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> The 50F unit will cool a smaller volume of air Your home's A/C unit can't just pick a particular volume of air and cool it. It takes in air from your entire
by pdonis 5y ago
> The 50F unit will cool a smaller volume of air
Your home's A/C unit can't just pick a particular volume of air and cool it. It takes in air from your entire house, cools it, and puts it back out into your house.
Moreover, your whole comparison is irrelevant to what I am saying. The objective is not to get the dewpoint (or temperature) as low as possible. The objective is to get the dewpoint (and temperature) just low enough for comfort (note that the target dewpoint might indeed be lower than the target temperature--see further comments below), and then keep it there by running the A/C continuously at a lower output, instead of running the A/C at full on for a while, which outputs air with a dewpoint (and temperature) lower than required, and then shutting off the A/C for a while, which lets the air's dewpoint (and temperature) rise to a level higher than desired. In other words, target the A/C's output to just where it needs to be, instead of cycling it on and off.
> There's an efficiency penalty to letting the temperature wander. But if it only wanders a couple degrees, it's not a huge penalty.
The main efficiency penalty is not from letting the temperature wander. It's from letting the dewpoint wander. Most of the work the A/C does is actually condensing extra water vapor out of the air, not cooling the air itself. If your room temperature rises from, say, 74 F to 76 F, that's not much more work for the A/C to do; but if the dewpoint of the air in the room rises from 50 F to 76 F (which it might well do on a hot, humid day during the off cycle of your A/C unit), that's a lot more work for the A/C to do to condense out all that extra water vapor.
> there's a penalty for keeping the fans and compressor going all the time, even if they're running at a lower power.
There's also a penalty for starting them up every time from off. For many such devices, given the number of times they cycle on and off during a hot, humid day, that penalty can easily average out to be larger than the cost of running the unit at lower power continuously.
> if you could leave the room at 73 with a low dew point
Which you can't if the system is off. The only way to control the dewpoint is to run the A/C continuously. As soon as you shut the A/C off, on a humid day, the dewpoint will start rising. And then your A/C will have to work harder during its next on cycle to lower it again.
If you run the A/C continously at lower power, then yes, you can adjust things so the heat coming into the house raises the temperature (from the A/C outlet temperature, which is basically the dew point) just enough to have the combination of temperature and dew point that you want. That's basically what I've been describing. The dewpoint of 68 F that I first threw out might be higher than optimal; the point was not to pick a specific optimal number but just to illustrate the concept.
- Dylan16807 5y ago> Your home's A/C unit can't just pick a particular volume of air and cool it. It takes in air from your entire house, cools it, and puts it back out into your house. I'm talking about how much air goes through the unit. By the time it's done, it will have emitted fewer cubic feet of air. > Moreover, your whole comparison is irrelevant to what I am saying. The objective is not to get the dewpoint (or temperature) as low as possible. The objective is to get the dewpoint (and temperature) just low enough for comfort (note that the target dewpoint might indeed be lower than the target temperature--see further comments below) It's not irrelevant if overshooting one target lets you undershoot the other target. > The main efficiency penalty is not from letting the temperature wander. It's from letting the dewpoint wander. Most of the work the A/C does is actually condensing extra water vapor out of the air, not cooling the air itself. Same difference. If you cycle the AC on and off every few minutes, neither one will wander much. > There's also a penalty for starting them up every time from off. For many such devices, given the number of times they cycle on and off during a hot, humid day, that penalty can easily average out to be larger than the cost of running the unit at lower power continuously. It only takes a few seconds of power to spin up. If that is the dominating inefficiency in the system, then it barely matters what you do. > Which you can't if the system is off. The only way to control the dewpoint is to run the A/C continuously. As soon as you shut the A/C off, on a humid day, the dewpoint will start rising. And then your A/C will have to work harder during its next on cycle to lower it again. If you run it a little bit every ten minutes it will do almost as good a job at preventing drifting. It will work "harder" but for less time, so averaged over the entire 10 minutes it's probably about the same. Might be more, might be less. Depends on how efficient the fans and compressor are at different speeds. And how efficient the evaporator is at different flow rates. > If you run the A/C continously at lower power, then yes, you can adjust things so the heat coming into the house raises the temperature (from the A/C outlet temperature, which is basically the dew point) just enough to have the combination of temperature and dew point that you want. That's basically what I've been describing. The dewpoint of 68 F that I first threw out might be higher than optimal; the point was not to pick a specific optimal number but just to illustrate the concept. The different modes having different dew points was a core part of your original suggestion. If you're changing that, and now we're talking about a system where "full on" is an optimal dew point but high air flow, and "energy saving mode" is the same dew point but with low air flow, then that sounds great. And if you had suggested that originally, I wouldn't have posted to question it.