Simply put, is it possible to design a dehumidifier that does not also generate a lot of heat?
I’ve got a basement that is nice and cool in the summertime, and it’s therefore a great place to escape the heat (my house does not have AC).
But the basement will get too damp unless I run my dehumidifier, which has an unfortunate side effect of blasting out air that is between 10 and 20 degrees hotter than the ambient air, thus significantly raising the overall temperature of the basement.
Now, my basement is no longer a cool place to be to escape the summer heat.
My dehumidifier is about 10 years old. Are newer ones able to do their job of drying the air without also heating up the air?
Or is this not possible due to the nature of the dehumidification process?
nope. a typical dehumidifier is just an air conditioner which doesn’t separate the hot and cold zones. they work by moving heat around, so in order to make the evaporator coils cold, the condenser coils have to get hot.
the only other way would be a desiccant of some sort, but those are limited in how much moisture they can hold before they stop working.
I was in a similar situation. My basement got humid in the summer, so much so that it had a musty smell. Running a dehumidifier non-stop removed the moisture, but increased the temperature to where it was uncomfortable. My water is heated with my natural gas boiler, so the furnace would run to heat our water, making the basement even warmer. I have central AC upstairs, but didn’t want to install ducts to serve the basement. And, yes, a dehumidifier is basically an air conditioner that vents the hot air out the back instead of outside - they will always heat up a room.
I ended up buying a hybrid (heat pump) water heater and installing it in my basement next to my existing gas water heater. The “hybrid” water heater heats the water with a heat pump, pulling heat and moisture from the air to heat the water in the tank. Problem solved. The water heater runs by removing heat and moisture, making the basement cool and dry, and heating my water in the summertime. Now, my furnace only turns on to heat the water when we have unusually large hot water demand, but otherwise stays off.
It wasn’t a cheap solution, but it is working well. I bought a GE Hybrid water heater four years ago from Lowe’s, and thankfully purchased the 10-year extended warranty. The heat pump on the water heater went kaput in year 3, and they refunded my money, and I replaced it with a new unit. The new unit is made in the US, while the older one was made in China. Lots of people had problems with the Chinese unit, and I’m hoping the new USA model is more durable. I bought a new 10-year warranty just in case.
Lowe’s has the GE Hybrid water heater for $999. With installation, you are probably looking at $1300 or so, but it will solve your heat and humidity problems. I got a $300 rebate from my electric company, reducing my net cost back down to $1,100 (including the now-9-year warranty) - don’t know if you can get an energy rebate in your area.
The most common household dehumidifier is basically an air conditioner or refrigerator. It utilizes the Ideal Gas law (when pressure increases, temperature increases, and vice versa) to create a hot side and cold side. The cold side dehumidifies the air by condensing water vapor. The pressure increase is achieved through a motor driven compressor. The motor requires energy, which is expels as heat.
There is also a desiccant dehumidifier, which works like the silica packets you get some packaged goods. The desiccant absorbs water vapor from the air. There is a limitation to how much water vapor the desiccant can absorb before it becomes saturated. In order to keep the desiccant active, a heater increases the temperature of the desiccant, so that it releases the moisture. This humidified air stream is channeled over the cold side of a thermoelectric plate/Peltier, which condenses the water vapor into water, and into a collector, to be emptied or pumped away. The heater and thermoelectric plate use energy, which is predictably expelled as heat.
The dehumidification process involves disrupting an equilibrium. It takes energy to maintain this continuous disequilibrium. The distinction of “continuous” is important.
I remember one of my college professors talking about a time before wide spread heating was available. People would leave rocks outside in the sun during the day, or heat them by the fire, to be brought into the house during night as a crude method of space heating.
I’ve always wondered if it would be feasible to create a “batch” method of dehumidifying or cooling the air. Perhaps you could freeze a large volume of water upstairs in the freezer, at night when it is cooler and the windows are open, then bring it downstairs to allow water vapor to condense upon it. Or, using a large container of desiccant and drying it in the oven upstairs. Of course, when you consider the manual nature of these, it’s probably a more efficient use of your time and money to just buy a dehumidifier and live with the temperature rise, or, buy a window A/C unit.
I know changing it from liquid to vapor (i.e. boiling) requires a lot of heat to accomplish, so I’d think that doing that phase change in reverse would liberate a lot of heat. ISTR reading something about it actually getting a little bit warmer during snowfalls for the same reason- changing the tiny water droplets to snowflakes liberates heat.
Anyway, maybe to some degree that’s what’s going on with the dehumidifier, considering that it doesn’t act like an air conditioner.
You could also buy one of those portable air conditioners with the large hose that expels the hot air outside through a window or vent hole. You would need to have a window or vent hole for the hose.
There are two sources for the heat that is being put out by a dehumidifier:
Latent heat. This is the energy that is recovered when water vapor is condensed into a liquid. If you live in a really humid climate, the majority of the work done by your home’s air conditioner is probably being used to remove moisture from the air, with a little bit of extra work being done to lower the temperature.
The work used to drive the refrigeration cycle. The laws of thermodynamics require that you expend energy in order to move heat from a cold place to a warm place. So when the DH moves that latent heat out of the water vapor to condense it into a liquid, it is sucking electrical power from your outlet to make that happen. All of that electrical power gets dumped out in the condenser along with the latent heat from the water vapor.
The only way around this is to use a desiccant dehumidifier. These are pellets of material that suck moisture right out of the air. But as noted upthread, these have a finite capacity for absorption; once they’re full loaded, you generally have to bake them at 300-400F to dry them out so you can reuse them. So they’re not really practical for continuous residential dehumidification.
You can duct the hot air from the DH to the great outdoors as kayaker suggests, but that will draw fresh air into your house through all of the cracks and crevices. If it’s really humid outside, this may be counterproductive.
The only way to really nail this thing shut is to physically separate the evaporator and condenser coils, which is what an air conditioner does. If your basement has a large enough window, a small window-based air conditioning unit may give you the results you’re after: cool, dry air in the basement, with heat being rejected to the great outdoors.
Odd, the air that my basement dehumidifier blows out is only slightly warmer than the ambient temp. Maybe you just need a newer, more efficient model. It works well too, I keep the humidity to 65% and have a full tank of water to dump on my plants every 3 or 4 days at the peak of summer.
The energy expelled is linearly proportional to air flowrate and temperature (among other things). So, two humidifiers could have the same power draw; one blows out a large amount of slightly warm air, the other blows out a small amount of very hot air.
It’s the principle as why a hair blow dryer could draw the same power as a toaster oven, even though one clearly gets hotter.
Same here. Perhaps my basement is way bigger than the OP’s (maybe like 1000 sq ft) but my dehumidifier (circa 2008 or so) is not bringing the temperature up in any meaningful way. Maybe you need to move it off to the side a bit?
I will try to vent the hot air flowing out the back of my unit to the outside. But then again, I might just as well buy the portable AC unit from Lowe’s if I’m going to have to mess with modifying a basement window for venting purposes.
I do have a rather small basement, so that’s why the temperature increase is so dramatic.
For now, I’m going to re-position the unit and aim the venting air up the basement steps.
I’m not an engineer, but it seems like there are at least three methods other than desiccants and refrigeration that would work.
Assuming that water is more liable to stick to other water than it is to stick to air, you could “wash” the air out by spraying water around a bunch, then flushing the water out quickly, before it could re-evaporate into the space.
Assuming that water less able to flow than air, if you circulate the air through a complex structure (like a long tube with lots of metal meshes through its length), I would expect the moisture to have a harder time making it through the structure than the air, and so it would peal off some of the water every time it passed through.
Convert your room into a giant centrifuge and use that to separate the water from the gas.
Are these wrong? I’ll grant that they might not form the foundation of a practical solution, but there do seem to be theoretical alternatives to those stated.
It is not. At room temperature, any collection of water molecules exhibit a distribution of thermal energy, i.e. some of them are hotter/colder than others. For a group of water molecules in liquid form, the hottest among them tend to spontaneously convert to vapor; conversely, for a group of water molecules in vapor form, the coldest among them tend to spontaneously condense into liquid.
When these two processes (evaporation/condensation) are in equilibrium, the air is said to be at 100% relative humidity, i.e. the amount of water vapor is as great as can possibly be. If any more liquid water evaporates, the rate at which vaporous water condenses will increase to offset it. Conversely, if the air is at less than 100% relative humidity and is exposed to some liquid water, the liquid will tend to evaporate faster than vapor will condense. So liquid water at room temperature is not a good desiccant.
Water vapor flows just as well as atmospheric air; it won’t condense onto flow obstructions.
If you want to separate water vapor from air, you either need to cool the mixture until it hits 100% relative humidity (and starts condensing the water out), or you need to expose it to a chemical that has a high affinity for water molecules (i.e. a desiccant).
I uh…oh, wait. Gas centrifuges have been used to separate gases containing two different isotopes of uranium. Your idea of a centrifuge is likely to be entirely impractical, but I wonder if the theory is sound.
Can a gas centrifuge be used to separate gases of differing molecular weights in the same manner as it’s used to separate gases containing multiple isotopes of one element?
I found this too, and installed a small air conditioner in one of my basement windows. Every once in a while, I turn off the dehumidifier and turn on the AC to help cool the place off during the hottest days.
This would be useless. A dehumidifier has one air intake and *one *air flow. The outflow contains the dried-out but warmer air. If you vent the outflow out of the basement then you never introduce drier air into the basement.
The reason an air conditioner vents to the outside is that it has *two *air flows. One flow goes over the condenser coils to cool down the heated refrigerant, and this air is vented to the outside. The other flow goes over the evaporator coils to make the air cool, which goes indoors.
I have nothing meaningful to add, so I’ll ramble along… but IF we could get stratification of different gasses from a mixture of dry air + water vapor, I’d think the air would begin to separate into its various constituents as well (nitrogen, oxygen, etc…). Also, what would happen to the water vapor? Would it separate into hydrogen and oxygen, or turn into steam, or condense…? If it did condense, introducing liquid loading inside of a room sized centrifuge spinning at 90K RPM would lead to a catastrophe that could launch OP’s couch onto someone’s lawn several states over!
FWIW, for a fixed DH efficiency and assuming a “well mixed” volume, the size of your basement shouldn’t have an effect on the temperature increase of your basement, since the energy consumption of your dehumidifier is directly proportional to the volume of the basement. Smaller basement = smaller amount of heat/smaller volume and vice versa for a large basement. The “smallness” or “largeness” falls out of the equation and you end up with a temperature increase regardless of whether the volume of your basement is 100 or 1000 sqft.
In reality, there are secondary effects that could make practical experience inconsistent with theoretical expectations; the operating efficiencies of even the same unit can be different (cycling on/off, dirty filters/coils…), basements are not always well mixed so there’s a gradient of temperatures, air volume-to-wall (which acts as a heat sink) ratios, etc…