5 ms·

I don't know much about how A/C works, whether the dehumidifier mechanism can be decoupled from the cooling mechanism. If it can't, then shouldn't the thermosta

I don't know much about how A/C works, whether the dehumidifier mechanism can be decoupled from the cooling mechanism. If it can't, then shouldn't the thermostat be coupled to a hygrostat so that that A/C runs when it is humid enough despite being cool in terms of temperature? On the other hand, today I learned that there's something called heat index and humidex.

pdonis 5y ago
> I don't know much about how A/C works, whether the dehumidifier mechanism can be decoupled from the cooling mechanism. It can't. When you cool air down to 50 degrees F (10 degrees C) or so (which is a typical outlet air temperature for a home A/C unit), it simply can't hold much water vapor, and the excess condenses out. That effectively dehumidifies the air--it lowers the dew point to whatever the outlet air temperature is. > shouldn't the thermostat be coupled to a hygrostat so that that A/C runs when it is humid enough despite being cool in terms of temperature? In principle it probably should, but in practice there is a close enough correlation between the two that it's not considered necessary. If energy savings is the goal, the real fix would be to use variable compressors in home A/C systems: that way, instead of the A/C running full on for a while, lowering the temperature (and dehumidifying), then shutting completely off for a while (so humidity can build up again), the A/C could just be running all the time, and changing the thermostat setting would simply change how much cooling and dehumidifying the A/C was doing--what the actual outlet air temperature/dewpoint was. "Full on" would still be, say, 50 degrees F outlet air/dewpoint, but most of the time you wouldn't need that; "energy saving mode" could be, say, 68 degrees F outlet air/dewpoint, which would still be reasonable in terms of comfort but would use a lot less energy, so it could be kept on continuously. (There are already systems like this in cars.)
- Dylan16807 5y ago
  > "Full on" would still be, say, 50 degrees F outlet air/dewpoint, but most of the time you wouldn't need that; "energy saving mode" could be, say, 68 degrees F outlet air/dewpoint If you do that, then the outlet air will have twice as much water. If you started with hot, humid air, that means you're still going to have quite a bit of humidity even as you drop the temperature through the 70s. So you're going to need to cool the air more to reach the same level of comfort. Are you sure the savings outweigh the costs?
  - pdonis 5y ago
    > If you do that, then the outlet air will have twice as much water. As long as that dewpoint is reasonably comfortable, that's not necessarily an issue. > If you started with hot, humid air, that means you're still going to have quite a bit of humidity even as you drop the temperature through the 70s. So you're going to need to cool the air more to reach the same level of comfort. You can't cool the air further without also further dehumidifying it. If you cool the air to, say, 60 F, the dewpoint can't be any higher than 60 F. There's no way to cool the air to 60 F but have a dewpoint of 68 F. That's physically impossible. > Are you sure the savings outweigh the costs? There is no such tradeoff: lower air temperature and more dehumidification always go together; they have to, as explained above. The savings comes from running continuously at a lower level of both cooling and dehumidification, one that is just sufficient to maintain comfort, instead of switching between full on (which cools more than needed to maintain comfort--it has to, to compensate for the period when the system is going to be off again) and full off (which doesn't cool or dehumidify at all, so the air warms up and gets more humid again until it reaches the point where the system has to switch back to full on). The switching on and off is less efficient for a given level of comfort.
    - Dylan16807 5y ago
      > You can't cool the air further without also further dehumidifying it. If you cool the air to, say, 60 F, the dewpoint can't be any higher than 60 F. There's no way to cool the air to 60 F but have a dewpoint of 68 F. That's physically impossible. The other way around. It's not that the dew point will be above the temperature, it's that it could go below the temperature. So as you suggested, let's have one AC that outputs air at 68F, and compare it to another that outputs at 50F. So clearly we have to be cooling the room to a temperature no lower than 68F. If we look at the initial cooling of hot wet air, there's not much difference. The 50F unit will cool a smaller volume of air and cool it to a lower temperature, and the resulting dew point is about the same for either unit. But as heat keeps leaking in through the walls and windows, the 68F unit will never get the dew point below 68F. While the 50F unit will keep outputting air with a dew point of 50F. If you had an airtight room with heat constantly leaking in, eventually the 50F unit would cycle all the air through it, and the dew point of the room would reach 50F, even though the temperature of the room might never go below 70. All rooms leak some air, so you'll never get quite that far, but the 50F unit will get the dew point a lot lower than the 68F unit. > The savings comes from running continuously at a lower level of both cooling and dehumidification, one that is just sufficient to maintain comfort, instead of switching between full on (which cools more than needed to maintain comfort--it has to, to compensate for the period when the system is going to be off again) and full off (which doesn't cool or dehumidify at all, so the air warms up and gets more humid again until it reaches the point where the system has to switch back to full on). The switching on and off is less efficient for a given level of comfort. There's an efficiency penalty to letting the temperature wander. But if it only wanders a couple degrees, it's not a huge penalty. On the other side, there's a penalty for keeping the fans and compressor going all the time, even if they're running at a lower power. Neither one of those factors is enormous. But if you could leave the room at 73 with a low dew point, instead of 70 with a high dew point, that could eventually save you a lot of energy. So that's why I'd want to see real-life efficiency numbers and comfort ratings. It could go either way on base principles.
- nitrogen 5y ago
  There are variable refrigerant flow heat pumps and variable speed blowers out there. And standalone dehumidifiers, where the hot and cold side of the heat pump are in the same air stream. The dehumidifier might have its own separate vents, or it can tie into the central vent system, or be part of an external vent system with an energy recovery ventilator. This channel has some interesting videos about the subject: https://youtube.com/playlist?list=PLDYh81z-RhxjQKC9CsJiJQHhyAA09-qPd https://youtube.com/playlist?list=PLDYh81z-RhxjQKC9CsJiJQHhy... https://youtu.be/ud5oAmTHcM4 https://youtu.be/ud5oAmTHcM4
iamthad 5y ago
Both Ecobee and Nest can do this: - Ecobee "AC Overcool / Dehumidify Using AC": https://support.ecobee.com/hc/en-us/articles/115000268887-How-to-use-AC-Overcool-Max-to-reduce-humidity https://support.ecobee.com/hc/en-us/articles/115000268887-Ho... - Nest "Cool to Dry": https://support.google.com/googlenest/answer/9294957?hl=en https://support.google.com/googlenest/answer/9294957?hl=en

pdonis 5y ago

> I don't know much about how A/C works, whether the dehumidifier mechanism can be decoupled from the cooling mechanism. It can't. When you cool air down to 50 degrees F (10 degrees C) or so (which is a typical outlet air temperature for a home A/C unit), it simply can't hold much water vapor, and the excess condenses out. That effectively dehumidifies the air--it lowers the dew point to whatever the outlet air temperature is. > shouldn't the thermostat be coupled to a hygrostat so that that A/C runs when it is humid enough despite being cool in terms of temperature? In principle it probably should, but in practice there is a close enough correlation between the two that it's not considered necessary. If energy savings is the goal, the real fix would be to use variable compressors in home A/C systems: that way, instead of the A/C running full on for a while, lowering the temperature (and dehumidifying), then shutting completely off for a while (so humidity can build up again), the A/C could just be running all the time, and changing the thermostat setting would simply change how much cooling and dehumidifying the A/C was doing--what the actual outlet air temperature/dewpoint was. "Full on" would still be, say, 50 degrees F outlet air/dewpoint, but most of the time you wouldn't need that; "energy saving mode" could be, say, 68 degrees F outlet air/dewpoint, which would still be reasonable in terms of comfort but would use a lot less energy, so it could be kept on continuously. (There are already systems like this in cars.)

Dylan16807 5y ago
> "Full on" would still be, say, 50 degrees F outlet air/dewpoint, but most of the time you wouldn't need that; "energy saving mode" could be, say, 68 degrees F outlet air/dewpoint If you do that, then the outlet air will have twice as much water. If you started with hot, humid air, that means you're still going to have quite a bit of humidity even as you drop the temperature through the 70s. So you're going to need to cool the air more to reach the same level of comfort. Are you sure the savings outweigh the costs?
- pdonis 5y ago
  > If you do that, then the outlet air will have twice as much water. As long as that dewpoint is reasonably comfortable, that's not necessarily an issue. > If you started with hot, humid air, that means you're still going to have quite a bit of humidity even as you drop the temperature through the 70s. So you're going to need to cool the air more to reach the same level of comfort. You can't cool the air further without also further dehumidifying it. If you cool the air to, say, 60 F, the dewpoint can't be any higher than 60 F. There's no way to cool the air to 60 F but have a dewpoint of 68 F. That's physically impossible. > Are you sure the savings outweigh the costs? There is no such tradeoff: lower air temperature and more dehumidification always go together; they have to, as explained above. The savings comes from running continuously at a lower level of both cooling and dehumidification, one that is just sufficient to maintain comfort, instead of switching between full on (which cools more than needed to maintain comfort--it has to, to compensate for the period when the system is going to be off again) and full off (which doesn't cool or dehumidify at all, so the air warms up and gets more humid again until it reaches the point where the system has to switch back to full on). The switching on and off is less efficient for a given level of comfort.
  - Dylan16807 5y ago
    > You can't cool the air further without also further dehumidifying it. If you cool the air to, say, 60 F, the dewpoint can't be any higher than 60 F. There's no way to cool the air to 60 F but have a dewpoint of 68 F. That's physically impossible. The other way around. It's not that the dew point will be above the temperature, it's that it could go below the temperature. So as you suggested, let's have one AC that outputs air at 68F, and compare it to another that outputs at 50F. So clearly we have to be cooling the room to a temperature no lower than 68F. If we look at the initial cooling of hot wet air, there's not much difference. The 50F unit will cool a smaller volume of air and cool it to a lower temperature, and the resulting dew point is about the same for either unit. But as heat keeps leaking in through the walls and windows, the 68F unit will never get the dew point below 68F. While the 50F unit will keep outputting air with a dew point of 50F. If you had an airtight room with heat constantly leaking in, eventually the 50F unit would cycle all the air through it, and the dew point of the room would reach 50F, even though the temperature of the room might never go below 70. All rooms leak some air, so you'll never get quite that far, but the 50F unit will get the dew point a lot lower than the 68F unit. > The savings comes from running continuously at a lower level of both cooling and dehumidification, one that is just sufficient to maintain comfort, instead of switching between full on (which cools more than needed to maintain comfort--it has to, to compensate for the period when the system is going to be off again) and full off (which doesn't cool or dehumidify at all, so the air warms up and gets more humid again until it reaches the point where the system has to switch back to full on). The switching on and off is less efficient for a given level of comfort. There's an efficiency penalty to letting the temperature wander. But if it only wanders a couple degrees, it's not a huge penalty. On the other side, there's a penalty for keeping the fans and compressor going all the time, even if they're running at a lower power. Neither one of those factors is enormous. But if you could leave the room at 73 with a low dew point, instead of 70 with a high dew point, that could eventually save you a lot of energy. So that's why I'd want to see real-life efficiency numbers and comfort ratings. It could go either way on base principles.
nitrogen 5y ago
There are variable refrigerant flow heat pumps and variable speed blowers out there. And standalone dehumidifiers, where the hot and cold side of the heat pump are in the same air stream. The dehumidifier might have its own separate vents, or it can tie into the central vent system, or be part of an external vent system with an energy recovery ventilator. This channel has some interesting videos about the subject: https://youtube.com/playlist?list=PLDYh81z-RhxjQKC9CsJiJQHhyAA09-qPd https://youtube.com/playlist?list=PLDYh81z-RhxjQKC9CsJiJQHhy... https://youtu.be/ud5oAmTHcM4 https://youtu.be/ud5oAmTHcM4

iamthad 5y ago

Both Ecobee and Nest can do this: - Ecobee "AC Overcool / Dehumidify Using AC": https://support.ecobee.com/hc/en-us/articles/115000268887-How-to-use-AC-Overcool-Max-to-reduce-humidity https://support.ecobee.com/hc/en-us/articles/115000268887-Ho... - Nest "Cool to Dry": https://support.google.com/googlenest/answer/9294957?hl=en https://support.google.com/googlenest/answer/9294957?hl=en