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> For every 40 pounds of 90°F/90% RH air that you want to make into comfortable inside air: That's a worst case scenario, though. For one, 90/90 would transla

> For every 40 pounds of 90°F/90% RH air that you want to make into comfortable inside air: That's a worst case scenario, though. For one, 90/90 would translate to a 87F dew point -- which is not a record, but it's pretty close, and there's a good chance it would be lethal to unprotected humans. So it does not happen too often, e.g. normal in a place like Phoenix Arizona might be 110F but 20% RH. Second, air conditioning doesn't cool outside air, it cools inside air. So on average it isn't going to have nearly that much water to pull out of the air.

sokoloff 5y ago
Typical house has between 0.5 (quite tight) and 2 air changes per hour. It’s true that the HVAC return air is coming from conditioned space, but it’s also true that that air is frequently exchanged to outside air, meaning it’s going to need to be continually dehumidified. I’ll assume you were kidding about a dew point of 87°F being lethal to unprotected humans. Make the assumptions such that you take 1 pound of water out of 200 pounds of air and (0.5% reduction in water content) and the latent and sensible heat are about equal. Dehumidifying is expensive energy-wise is the point.
- crazygringo 5y ago
  90/90 is lethal, isn't it? That's a heat index of 122°F. You're literally able to die because you're unable to evaporate away body heat quickly enough if you don't take precautions. On the NWS's heat index chart, that's color-coded "danger" while a single tick further up in either humidity or temperature is coded "extreme danger". So I don't see this as kidding... it's some pretty extreme conditions you chose. :)
  - pdonis 5y ago
    > 90/90 is lethal, isn't it? If you stay out in it without taking any precautions for long enough, it could be. But that doesn't mean that as soon as you step out of your front door in the summer in, say, Houston, Texas, you fall over and die. Millions of people in the US (quite possibly billions if you count the rest of the world) live in places where the outside conditions are 90/90 (or worse) frequently in the summer. > it's some pretty extreme conditions you chose As I have already posted elsewhere in this thread, 90/90 is a condition that is commonly tested for HVAC systems; it's not at all "extreme".
    - deleted 5y ago
      [deleted]
pdonis 5y ago
> 90/90 would translate to a 87F dew point -- which is not a record, but it's pretty close, and there's a good chance it would be lethal to unprotected humans That would come as a great surprise to the millions of people who live in places like Houston, Texas or Miami, Florida, where 90/90 is common. (And that's only places in the US, since those are what I'm familiar with from doing HVAC testing years ago. Around the world there are many places that are even worse.) > normal in a place like Phoenix Arizona might be 110F but 20% RH. Yes, but Phoenix is a desert climate, not a hot, humid climate. There are many places that have hot, humid climates.

sokoloff 5y ago

Typical house has between 0.5 (quite tight) and 2 air changes per hour. It’s true that the HVAC return air is coming from conditioned space, but it’s also true that that air is frequently exchanged to outside air, meaning it’s going to need to be continually dehumidified. I’ll assume you were kidding about a dew point of 87°F being lethal to unprotected humans. Make the assumptions such that you take 1 pound of water out of 200 pounds of air and (0.5% reduction in water content) and the latent and sensible heat are about equal. Dehumidifying is expensive energy-wise is the point.

crazygringo 5y ago
90/90 is lethal, isn't it? That's a heat index of 122°F. You're literally able to die because you're unable to evaporate away body heat quickly enough if you don't take precautions. On the NWS's heat index chart, that's color-coded "danger" while a single tick further up in either humidity or temperature is coded "extreme danger". So I don't see this as kidding... it's some pretty extreme conditions you chose. :)
- pdonis 5y ago
  > 90/90 is lethal, isn't it? If you stay out in it without taking any precautions for long enough, it could be. But that doesn't mean that as soon as you step out of your front door in the summer in, say, Houston, Texas, you fall over and die. Millions of people in the US (quite possibly billions if you count the rest of the world) live in places where the outside conditions are 90/90 (or worse) frequently in the summer. > it's some pretty extreme conditions you chose As I have already posted elsewhere in this thread, 90/90 is a condition that is commonly tested for HVAC systems; it's not at all "extreme".
  - deleted 5y ago
    [deleted]

pdonis 5y ago

> 90/90 would translate to a 87F dew point -- which is not a record, but it's pretty close, and there's a good chance it would be lethal to unprotected humans That would come as a great surprise to the millions of people who live in places like Houston, Texas or Miami, Florida, where 90/90 is common. (And that's only places in the US, since those are what I'm familiar with from doing HVAC testing years ago. Around the world there are many places that are even worse.) > normal in a place like Phoenix Arizona might be 110F but 20% RH. Yes, but Phoenix is a desert climate, not a hot, humid climate. There are many places that have hot, humid climates.