This One Is For You Science Types

Back when I was in law school, I was really really poor. Like, poorer than I am now. Yes, really.

Anyway, one day, it must have been during the winter, I found myself reading a website with tips on how to save money on your energy bill (instead of reading cases like I was supposed to, I'm sure).

One of the tips was that, during the winter, if you take a bath, instead of just draining the tub when you get out, you should leave the water in the tub until it cools down to room temperature. This was followed with some science mumbo jumbo about it giving off the heat into your home, blah blah blah. (Gee, wonder why I couldn't finish that pre-med major?) And, it even said that this will add a "moist heat," as opposed to a "dry heat," so that it would not only heat your home, it could also replace the need for a humidifier. And it also said (yes, it gets even better), that you could even do this when taking a shower, by letting the tub fill when you take a shower, not just baths. Which, I think would be kind of icky.

This led to a lot of debate between the boyfriend and me. No doubt this debate began when he went into the bathroom, found the tub still full, and said, "Is the drain clogged or something?" And I responded, "No, I'm saving money, dammit!"

So, the first set of questions are (1) Has anyone ever heard of this before? (2) Does this sound like it could be real, scientifically or whatnot?

And, I understand that the effectiveness would vary depending on the size of your home, the location of the bathroom (whether it is in the center of the home or not), etc. And I'm not saying that it would be able to heat your entire home, but the point, I think, of the save-money-on-energy article, was that if you're paying to heat that water anyway, why let it go down the drain?

Now for the next part... assuming you believe this whole tub-effecting-the-temperature thing, what about during the summer?

Let's say that it's 90 degrees in my tiny little apartment. And I fill the bathtub (located almost in the center of the tiny apartment) with ice cold water. Maybe even a few pounds of ice. Is that going to cool off the apartment at all?

And, if not, where can I get one of those HUGE blocks of ice that I imagine they used to put in front of fans in the olden days (or, at least, in cartoons they did)?

I believe this calls for a blogpoll and a joke...

Knock Knock
Who's there?
Dwayne who?
Dwayne the tub, I'm dwoning!


  1. I think that filling your tub with ice and pointing a fan that blows air across the top of it would create a cooling effect in your casa (similair to a Swamp Cooler).

    Also, it would work great for storing booze during parties and betting to see how long your drunk friends can stay in the ice water...not that I've ever done that.

    So pre-med huh?

  2. Great tip: when it's really hot, spray water next to a fan and feel the cooling effect!

  3. I've filled the tub when I'm in a hotel and don't have my humidifier (to take care of dryness). Theoretically the hot water should also warm the air a bit, but I would doubt it makes a noticeable difference in the winter. Who takes baths anyway?

  4. The heating effect would be real, but I doubt it's very large, unless you take a lot of baths every day.

    The cooling effect is real too, but I'm pretty sure it's also small. I wouldn't spend any money to bring in ice if I were you.

    By the way, making the ice yourself might save money, but it won't help at all: A freezer cools stuff by pumping the heat out of the freezer compartment and into the coils on the back (feel 'em, they get hot). The coils will warm the apartment as much as the ice will cool it. This is why you can't permanently cool your apartment by leaving the 'fridge open, although a brief blast can feel real good on a hot day.

    What would work is a refrigerator that sticks out of your house, with the door open inside your house and the coils outside. Give or take a few parts, that's basically what an air conditioner is.

    Yes, I was a science geek, how did you guess?

  5. I agree with WP that the effect is likely to be small, unless you have a small apartment maybe, and depending on the location of your bathroom. Note, however, that leaving a bunch of water standing around could lead to the formation of mold. It thrives in warm, moist environments. My lease actually has a bunch of rules about avoiding mold. While it doesn't mention draining the tubs, it does tell me to run an exhaust fan to remove excess moisture after bathing, and to wipe up any spilled water.

    Also, my Mom likes to put a half-full pot of water on the stove, at a low boil, to work as a cheap humidifier. Just keep an eye on it to make sure it doesn't boil dry.

  6. If nothing else, if it's really hot in the summer and you have a tub filled with cold water and ice, I imagine standing in that water or dunking your hands and wrists for a couple of minutes in will help cool you down, at least temporary. When I was a kid, my grandmother always said I should run my wrists under cold water for one minute... that was enough time for all the blood in my body to get hit by the cold water, thus cooling me off. Who knows.

  7. Assuming 30 gal of water at a temperature which differs from that of your
    apartment, I calculate[1] about 2.2 kW-hours of energy. The cost of 1 kW-hour
    is probably around 5-15 cents, so your net gain per bath is about 12-33 cents.
    Split the difference and call it a quarter. If you take 30 baths per month,
    you save $7.50/month.

    [1] Q = mc Delta T

    Q = heat (joules, kW-hours, etc.)
    m = mass of water (kg, lbs, etc.)
    c = heat capacity of water, 1 btu/lb-degF
    Delta T = Temperature differential (deg C, F, etc.)

    water has a density of 1g/cc or 8.34 lbs/gallon

    Obviously, you have to spend money to heat the water from the tap water temperature,
    so you'll recover the heat as it cools down. A better way to increase energy efficiency,
    would be to integrate the hot water heater with airconditioning so that instead of dumping the
    heat to the outside, the the tap water be heated using the heat removed from your home.
    Unfortunately, that sort of thing would need to be implemented in a design of a utilities
    package as whole rather than in the way utilities for houses are usually conceived.

  8. Yes! Science! That is exactly what I wanted to hear!

  9. The big difference between the cold ice and the hot bath water is that the hot bathwater is a by-product and the ice is not, unless you are talking about leaving the ice in your glass after drinking your last whisky of the night, instead of washing it down the drain.