4 ms·

Might be misunderstanding something but wouldn't the condensation happen after the AC shut down (presumably after over cooling the room)? A _cool_ story nevert

Might be misunderstanding something but wouldn't the condensation happen after the AC shut down (presumably after over cooling the room)? A _cool_ story nevertheless

hinkley 4y ago
Don't think I've ever thought too hard about it. Thinking 'out loud': If you heat or cool a whole room by changing out the air in it, then the humidity or dew point should be whatever was in the heat pump, so you might get condensation on glass walls between rooms but you shouldn't get any in the room. But in general you're pumping really cold or really hot air into a room and doing only a partial air change and the blower is supposed to stir the room up to even out the temperature. I had to look it up because I wasn't sure, but the amount of humidity air can hold is in fact a curve, not a flat line. So if you mix one unit of very cold air (below the dew point) with four units of ambient air (also below the dew point), it does seem like the dew point of the resulting air might be higher than the temperature. Some server rooms get too dry and moisture has to be added back in to prevent static discharge damaging electronics. That might be the sort of thing he wouldn't have been aware of but was part of the equation. There's also the possibility that he was fucking with me.
- toast0 4y ago
  You don't generally exchange air with the heatpump, you're blowing the conditioned air through a radiator and the heat transfers from the air to the coolant which evaporates; the coolant is piped (pumped) to the outside equipment where there's a condensor and another radiator where the heat is transfered to the outside air. Anyway, if everything is working properly, you more or less get a fixed delta T between the inlet and the outlet of the air handler while the system is running. If you eliminate a lot of the heat load, your inlet temperature might not get as hot, also even if it does get as hot when the A/C is off, when it turns on, it may cool down much quicker and then you may overcool; especially if the A/C has a minimum cycle time. As you say, when you cool air, the humidity capacity drops. Most air handlers have some capacity to remove humidity, but it's no absolute. If you drop the temperature too much, relative humidity will be really high and it will condense on all sorts of surfaces.
  - hinkley 4y ago
    Yeah I assumed but did not state that any condensation in the exchanger would end up down a drain. It's just the room that's the problem.
  - StillBored 4y ago
    Gosh, I wonder where you guys live.. The AC unit pulls the humidity down as it cools (actually it _has_ to remove the moisture first). Which means that _all_ the excess humidity/condensate goes down the AC drain from the evaporator which will be 20-40F (more or less, depending on system/etc, it might be more) cooler than the inlet. If you live say in the southern US and have ever had to use aux cooling in a datacenter you will be shocked at how much you have to empty the condensate collectors. Even on fairly small units you can fill up 55 gallon trash cans in a couple hours. AKA even if its way overspeced you won't get a relative humidity > 100% which is what is required for everything in the room to start condensing. So, if the outlet is really cold, and the humidity/room temp is high you can actually get a small amount of visible condensation as the cold air blows into a hot humid room, but I've never see that stuff manage to get more than a foot or two before it turns back into vapor. Basically a tiny cloud. Which would be how you _might_ get some condensate on something.. If your blowing the cold air against something (or its discharging up through a raised floor vent into a machine cabinet) and the humidity is high in the room it might be possible, but this is why you either mix the room/ac air first, or you seal the cold isle/machine cabinet/floor to avoid cold->hot leaks. If the AC is flowing from the cool/dry side through the machine the machine even if its not heating the air much will just raise the temp, not the humidity. Which lowers the RH in the room. Also, the condensate on a machine room window, will be on the _outside_ where its warmer/wetter because the window is _cooler_ than the outside ambient. Also, unless there is a huge air leak in the room, the machines will act like a heater, and the room will tend to have a fairly low relative humidity. AKA the AC will pull out the moisture for a given temp, and the machines/walls raise the temp of that air, so you have air with low relative humidity. So, the only way the room starts to condense is if a bunch of external warm+humid air is added to a room, but that can happen independent of how much over/under cooling is happening. Running a perfectly setup room at say 70F, and throwing a the doors open to 100F 95% RH air and replacing the air faster than everything in the room can warm up will condense.
- _carbyau_ 4y ago
  And it may have been seen when people walk into the room. IE open the door, let in warmer air. Similar to a walk in refrigerator. > There's also the possibility that he was fucking with me. That too. I've seen parents do this over random things. I hope not to (I don't want to betray trust) but I may be tempted one day if my kid is being willfully ignorant over something that matters.
ahepp 4y ago
It may be overshoot, if the AC is so powerful that when it’s triggered to turn on, it’s tuned to cool WAY too much.
treis 4y ago
Probably getting the windows cold enough so that the humid/warm air outside condensates on the window.

Might be misunderstanding something but wouldn't the condensation happen after the AC shut down (presumably after over cooling the room)? A _cool_ story nevertheless

hinkley 4y ago

Don't think I've ever thought too hard about it. Thinking 'out loud': If you heat or cool a whole room by changing out the air in it, then the humidity or dew point should be whatever was in the heat pump, so you might get condensation on glass walls between rooms but you shouldn't get any in the room. But in general you're pumping really cold or really hot air into a room and doing only a partial air change and the blower is supposed to stir the room up to even out the temperature. I had to look it up because I wasn't sure, but the amount of humidity air can hold is in fact a curve, not a flat line. So if you mix one unit of very cold air (below the dew point) with four units of ambient air (also below the dew point), it does seem like the dew point of the resulting air might be higher than the temperature. Some server rooms get too dry and moisture has to be added back in to prevent static discharge damaging electronics. That might be the sort of thing he wouldn't have been aware of but was part of the equation. There's also the possibility that he was fucking with me.

toast0 4y ago
You don't generally exchange air with the heatpump, you're blowing the conditioned air through a radiator and the heat transfers from the air to the coolant which evaporates; the coolant is piped (pumped) to the outside equipment where there's a condensor and another radiator where the heat is transfered to the outside air. Anyway, if everything is working properly, you more or less get a fixed delta T between the inlet and the outlet of the air handler while the system is running. If you eliminate a lot of the heat load, your inlet temperature might not get as hot, also even if it does get as hot when the A/C is off, when it turns on, it may cool down much quicker and then you may overcool; especially if the A/C has a minimum cycle time. As you say, when you cool air, the humidity capacity drops. Most air handlers have some capacity to remove humidity, but it's no absolute. If you drop the temperature too much, relative humidity will be really high and it will condense on all sorts of surfaces.
- hinkley 4y ago
  Yeah I assumed but did not state that any condensation in the exchanger would end up down a drain. It's just the room that's the problem.
- StillBored 4y ago
  Gosh, I wonder where you guys live.. The AC unit pulls the humidity down as it cools (actually it _has_ to remove the moisture first). Which means that _all_ the excess humidity/condensate goes down the AC drain from the evaporator which will be 20-40F (more or less, depending on system/etc, it might be more) cooler than the inlet. If you live say in the southern US and have ever had to use aux cooling in a datacenter you will be shocked at how much you have to empty the condensate collectors. Even on fairly small units you can fill up 55 gallon trash cans in a couple hours. AKA even if its way overspeced you won't get a relative humidity > 100% which is what is required for everything in the room to start condensing. So, if the outlet is really cold, and the humidity/room temp is high you can actually get a small amount of visible condensation as the cold air blows into a hot humid room, but I've never see that stuff manage to get more than a foot or two before it turns back into vapor. Basically a tiny cloud. Which would be how you _might_ get some condensate on something.. If your blowing the cold air against something (or its discharging up through a raised floor vent into a machine cabinet) and the humidity is high in the room it might be possible, but this is why you either mix the room/ac air first, or you seal the cold isle/machine cabinet/floor to avoid cold->hot leaks. If the AC is flowing from the cool/dry side through the machine the machine even if its not heating the air much will just raise the temp, not the humidity. Which lowers the RH in the room. Also, the condensate on a machine room window, will be on the _outside_ where its warmer/wetter because the window is _cooler_ than the outside ambient. Also, unless there is a huge air leak in the room, the machines will act like a heater, and the room will tend to have a fairly low relative humidity. AKA the AC will pull out the moisture for a given temp, and the machines/walls raise the temp of that air, so you have air with low relative humidity. So, the only way the room starts to condense is if a bunch of external warm+humid air is added to a room, but that can happen independent of how much over/under cooling is happening. Running a perfectly setup room at say 70F, and throwing a the doors open to 100F 95% RH air and replacing the air faster than everything in the room can warm up will condense.
_carbyau_ 4y ago
And it may have been seen when people walk into the room. IE open the door, let in warmer air. Similar to a walk in refrigerator. > There's also the possibility that he was fucking with me. That too. I've seen parents do this over random things. I hope not to (I don't want to betray trust) but I may be tempted one day if my kid is being willfully ignorant over something that matters.

ahepp 4y ago

It may be overshoot, if the AC is so powerful that when it’s triggered to turn on, it’s tuned to cool WAY too much.

treis 4y ago

Probably getting the windows cold enough so that the humid/warm air outside condensates on the window.