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imagine that the reduction of energy consumption is the main goal. Is it more efficient to leave the dehumidifier to work over a longer period of time to keep the specific set point or it is more efficient to turn the dehumidifier completely off and turn it on to reach the set point only when it is needed.

as an example we are away from home, if the ideal set point is RH% equal to 50%, and the ambient has the potential to increase the RH% to 80%, is it better that the dehumidifier works continuously to keep this 50%? or turn it on when we arrive home which probably the RH is 80% and it should work to reach 50%.

thank you!

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    A better question is whether you're willing to tolerate uncomfortably high humidity while you're waiting for the unit to recover (if it ever does), and whether you'll end up with more mildew and mold in your home as well. Dehumidifiers can't use more than a few dollars worth of power per month, right? – isherwood Jul 26 '16 at 14:36
  • I think there are too many variables to accurately answer this question. This might be better suited to a home project with some electricity consumption monitor like a "Kill-a-watt". There are many theories to this, as well as practical limitations to maintaining your controls versus variables. Sounds like a good HS/College Physics research project though. – BrownRedHawk Jul 26 '16 at 15:36
  • @isherwood Typical household humidifiers draw 400-500W and can do so continuously if humidity is high and air is penetrating from outdoors. Running full tilt this translates to about $30-40 a month in energy usage (assuming typical $.12/kwh) – Jeff Meden Jul 26 '16 at 15:48
  • I understand, but obviously the OP doesn't have a "full tilt" situation. Maybe my use of "a few" wasn't reasonable, but we're probably talking about $10-20. How much does a person stand to save by fretting over timelines? 10% of that? 20? Now we're in the single digits. That's a pretty small savings for the effort. – isherwood Jul 26 '16 at 15:53
  • @isherwood - an easy way around that problem is to put it on a timer, set to turn on an hour or so before they come home. – Johnny Jul 26 '16 at 17:34
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This is a really common question, that applies to many things: heat, air conditioning, dehumidifiers, lighting and even PCs.

"Isn't it better to just leave the machine running continuously? The energy costs of restarting it must be enormous, surely they must be more than the costs of leaving it running."

Almost universally, the answer is nope. "Leave it running" is just an excuse/rationalization. What's really true is you want the facility to be already comfortable when you return, but you are bothered by the "guilt" of leaving equipment running unnecessarily. So you find a way to "believe" you are doing the right thing. It's a convenient untruth.

WHY? Consider: when you turn off the heat/cool/dehumidify, it has a "ramp" period where the house gets worse. Then it has an "equilibrium" period where the house has hit bottom, and doesn't get any worse. Then another "ramp" period when you start the machine back up, to bring it to your preferred level.

First, let's look at those ramp periods. Surely they cancel each other out, yes? Not quite. Actually, A/C and dehumidifiers work more efficiently when conditions are worse. That's readily obvious on a dehumidifer; it's much easier for the unit to get from 80% to 70% than it is to get from 50% to 40%. An A/C coil also has a much easier time cooling air from 100 to 90 than from 78 to 68. Heat transfers more easily when temperature differentials are higher.

Second, for that same reason, insulation works better when there's less temperature differential across it. The houses loses much less "cool" at 90 than it does at 70.

Third, that equilibrium period is absolutely free. The house doesn't get any worse, even though you aren't paying to run the machine.

So go ahead and shut it off if you don't need it. If you are annoyed having to suffer through the down-ramp period, then consider a Nest style thermostat or other "smart home" type automation that lets you turn it on via a timer, or auto-learning process, or remotely with your phone.

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    Excellent answer. I was about to write up something about evaporation rates and differentials, but you covered it all and then some. Bonus kudos for tying in human nature. We are a species that loves to rationalize what we want to be true. – isherwood Jul 26 '16 at 16:43
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It is always at least as efficient to leave the dehumidifier off while you're away, as it thus won't use any electricity. If you leave it on, assuming ambient is always at 80% RH, each time the humidity gets down to 50% RH, it will eventually go back up to 80%, causing the dehumidifier to run again until it's down to 50%. Rinse and repeat. How often it runs in this case depends on the efficiency of the dehumidifier and how quickly the humidity rises.

The actual savings, on the other hand, are not so simple to calculate. It depends on how quickly the humidity rises (which in turn depends on the weather), energy usage of the dehumidifier, cost of electricity, and how quickly the dehumidifier removes moisture from the air.

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