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I've often wondered if opening windows while AC is running and outside temp is below inside temp is less efficient than keeping them closed. My thought is I sh

I've often wondered if opening windows while AC is running and outside temp is below inside temp is less efficient than keeping them closed. My thought is I should keep them closed due to extra load on the AC to dehumidify the outside air. Or open windows and turn AC to fan-only mode to prevent stagnant air in rooms without windows.

soco 4y ago
Is the AC dehumidifier an extra feature/task, so actively followed, or is it doing dehumidification just as a byproduct happening while cooling?
- rstupek 4y ago
  It's a byproduct of the process of cooling the air
- yetihehe 4y ago
  Some AC units have two modes - cooling and dehumidifying. When in cooling mode it will blow a lot of air to keep evaporator as hot as possible, so less humidity is condensing on it (but there will still be some). In dehumidifying mode it will blow less air but keep evaporator cool, so more condensation for the same amount of air cooling.
  - teo_zero 4y ago
    Do you have any source for this? It's a topic I've always wanted to understand better. Especially, it's not clear which mode consumes less.
    - yetihehe 4y ago
      Source - I have Kaisai mini-split at work and I've observed how each mode works (with my hand). There's probably more energy used per cooling amount during dehumidifying, because it's bigger temperature differnce (which means lower COP) PLUS condensing water (water phase change energy is big).
jdmichal 4y ago
Unless you have a very strange setup, centralized A/C does not have humidity sensors nor run just to dehumidify. Nor does removing humidity increase the load on the A/C system. Removing humidity is just a happy by-product of how centralized A/C works. So no, you would not be increasing load for that reason.
- TeMPOraL 4y ago
  Happy in some times, unhappy in others. There are days I'd love if the A/C would have a humidifier built-in, for those winter months when you might want to use A/C to warm the interior.
  - ComputerGuru 4y ago
    A good chunk of the United States (and a number of other countries) uses forced air natural gas heating. If you fall into that group, you can install a "whole-house humidifier" that injects steam into the hot air plenum, distributing it around the house. This makes the air less dry (obviously) but also has the side-effect of actually reducing the need for heating because air holding moisture feels warmer than air without (hence the oppressive heat in the humidity of summer), meaning you can lower the thermostat. Unfortunately I have only seen commercial humidification units that boil water into steam with the use of natural gas. Without exception, everything for sale for home use uses electricity (you'll need 230V minimum, single phase will do just fine) to boil the water, which is costly (though this year natural gas prices have risen or even doubled, but even in states with cheap electricity it's probably still cheaper to use natural gas).
    - mmaurizi 4y ago
      We run our steam humidifier just fine on 120v, but we have a relatively small townhome at 1400sqft
      - ComputerGuru 4y ago
        Be careful you’re not running it independently of whether or not the furnace is actually on. 120V generally can’t evaporate enough water in the short window while the furnace is running and many techs will set it up to run at all times (with the blower but not the furnace) to compensate. This lets mildew or mold grow in your ducts.

soco 4y ago

Is the AC dehumidifier an extra feature/task, so actively followed, or is it doing dehumidification just as a byproduct happening while cooling?

rstupek 4y ago
It's a byproduct of the process of cooling the air
yetihehe 4y ago
Some AC units have two modes - cooling and dehumidifying. When in cooling mode it will blow a lot of air to keep evaporator as hot as possible, so less humidity is condensing on it (but there will still be some). In dehumidifying mode it will blow less air but keep evaporator cool, so more condensation for the same amount of air cooling.
- teo_zero 4y ago
  Do you have any source for this? It's a topic I've always wanted to understand better. Especially, it's not clear which mode consumes less.
  - yetihehe 4y ago
    Source - I have Kaisai mini-split at work and I've observed how each mode works (with my hand). There's probably more energy used per cooling amount during dehumidifying, because it's bigger temperature differnce (which means lower COP) PLUS condensing water (water phase change energy is big).

jdmichal 4y ago

Unless you have a very strange setup, centralized A/C does not have humidity sensors nor run just to dehumidify. Nor does removing humidity increase the load on the A/C system. Removing humidity is just a happy by-product of how centralized A/C works. So no, you would not be increasing load for that reason.

TeMPOraL 4y ago
Happy in some times, unhappy in others. There are days I'd love if the A/C would have a humidifier built-in, for those winter months when you might want to use A/C to warm the interior.
- ComputerGuru 4y ago
  A good chunk of the United States (and a number of other countries) uses forced air natural gas heating. If you fall into that group, you can install a "whole-house humidifier" that injects steam into the hot air plenum, distributing it around the house. This makes the air less dry (obviously) but also has the side-effect of actually reducing the need for heating because air holding moisture feels warmer than air without (hence the oppressive heat in the humidity of summer), meaning you can lower the thermostat. Unfortunately I have only seen commercial humidification units that boil water into steam with the use of natural gas. Without exception, everything for sale for home use uses electricity (you'll need 230V minimum, single phase will do just fine) to boil the water, which is costly (though this year natural gas prices have risen or even doubled, but even in states with cheap electricity it's probably still cheaper to use natural gas).
  - mmaurizi 4y ago
    We run our steam humidifier just fine on 120v, but we have a relatively small townhome at 1400sqft
    - ComputerGuru 4y ago
      Be careful you’re not running it independently of whether or not the furnace is actually on. 120V generally can’t evaporate enough water in the short window while the furnace is running and many techs will set it up to run at all times (with the blower but not the furnace) to compensate. This lets mildew or mold grow in your ducts.