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Something similar happened when I put a dehumidifier in my bedroom. Basically, I noticed that the room was heating significantly when I turned it on. Way faste

Something similar happened when I put a dehumidifier in my bedroom. Basically, I noticed that the room was heating significantly when I turned it on. Way faster than what you should expect from its power usage (about 250W avg.). Correct me if I'm wrong, but by converting air moisture to liquid water using a compressor, the dehumidifier is releasing the latent energy of water vapor (0.627Wh/L) in addition to its own energy usage. I measured that my dehumidifier required +/- 1kW/h to get 1L of liquid water from the room. This it's overall heating efficiency is about 1.6x, which is way better than any electric heating system that is not an heat-pump.

Ekaros 3y ago
Dehumidifiers are heat pumps. The heat source is latent energy in water vapour.
NegativeK 3y ago
Given that a dehumidifier is doing the opposite of a swamp (evaporative) cooler, this makes sense.
laweijfmvo 3y ago
doesn't humid air "feel" warmer, though?
- AnimalMuppet 3y ago
  Warmer if it's warm, colder if it's cold. I think the deal is that it's more efficient at transferring heat to/from your skin.
  - pdonis 3y ago
    > Warmer if it's warm Yes, but not quite for the reason you give. See below. > colder if it's cold. This I don't agree with. Cold air can't hold any significant amount of water vapor to begin with. It is true that a cold, rainy day feels colder than a cold, dry day with the same air temperature, but rain isn't humidity; rain is water, not water vapor. > I think the deal is that it's more efficient at transferring heat to/from your skin. Not quite. The deal is that your body sheds heat by evaporation of water (sweat), but water can only evaporate if the surrounding air is not already saturated, and the closer the surrounding air is to being saturated, i.e., the more humid it is, the less heat your body can shed before the air is saturated. In cold conditions your body wants to retain heat, not shed heat, and the humidity in the cold air doesn't really affect that at all, because, as above, there just isn't enough of it in cold air to make a difference. What does affect your body's ability to retain heat is dampness or rain, i.e., water, not water vapor; the extra water (far more water than you would generate by sweat on a hot day, or than exists as water vapor in cold air) sucks heat out of your body because the water is colder than you are and has such a high heat capacity (even if it doesn't evaporate).
    - AnimalMuppet 3y ago
      Good explanation on the warm. I still disagree on the cold. The cold in the midwest cuts through you in a way that it doesn't in the dry. Air temperature's the same, but the difference is noticeable. Why is that? If not humidity, then what is it?
      - pdonis 3y ago
        See my response to the sibling post to yours. My experience in the Midwest (and elsewhere) on colder days is that the biggest factor is wind. Even a relatively light wind on a cold day can make a big difference in draining heat from your body. Convection makes a big difference in the heat transfer properties of air. Btw, you say "in the dry", but have you ever been in the desert on a cold night? I think you might reconsider just how cold "dry cold" can feel.
        AnimalMuppet 3y ago
        > Btw, you say "in the dry", but have you ever been in the desert on a cold night? I think you might reconsider just how cold "dry cold" can feel. Lived in Salt Lake City for 60 years, so yes. And I would say that, to me, the cold in the Midwest is worse than the cold in the West, even at the same temperature and wind. But after reading your response, another variable has occurred to me. The west isn't just dry, it's high and dry. Higher elevation means lower air pressure. Lower air pressure means fewer molecules hitting each square inch each second, carrying off heat. Is lower air pressure part of why the cold doesn't cut through you as much out west?
        pdonis 3y ago
        > Is lower air pressure part of why the cold doesn't cut through you as much out west? Could be, yes. The lower air pressure means less heat transfer out of your body, as you say.
    - nlh 3y ago
      > This I don't agree with. Cold air can't hold any significant amount of water vapor to begin with. It is true that a cold, rainy day feels colder than a cold, dry day with the same air temperature, but rain isn't humidity; rain is water, not water vapor. I have a question on this -- I've never quite been able to put my finger on it (ha) but there is some atmospheric effect that makes SOME cold days feel, to use a phrase "bone-chilling cold", vs other cold days that just feel chilly. And I'm not talking about 10 degrees F vs. 40 degrees F. There are 45-degree days that just feel miserable and 45 degree days that just feel cool. On those bone-chilling days, it almost feels as if the cold is sticking to my skin. I've always thought the difference was humidity -- similar to how a hot humid day is just 10x more miserable than the venerable "dry heat" of a desert, which can feel significantly more comfortable even at many degrees hotter. If it's not humidity on those cold days, what is it? [Edit: Funny - the sibling commenter had basically the same question at the exact same time]
- wuschel 3y ago
  In simple terms, water is more than 20 times more efficient in transferring heat energy than air. The more moisture in the air, the more heat energy transmitting matter there is per volume, the more you feel heat or cold. That is why it can feel less cold when temperatures fall lower than 0 degrees Celsius (32 Fahrenheit): Most of the moisture in the air becomes solid and falls to the ground. Don't we all love those wet cold days?
  - Filligree 3y ago
    > Don't we all love those wet cold days? Coming from Norway to Dublin, one of my biggest annoyances is the winter is 5-6 degrees warmer... which puts it right above zero... which makes it feel much, much colder. The instant you hit freezing temperatures the absolute humidity falls off a cliff.
- WalterSear 3y ago
  It makes cooling via evaporation less effective, and is more effective at absorbing heat when it's colder than you.
grammers 3y ago
That's interesting, will test this right away myself!
pdonis 3y ago
> by converting air moisture to liquid water using a compressor, the dehumidifier is releasing the latent energy of water vapor The latent heat does get released, but the question is, where does it go? In an air conditioner, which also condenses water vapor from the air, the heat released goes into the refrigerant, which means it ultimately gets put into the atmosphere outside from the condenser. A dehumidifier is basically an air conditioner without an outside unit, so the heat from the condensation of water vapor goes into the refrigerant, but then gets put into the room from the condenser, instead of outside.
- BobaFloutist 3y ago
  I guess it's like a reverse swamp-cooler?
  - pdonis 3y ago
    Sort of, if you think of a swamp cooler as something like a "rehumidifier".
qqqwerty 3y ago
Interesting. I am surprised this doesn't seem to be more widely known. We have an ancient gas wall heater in our living room and due to a cat with CKD, we need to keep the bedroom doors closed. We have some small electric heaters, but those are not safe to leave on in our toddlers bedroom and are expensive to operate. I have been looking into getting some window mounted heat pumps, but they are expensive, bulky, and don't really work with our windows. We live by the coast, so we have a few small dehumidifiers for mold prevention but have not been using them much. I am going to test this out and see if it works. I don't particularly mind bundling up, but the toddler can't make it through the night with the blankets on. So if this does work, seems like it would be the perfect solution.

Ekaros 3y ago

Dehumidifiers are heat pumps. The heat source is latent energy in water vapour.

NegativeK 3y ago

Given that a dehumidifier is doing the opposite of a swamp (evaporative) cooler, this makes sense.

laweijfmvo 3y ago

doesn't humid air "feel" warmer, though?

AnimalMuppet 3y ago
Warmer if it's warm, colder if it's cold. I think the deal is that it's more efficient at transferring heat to/from your skin.
- pdonis 3y ago
  > Warmer if it's warm Yes, but not quite for the reason you give. See below. > colder if it's cold. This I don't agree with. Cold air can't hold any significant amount of water vapor to begin with. It is true that a cold, rainy day feels colder than a cold, dry day with the same air temperature, but rain isn't humidity; rain is water, not water vapor. > I think the deal is that it's more efficient at transferring heat to/from your skin. Not quite. The deal is that your body sheds heat by evaporation of water (sweat), but water can only evaporate if the surrounding air is not already saturated, and the closer the surrounding air is to being saturated, i.e., the more humid it is, the less heat your body can shed before the air is saturated. In cold conditions your body wants to retain heat, not shed heat, and the humidity in the cold air doesn't really affect that at all, because, as above, there just isn't enough of it in cold air to make a difference. What does affect your body's ability to retain heat is dampness or rain, i.e., water, not water vapor; the extra water (far more water than you would generate by sweat on a hot day, or than exists as water vapor in cold air) sucks heat out of your body because the water is colder than you are and has such a high heat capacity (even if it doesn't evaporate).
  - AnimalMuppet 3y ago
    Good explanation on the warm. I still disagree on the cold. The cold in the midwest cuts through you in a way that it doesn't in the dry. Air temperature's the same, but the difference is noticeable. Why is that? If not humidity, then what is it?
    - pdonis 3y ago
      See my response to the sibling post to yours. My experience in the Midwest (and elsewhere) on colder days is that the biggest factor is wind. Even a relatively light wind on a cold day can make a big difference in draining heat from your body. Convection makes a big difference in the heat transfer properties of air. Btw, you say "in the dry", but have you ever been in the desert on a cold night? I think you might reconsider just how cold "dry cold" can feel.
      - AnimalMuppet 3y ago
        > Btw, you say "in the dry", but have you ever been in the desert on a cold night? I think you might reconsider just how cold "dry cold" can feel. Lived in Salt Lake City for 60 years, so yes. And I would say that, to me, the cold in the Midwest is worse than the cold in the West, even at the same temperature and wind. But after reading your response, another variable has occurred to me. The west isn't just dry, it's high and dry. Higher elevation means lower air pressure. Lower air pressure means fewer molecules hitting each square inch each second, carrying off heat. Is lower air pressure part of why the cold doesn't cut through you as much out west?
        pdonis 3y ago
        > Is lower air pressure part of why the cold doesn't cut through you as much out west? Could be, yes. The lower air pressure means less heat transfer out of your body, as you say.
  - nlh 3y ago
    > This I don't agree with. Cold air can't hold any significant amount of water vapor to begin with. It is true that a cold, rainy day feels colder than a cold, dry day with the same air temperature, but rain isn't humidity; rain is water, not water vapor. I have a question on this -- I've never quite been able to put my finger on it (ha) but there is some atmospheric effect that makes SOME cold days feel, to use a phrase "bone-chilling cold", vs other cold days that just feel chilly. And I'm not talking about 10 degrees F vs. 40 degrees F. There are 45-degree days that just feel miserable and 45 degree days that just feel cool. On those bone-chilling days, it almost feels as if the cold is sticking to my skin. I've always thought the difference was humidity -- similar to how a hot humid day is just 10x more miserable than the venerable "dry heat" of a desert, which can feel significantly more comfortable even at many degrees hotter. If it's not humidity on those cold days, what is it? [Edit: Funny - the sibling commenter had basically the same question at the exact same time]
wuschel 3y ago
In simple terms, water is more than 20 times more efficient in transferring heat energy than air. The more moisture in the air, the more heat energy transmitting matter there is per volume, the more you feel heat or cold. That is why it can feel less cold when temperatures fall lower than 0 degrees Celsius (32 Fahrenheit): Most of the moisture in the air becomes solid and falls to the ground. Don't we all love those wet cold days?
- Filligree 3y ago
  > Don't we all love those wet cold days? Coming from Norway to Dublin, one of my biggest annoyances is the winter is 5-6 degrees warmer... which puts it right above zero... which makes it feel much, much colder. The instant you hit freezing temperatures the absolute humidity falls off a cliff.
WalterSear 3y ago
It makes cooling via evaporation less effective, and is more effective at absorbing heat when it's colder than you.

grammers 3y ago

That's interesting, will test this right away myself!

pdonis 3y ago

> by converting air moisture to liquid water using a compressor, the dehumidifier is releasing the latent energy of water vapor The latent heat does get released, but the question is, where does it go? In an air conditioner, which also condenses water vapor from the air, the heat released goes into the refrigerant, which means it ultimately gets put into the atmosphere outside from the condenser. A dehumidifier is basically an air conditioner without an outside unit, so the heat from the condensation of water vapor goes into the refrigerant, but then gets put into the room from the condenser, instead of outside.

BobaFloutist 3y ago
I guess it's like a reverse swamp-cooler?
- pdonis 3y ago
  Sort of, if you think of a swamp cooler as something like a "rehumidifier".

qqqwerty 3y ago

Interesting. I am surprised this doesn't seem to be more widely known. We have an ancient gas wall heater in our living room and due to a cat with CKD, we need to keep the bedroom doors closed. We have some small electric heaters, but those are not safe to leave on in our toddlers bedroom and are expensive to operate. I have been looking into getting some window mounted heat pumps, but they are expensive, bulky, and don't really work with our windows. We live by the coast, so we have a few small dehumidifiers for mold prevention but have not been using them much. I am going to test this out and see if it works. I don't particularly mind bundling up, but the toddler can't make it through the night with the blankets on. So if this does work, seems like it would be the perfect solution.