months (cooling from 90°F to 70°F = -20°F). If your math were correct, it would be roughly the opposite. As far as I can tell, the cost of running a heat pump is basically the same no matter which direction you're running it.
- sokoloff 5y ago
  In HVAC design, the two concepts are called sensible heat (temperature change) and latent heat (dehumidification). If you just cool the air without removing water vapor, you will drive up the relative humidity and often decrease occupant comfort.
  - crazygringo 5y ago
    That doesn't address anything I said about costs. And you often can't cool air without removing water vapor, the water condenses whether you want it to or not.
    - sokoloff 5y ago
      You appear to have previously to be laboring under the misconception that latent heat (the heat required to condense water in gaseous form into water in liquid form) didn’t exist or that humidity isn’t gaseous water. My comment was directly responsive to whichever of those misconceptions was underlying your comment above. That’s why I gave you the term, in an effort to show you politely that you were wrong enough that the HVAC industry has an entire term specific to this concept and that suggests that it’s more than a rounding error. It appears that you at least now agree that water in gas form is condensing into liquid form (which requires the removal of the heat of vaporization [what science calls it] or the latent heat [what HVAC calls it]).
      - crazygringo 5y ago
        Ah ha, thank you! I was indeed under the misperception that latent heat was associated only with evaporation/condensation at high-energy boiling points, not at e.g. room temperature for water. Turns out it's fiendishly difficult to find anything on Google or educational materials about phase changes or latent heat that aren't at a boiling (or freezing) point -- no wonder I was never taught it. (Finally found [1].) But in any case, I finally found them and ran the numbers. The latent heat of water is ~4x that of air, but even on a really hot humid summer day water is only 3% of air by mass, which means in a worst-case common scenario, 11% of energy is going to condensing the water, while 89% is simply cooling the air and (non-condensing) humidity. So if we round, we can say that a heat pump cooling humid summer air rather than drier winter air uses up to around 10% more energy (since winter air has a bit of humidity too), which means heating/cooling are still roughly the same for most purposes. Whereas the commenter I was originally replying to seemed to be suggesting cooling takes 3.5x as much energy, which doesn't seem to match summer/winter energy bills at all. Does that cover it, or is there something else my math is missing? Thanks again. [1] https://en.wikipedia.org/wiki/Latent_heat#Specific_latent_heat_for_condensation_of_water_in_clouds https://en.wikipedia.org/wiki/Latent_heat#Specific_latent_he...

by pdonis 5y ago

crazygringo 5y ago

Do you have a reputable source for that? I've never heard that before, it seems to defy intuition in a couple of ways, and it defies my own electricity bill. First, water vapor in air is usually less than 2% of air by mass, and contrary to what you say there's absolutely no phase change involved -- water vapor isn't steam, it's far far below boiling. And AC usually doesn't condense more than about 25% of humidity, so it's really a tiny, tiny amount of water being cooled relative to air being cooled. So I'm missing what could possible be so energy-intensive about cooling water vapor, or why condensation would be relevant to energy usage at all? (Cool condensation still helps "chill" the room.) And second, even if that were the case, wouldn't it be the identical energy loss when heating? A heat pump in the winter is literally just air-conditioning the outside, and produces similar condensation in the unit outdoors. In fact, my heat pump runs in reverse for ~15 min once or twice a day on really cold days (suddenly it blows cold air inside and warm air outside) in order to melt the accumulated ice from condensation, to continue operating safely. So I don't see at all how how cooling somehow uses 3.5x more energy than heating. My personal electric bill with my heat pump is also about 2x as expensive in the coldest winter months (heating from 25°F to 70°F = +45°F) as in the warmest summer months (cooling from 90°F to 70°F = -20°F). If your math were correct, it would be roughly the opposite. As far as I can tell, the cost of running a heat pump is basically the same no matter which direction you're running it.

sokoloff 5y ago
In HVAC design, the two concepts are called sensible heat (temperature change) and latent heat (dehumidification). If you just cool the air without removing water vapor, you will drive up the relative humidity and often decrease occupant comfort.
- crazygringo 5y ago
  That doesn't address anything I said about costs. And you often can't cool air without removing water vapor, the water condenses whether you want it to or not.
  - sokoloff 5y ago
    You appear to have previously to be laboring under the misconception that latent heat (the heat required to condense water in gaseous form into water in liquid form) didn’t exist or that humidity isn’t gaseous water. My comment was directly responsive to whichever of those misconceptions was underlying your comment above. That’s why I gave you the term, in an effort to show you politely that you were wrong enough that the HVAC industry has an entire term specific to this concept and that suggests that it’s more than a rounding error. It appears that you at least now agree that water in gas form is condensing into liquid form (which requires the removal of the heat of vaporization [what science calls it] or the latent heat [what HVAC calls it]).
    - crazygringo 5y ago
      Ah ha, thank you! I was indeed under the misperception that latent heat was associated only with evaporation/condensation at high-energy boiling points, not at e.g. room temperature for water. Turns out it's fiendishly difficult to find anything on Google or educational materials about phase changes or latent heat that aren't at a boiling (or freezing) point -- no wonder I was never taught it. (Finally found [1].) But in any case, I finally found them and ran the numbers. The latent heat of water is ~4x that of air, but even on a really hot humid summer day water is only 3% of air by mass, which means in a worst-case common scenario, 11% of energy is going to condensing the water, while 89% is simply cooling the air and (non-condensing) humidity. So if we round, we can say that a heat pump cooling humid summer air rather than drier winter air uses up to around 10% more energy (since winter air has a bit of humidity too), which means heating/cooling are still roughly the same for most purposes. Whereas the commenter I was originally replying to seemed to be suggesting cooling takes 3.5x as much energy, which doesn't seem to match summer/winter energy bills at all. Does that cover it, or is there something else my math is missing? Thanks again. [1] https://en.wikipedia.org/wiki/Latent_heat#Specific_latent_heat_for_condensation_of_water_in_clouds https://en.wikipedia.org/wiki/Latent_heat#Specific_latent_he...