Lowering relative humidity in a room

A student living in a 1 bedroom apartment (about 750 square feet) wants to lower the RH in his room from the current 75% to a more comfortable 50-60% range. Indoor temperature is about 28C/82F, outdoor conditions are about the same temperature with RH in the high 90’s during the rainy season. His idea is to freeze 2-litre water bottles and place them in a pan in front of a fan to condense and collect water, acting like a crude dehumidifier. Of course all windows will be closed.

I suppose in theory this would work but how (in)efficient would this be? How many frozen bottles would it take to reduce RH 10%? How much humidity can one bottle reduce? The thermal and electrical cost of freezing the water bottles can be ignored for now.

BTW, he has an air conditioner but doesn’t want to use it for health and cost reasons.

I can run the numbers for you if you provide some more information :

  1. How much is the heat loss (insulation losses) in the room ? Most stored heat in a room is not in the air; but in the floor, the walls, furniture …

  2. If the frozen ice won’t change the temperature of the room (poor insulation or high heat capacity of floor ) then the RH in the room will increase from all the water evaporating once the ice melts.

  3. I’d the freezer (refrigerator) is in the room then it would end up heating the room and thereby evaporating more water I.e. higher humidity.

As the term implies, relative humidity is the measure of moisture in the air relative to its maximum moisture capacity. As temperature of the air increases, so too does its moisture capacity.

Using a psychrometric calculator, we find that at 82F and 75% RH, the specific humidity is 124 grains per pound (that’s grains of water per pound of air). For a 10% RH reduction (to 65% RH) we need a decrease of about 17 grains per pound.

There are 7000 grains in a pound, and each pound of air might occupy 12-13 cubic feet. A 750 sf apartment with 8 ft ceilings is about 6000 cf, so we guess there’s about 480 lbs of air in the apartment. For an overall reduction of 10% RH, we need to remove a total of about 1.2 lbs, or a little over 1 pint of water. This is all napkin math so take it with a grain of salt.

Whether or not that’s doable with a frozen-bottle dehumidifier (w/fan), I don’t know. It’ll be a lot of energy and diligence to keep it functioning. I’m concerned the condensate will quickly re-evaporate into the air. It’ll also be a constant effort, as moisture from outside will constantly re-enter the apartment to equalize the humidity.

As a comparison, most regular dehumidifiers can remove several pints of water each day and can achieve 10% RH reductions pretty reliably. The ones I’ve worked with in the past have had the tendency to heat up the air over time due to the nature of their operation.

You can use rock salt or calcium chloride. Get two buckets (those plastic pails Home depot should work. Drill holes in the lower half of one bucket and place it into the second bucket. Pour the salt or calcium in the inner bucket. The salts will absorb water out of the air and the excess will drip into the outer bucket which can be emptied as needed. There are also commercial bags of desiccant meant to do this but they would be more expensive.

Personally I think it is time to buy a $110 cheapo air conditioner at Walmart. Or a used one off Craigslist.

Thanks for the calculation. At least we now have a ballpark figure of how much water it takes to reduce RH. Now we have to figure out how many frozen bottles is going to achieve that. A couple I can see being worthwhile, but not if you need dozens of them.

The only energy needed to keep this running is a low-wattage fan, apart from the energy needed to freeze the bottles that is. I’m not too worried about the condensate evaporating that quickly as long as he regularly dumps the collected water. I don’t know how draughty his room is but modern homes are well-sealed and I know in my case that if I shut the windows, the RH doesn’t rise (or lower) by any significant amount.

Yes I know about calcium chloride, thank you. I’m not really looking for a dehumidifying solution. A work colleague was talking about his son trying something which I just kind of laughed off and then started wondering if it would work. I thought the theory was sound. That’s all.

For me, living in a sweat-box at 28C would be far more damaging to my health and welfare than running ab air con. At the very least, I would have a fan to evaporate the sweat on my body.

The thing that suck about a room that size is you can’t even run an actual dehumidifier unless you’re willing to run an exhaust hose out the window.

Dehumidifiers are basically air conditioners that dump their waste heat back into the air that they just dehumidified. They’re handy when you want to take moisture out of a space without cooling it (e.g. damp basements). If you’ve got a living space that’s already 82F, you might as well reap the benefits of cooling along that dehumidifying action - IOW, don’t get a dehumidifier, get an AC.

As noted upthread, the OP’s idea to freeze bottles of water and use them as water-magnets is basically a dehumidifier, since the freezer will be dumping its waste heat into the room. That is, unless the freezer is actually outside the room, in which case this scheme would be functioning as an air conditioner.

OK, so we need to take a pint of water out of the air? For simplicity, let’s only consider the dehumidifying potential due to the melting of our ice bottles, and not the dehumidifying potental due to their eventual warming (after they’ve melted completely).

Heat of fusion of water is 334 joules per gram. That is, if you want to melt a gram of ice, you need to deliver 334 joules of heat to it.

Heat of vaporization of water is 2230 joules per gram. That is, if you want to condense a gram of water vapor, you need to pull 2230 joules out of it.

So to condense a pint of water out of the air, you’ll need to enough ice to melt into 1* 2230/334 = 6.7 pints of liquid water. Ice is about 90% as dense as liquid water, so you’ll need to start with about 7.4 pints of ice.

This disregards several factors:

-the infiltration of humid air into the residence from the great outdoors

-the exhaled moisture and evaporated sweat from occupants

-evaporation from toilets and from whatever vessel is collecting the condensate from the ice bottles

This will be tough to do. Unless you put out many bottles of ice at once, you’re not likely to get the RH from 75% down to 65%. Although if you try, you might actually cool the place down. But your refrigerator will be drawing a lot of electrical energy, and you’ll be very busy; at that point, your life will be simpler if you just turn on the AC instead.

In my own experience with dehumidifying it takes a long time to initially get the RH to start moving down. I have assumes that the water in the air has come into a equilibrium with the water in the sheetrock and wood and other substances that can hold water. As the air is dried water comes from them back into the air. So it is a drying out of those materials that take so long. Once dried out and the RH is allowed to go back up, it is much quicker to get it back down.

So even if your son’s method may work, it will be during that initial phase where one may not see results for quite some time. Adding that his method is much lower in capacity then a standard dehumidifier he may not see results on RH even if he gets the puddle.

I’ve been in the same boat. As I’m sure you know, A/C units have settings. Blasting the unit at even the medium level will be costly, but a unit set to minimum will not necessarily make the room “comfortable”, but it will allow you to remain in the room with some comfort and not break the bank. If, at the end of your electric billing period, you find you can’t afford even using the A/C at the minimum setting, moving might be the next step.

This is great, thank you. 7.4 pints of ice can be made with two 2-litre bottles and not the dozens I imagined it would take. It’s somewhere to start. I’ll tell the father what I got from this thread and see if it works.

Given that this system operates pretty much the same as a commercially available dehumidifier (but with considerably more hassle and no less electricity cost), can you explain why it’s preferable?

Moreover, given that an air conditioner not only dehumidifies, but has the added benefit of actually cooling the place down, why is the AC not preferable? What are the health reasons you mentioned?

The father mentioned not wanting to use the air conditioner to save electricity and not liking the dry stale air. Whether he has allergies or asthma, I don’t know. But even without any underlying condition I can understand his dislike of air conditioning. If it wasn’t for my wife or company coming over, I wouldn’t use it as much. I’d much prefer to open the windows for fresh ventilation and put up with some heat. I find air conditioning to be too artificially cold and dehydrating. And as far as I know, he doesn’t have a dedicated dehumidifier and thus this idea to use frozen water bottles.

You will use more electricity making ice.

If you hate air conditioning (I.e. cooling the air), then look into buying a dessicant dehumidifier. They are in the $150 to $250 range. Instead of the ice bottle, you will have something like silica gel that will absorb the moisture. The silica gel is regenerated at night.

Technically, reducing the humidity by any means is still air conditioning.

I have found the opposite, that humidity drops very quickly when the A/C is turned on. But I am starting with a room that is usually dehumidified as you suggest.

Actual data point: Last night it was not real hot but somewhat humid in the house. So I set the timer on the bedroom A/C to come on 30 minutes before I went to bed. The 400 square foot room started out at 76 F and 57% RH. When I got to bed after 35 minutes of run time the room was 73F and 40% Rh.

So to the OP I would suggest a short run of the A/C is going to do way more than a bunch of frozen bottles of water. Anyway, I hope you keep us posted if they actually try the method.

Would OP consider a de-humidifier? Dries the air without resorting to ice, and raises the temperature in the room. Both effects, heating and drying, lower the RH.

I guess it’s been a few days, but I’m guessing the two 2-liter bottles didn’t work? I would think anybody who’s lived in the South, especially in the pre-A/C days, could tell you this.

The assumption given was that no outside (hence very humid) air would get in, which is a virtual impossibility for a room intended for habitation. Also, it’ll help in the immediate vicinity of the bottles but the air won’t be perfectly mixed from the start, i.e. it’ll be cooler/drier near the bottles but likely to still be hot/humid away from them. You’d want a fan to circulate that dry air from the bottles around the room, and then all you’ve done is made yourself an inefficient A/C system.

2 two litre bottles are not going to have much surface area, therefore little heat transfer.

In the spirit of what was asked, vs what was probably meant…
The simplest way to reduce the relative humidity in a room is to raise the temperature.
What the OP seems to really want to ask is how to lower Absolute Humidity.