Author Topic: A/C vs Dehumidifier (I want this to work, but I don't think it will)  (Read 2826 times)

igthebold

  • Bristles
  • ***
  • Posts: 470
  • Age: 41
  • Location: NC Piedmont
OK, thermodynamics nerds.

Here's what I *want* to do: I want to open the window if it's cool enough outside and pull air through regardless of the humidity. Then I want to run a dehumidifer to remove the humidity from the air and keep my A/C at 82F/28C. Or higher.

Here's why I think it works: I just learned about the concepts of latent heat (humidity) and sensible heat (temperature). By pulling air in from outside, I decrease the sensible heat, and either increase or maintain latent heat.

A/C does two jobs: remove sensible heat and remove latent heat. Dehumidifers do one job: remove latent heat. By pulling cool air in, I obviate the need to remove sensible heat. Ideally, the dehumidifier spends less energy doing so.

Here's why I'm skeptical: I'm essentially running a limited purpose heat pump inside my house. While it might be more efficient, I'm guessing it's not *that* much more efficient.

What do you think?

velocistar237

  • Handlebar Stache
  • *****
  • Posts: 1422
  • Location: Metro Boston
Re: A/C vs Dehumidifier (I want this to work, but I don't think it will)
« Reply #1 on: June 07, 2013, 06:09:56 AM »
It won't work. The latent load you would pull through the windows is greater than the nighttime difference in sensible heat. Also, you can think of a dehumidifier as a very efficient heater.

Rural

  • Magnum Stache
  • ******
  • Posts: 4932
Re: A/C vs Dehumidifier (I want this to work, but I don't think it will)
« Reply #2 on: June 07, 2013, 06:16:39 AM »
I'll second the dehumidifier as heater. We don't yet have our through the wall heat pumps installed here and are doing what you describe with a portable dehumidifier when the humidity gets very high. It's a net heat gain, no doubt, and the end result is that the house is hotter during the day than it would be with nothing and wetter at night than is safe from a mold-growth perspective. Heat pumps are on the "do it soon" list.

velocistar237

  • Handlebar Stache
  • *****
  • Posts: 1422
  • Location: Metro Boston
Re: A/C vs Dehumidifier (I want this to work, but I don't think it will)
« Reply #3 on: June 07, 2013, 07:17:33 AM »
Raleigh, NC, has an average overnight low of 70 degF in July and August. I'll assume a corresponding relative humidity of 95%. You want inside air to be 82 degF and 50% relative humidity. The sensible load in your favor from the 12 degF temperature difference would be about 1300 BTU/h per 100 cfm of air. The latent difference required to remove 25 grains/lb of excess humidity from 100 cfm of air would be about 1600 BTU/hr. Add to that the 400 W (~1300 BTU/hr) from a typical small dehumidifier, since the dehumidifier turns that electricity into heat; the extra heat from temperatures higher than the overnight low (most of the time); and the heat gain from sun, walls, windows, appliances, and people. In the middle of summer, you would probably end up with an inside temperature well over 100 degF.

If you're the DIY type, you can get a modestly efficient ductless mini-split online for about $1500 and install it yourself. If you do that, let us know how it goes.

igthebold

  • Bristles
  • ***
  • Posts: 470
  • Age: 41
  • Location: NC Piedmont
Re: A/C vs Dehumidifier (I want this to work, but I don't think it will)
« Reply #4 on: June 07, 2013, 11:00:08 AM »
Thanks for the info and number crunching. As much as I would like to keep the temperature up, I'm probably going to have to cool the house down just so I'm a responsible homeowner. We have an almost new A/C system, and that's probably the sanest thing to use.