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I think the main problem would be humidity issues. The outside air swings so wildly in humidity. I was growing weed outside in my state and surprised to watch

I think the main problem would be humidity issues. The outside air swings so wildly in humidity. I was growing weed outside in my state and surprised to watch the humidity on the meter regularly go from 21% to nearly 100% on day night cycles.

amluto 4y ago
Look at dew point or some absolute humidity measure instead. As air gets colder, the amount of water vapor it can hold (the denominator in relative humidity) decreases dramatically. Introducing cold, saturated air to your warm house will not, in general, make it feel nasty and humid.
- JohnJamesRambo 4y ago
  Some googling of experts shows it can be quite a concern. https://www.eng-tips.com/viewthread.cfm?qid=188008 https://www.eng-tips.com/viewthread.cfm?qid=188008
  - Scoundreller 4y ago
    I can only barely understand what’s going on in that link, but I think it really depends on target indoor environment. While I don’t want the walls sweating, I could imagine there are industrial or commercial environments where moisture level matters a lot. Electronics plant: dry is always bad! Want some moisture to dissipate static. A library wants low, but not zero moisture. A manufacturing plant wants it to be very consistent. A fruit market wants its humidity high. In a residential environment, it’s a toss up: some stuff wants to be dryer while other stuff doesn’t.
    - amluto 4y ago
      I don’t think you’re likely to make the walls sweat by running an economizer or, for that matter, opening a window while it’s cooler outside than inside. You may end up with more humidity inside than you like, though, depending on the weather. That link seems to be discussing a situation where there is too much humidity coming in relative to the amount of heat the A/C needs to remove, and that hits my pet peeve about air conditioning design. Air conditioners generally run at a roughly constant coil temperature, which means that there is no independent control over dehumidification versus cooling. If you happen to be at the design conditions, great. Otherwise you don’t end up with the humidity you want. I know of one system (“dynamic humidity control” by Chiltrix) that adjusts the coil temperature to control humidity. As far as I know, any hydronic system can do this with minimal modification, but I don’t think it’s popular.
      - Scoundreller 4y ago
        I have a similar issue with thermostats in general: set points without a humidity coefficient is a constant battle. 26C and dry is comfortable: Stay off. 26C and humid is sweltering: Power up. My "smart" thermostat has a settings for this, but I seem to be at its algorithm's mercy instead of specifying: "50% RH or less, no change, 50-70%, -2C to the set points, 70%+, -3C to the set points". Let alone being able to do dehumidification only.
        amluto 4y ago
        This isn’t entirely a problem with your thermostat. If it’s 26C and humid, all your thermostat can do is to call for cooling. (Maybe it can call for stage 1 or stage 2. You are unlikely to have anything that can do stand-alone dehumification.) So it can’t actually achieve a comfortable and efficient target.
        Scoundreller 4y ago
        Would still be more comfortable and efficient if could aim for what I'd be equally happy with: 23C + humid or 26C + dry
pkulak 4y ago
That’s just because the temperature swings wildly. Water isn’t going in and out of the air.
quickthrower2 4y ago
Cold air holds less water, so that makes sense. It is like pouring 200ml of water from a 1000ml jug into a 200ml jug. Same water, higher % fillage (to use a technical term).
stephen_g 4y ago
If you're in a climate where that is a problem, the solution is to use an energy recovery ventilator (ERV) along with your air conditioning to use the conditioned inside air being exhausted to pre-treat the incoming air using an enthalpy heat exchanger. Heat recovery ventilation (HRV), which uses a sensible heat exchanger, is sufficient in temperate climates but otherwise you need to be able to transfer latent heat also to reduce humidity, so need the ERV.
- JohnJamesRambo 4y ago
  Really neat, thank you! I knew about the HRV but not the ERV.

amluto 4y ago

Look at dew point or some absolute humidity measure instead. As air gets colder, the amount of water vapor it can hold (the denominator in relative humidity) decreases dramatically. Introducing cold, saturated air to your warm house will not, in general, make it feel nasty and humid.

JohnJamesRambo 4y ago
Some googling of experts shows it can be quite a concern. https://www.eng-tips.com/viewthread.cfm?qid=188008 https://www.eng-tips.com/viewthread.cfm?qid=188008
- Scoundreller 4y ago
  I can only barely understand what’s going on in that link, but I think it really depends on target indoor environment. While I don’t want the walls sweating, I could imagine there are industrial or commercial environments where moisture level matters a lot. Electronics plant: dry is always bad! Want some moisture to dissipate static. A library wants low, but not zero moisture. A manufacturing plant wants it to be very consistent. A fruit market wants its humidity high. In a residential environment, it’s a toss up: some stuff wants to be dryer while other stuff doesn’t.
  - amluto 4y ago
    I don’t think you’re likely to make the walls sweat by running an economizer or, for that matter, opening a window while it’s cooler outside than inside. You may end up with more humidity inside than you like, though, depending on the weather. That link seems to be discussing a situation where there is too much humidity coming in relative to the amount of heat the A/C needs to remove, and that hits my pet peeve about air conditioning design. Air conditioners generally run at a roughly constant coil temperature, which means that there is no independent control over dehumidification versus cooling. If you happen to be at the design conditions, great. Otherwise you don’t end up with the humidity you want. I know of one system (“dynamic humidity control” by Chiltrix) that adjusts the coil temperature to control humidity. As far as I know, any hydronic system can do this with minimal modification, but I don’t think it’s popular.
    - Scoundreller 4y ago
      I have a similar issue with thermostats in general: set points without a humidity coefficient is a constant battle. 26C and dry is comfortable: Stay off. 26C and humid is sweltering: Power up. My "smart" thermostat has a settings for this, but I seem to be at its algorithm's mercy instead of specifying: "50% RH or less, no change, 50-70%, -2C to the set points, 70%+, -3C to the set points". Let alone being able to do dehumidification only.
      - amluto 4y ago
        This isn’t entirely a problem with your thermostat. If it’s 26C and humid, all your thermostat can do is to call for cooling. (Maybe it can call for stage 1 or stage 2. You are unlikely to have anything that can do stand-alone dehumification.) So it can’t actually achieve a comfortable and efficient target.
        Scoundreller 4y ago
        Would still be more comfortable and efficient if could aim for what I'd be equally happy with: 23C + humid or 26C + dry

pkulak 4y ago

That’s just because the temperature swings wildly. Water isn’t going in and out of the air.

quickthrower2 4y ago

Cold air holds less water, so that makes sense. It is like pouring 200ml of water from a 1000ml jug into a 200ml jug. Same water, higher % fillage (to use a technical term).

stephen_g 4y ago

If you're in a climate where that is a problem, the solution is to use an energy recovery ventilator (ERV) along with your air conditioning to use the conditioned inside air being exhausted to pre-treat the incoming air using an enthalpy heat exchanger. Heat recovery ventilation (HRV), which uses a sensible heat exchanger, is sufficient in temperate climates but otherwise you need to be able to transfer latent heat also to reduce humidity, so need the ERV.

JohnJamesRambo 4y ago
Really neat, thank you! I knew about the HRV but not the ERV.