How can I keep my bath warm for longer periods of time?
March 9, 2013 4:21 PM   Subscribe

- I live in an apartment and the water heater does not give an entire 40g of hot water [the capacity of the water heater and approximate capacity of the tub]. - I would like to relax in the bath for at least 20 minutes or more. - I've tried boiling water, aquarium heater [fail, takes to long to heat], basalt stones [too small to matter]. Do you have any ideas for lengthening my bath time? My water goes lukewarm after ten minutes, almost chilly.
posted by squirbel to Home & Garden (23 answers total) 7 users marked this as a favorite
Best answer: This happened at our house, and it turned out the tank has two separate heaters - one to keep the water at the top warm, and one at the bottom - and the top one was out. That's why we'd have no hot water for anything else after running the washer or the dishwasher or taking a shower. Call someone in to look at your tank/heater.
posted by peagood at 4:29 PM on March 9, 2013 [2 favorites]

Is the bathroom cold in general? You might try either upping the thermostat for bath time if it heats the bathroom or getting a space heater. Of course position the space heater far away from where it might get wet, blah blah. You could also try draping a towel or 2 over the tub where your legs are. If you have one of those old claw foot tubs, you can try wrapping the whole thing in a blanket.
posted by tealcake at 4:34 PM on March 9, 2013

Hot water heaters fill with gunk and sediment over time and don't really fill. Also, hot water heaters have thermostats themselves. Have the landlord check its age and/or turn it up hotter.
posted by Cool Papa Bell at 4:40 PM on March 9, 2013 [2 favorites]

Response by poster: It isn't the air that is cold [the vent blows right into the bath area], but the water. I am going to try a bucket immersion heater, and boil the water before I get in, lol. It'll hopefully last or cook me. :)
posted by squirbel at 4:41 PM on March 9, 2013

You can also check the temperature that your hot water heater is reaching. Because you are almost always mixing hot water with cold water to get the temp you want for a bath. Unless your water heater is set too cold.

If you are using every drop of the "hot" water in the tank to draw a bath, I'm willing to bet that your temperature is either set below 130 or, as mentioned above, one of the heaters for the tank has failed.

But my money is on the thermostat, because it's a common adjustment people make to save money.

If you own the property, look into hiring an electrician to install a wall switch for your hot water heater. Because (as acquaintances taught me) you only really need hot water for laundry, bathing and washing dishes. And you can schedule these things. (the way it worked for them was they woke up, flipped the water switch, took the dogs out, cooked breakfast, watched some tv, whatever, and then they used their water before turning the water heater off. In the evenings, get home from work, turn on the switch, make dinner, do homework, etc, use the water, flipping the switch again before bedtime.)
posted by bilabial at 4:50 PM on March 9, 2013 [3 favorites]

Also you lose a lot of heat to warm up the cold bathtub when you first fill it. So if you can heat up the tub first, the water will stay warmer. You can use a hairdryer for this. Blow it all over the bathtub until the whole surface is warm to the touch, and THEN fill it with hot water.
posted by lollusc at 5:16 PM on March 9, 2013 [9 favorites]

I just add more hot water periodically (and drain out some of the cold). The water heater should be heating new warm water for you while you soak. Have you tried that?
posted by Jahaza at 5:34 PM on March 9, 2013 [1 favorite]

I used to put two large pots of water on the stove, like 10 quart (about 10 liter) capacity, and bring to a boil (lids help speed this process). Pour them each into the stoppered tub, pouring it over as much of the surface as you can to warm the tub up. Then run your bath water. Keep doors and shower curtains closed to retain heat. It makes a difference.
posted by Michele in California at 5:36 PM on March 9, 2013 [2 favorites]

Best answer: it turned out the tank has two separate heaters - one to keep the water at the top warm, and one at the bottom

In the temperature range you'd find inside a domestic water heater, warming water lowers its density. Water is also quite a poor conductor of heat, so provided there is minimal turbulence inside a tank, warming water will make it rise and form a stable layer that floats on the cooler water below.

So when a heater element is active, there will be a plume of warmed water rising from it, and that water will keep rising until it hits either the top of the tank, or the bottom of the warm water layer above.

When the boundary between the warm layer and the cool layer gets pushed down below the heater element, the element starts to reheat water that's already been heated and a new warmer-over-warm boundary layer forms near the top of the tank. The water below the heater element remains almost completely undisturbed, and the new boundary layer makes its way down toward the heater element. This process continues, making the temperature at the top of the tank rise roughly stepwise, until the thermostat detects that the heater element, and therefore the bottom of the warmed layer above it, has reached the desired holding temperature.

When you open the hot water tap, hot water is drawn off from the top of the tank, to be replaced by cold water entering at the bottom. There is a diffuser on the bottom inlet to discourage swirling and mixing, so the tank will maintain a fairly well-defined boundary between cold water below and hot water above until almost all the hot water has been drawn off and the hot water "goes cold".

Tanks with two heaters typically have the bottom one wired via a timer and the top one wired directly to the mains. The bottom heater runs overnight, to heat the entire tank using cheap off-peak electricity. The top heater only comes on when the boundary layer between hot and cold water rises above it, typically meaning that well over half the tank's hot water has been drawn off.

The top heater won't heat the whole tank, only the water above it; when the top heater's thermostat switches it off, there will be a hot-on-cold boundary layer just below it. The top heater's thermostat is also typically set a fair bit cooler than the bottom heater's, to make sure that the peak-electricity booster heater doesn't operate while there is still plenty of off-peak-heated water available in the tank.

If your hot water consistently runs out well before you've drawn off the tank's expected capacity, it may well be that the bottom heater has failed - in which case your hot water service is now not only operating at much reduced effective capacity, but is doing all its heating with relatively expensive peak electricity. You might also be able to provoke this behaviour by setting the bottom heater's thermostat (which may be the only adjustable one) lower than the top heater's, in what turns out to be a counter-productive attempt to save electricity.

If the top heater fails, you should still get your full 40 gallons of hot water but that would be it for the rest of the day; to get more, you'd need to wait until the tank had heated up again overnight. The "hot" water would also be exactly as cold as the cold water in this scenario, because there would not be an active top element struggling to make it even lukewarm.

Because the bottom heater does most of the heating by design, it's the one most likely to fail.

Getting your water heater running properly is by far the least expensive way to get your bath the way you like it.
posted by flabdablet at 5:42 PM on March 9, 2013 [17 favorites]

I've noticed that the empty tub is cold, and pulls a lot of heat out of the water. My workaround has been to run pure hot hot hot water into the tub, fill it about four inches high, and let that sit for a few minutes. I brush my teeth, light some candles, etc. Then I fill the tub up with regular temperature water. This extends the warmth of the bath a good 20 minutes.
posted by ambrosia at 5:42 PM on March 9, 2013 [4 favorites]

I boil the tea kettle and pour it in the tub first, which helps some.

We have a clawfoot tub. If you have one of those, you can put sterno or something under your knee section, or wherever your body isn't touching the tub. Candles work but they're a little wimpy. Alcohol works pretty well- not as scorching as sterno, not as cool as candles.
posted by small_ruminant at 5:44 PM on March 9, 2013

Make sure your clawfoot tub still has the original enamel finish if you're going to do violent things to its temperature like pouring in boiling water or putting flames underneath it for that cannibal-cookpot effect. If it's been refinished with two-pack or acrylic, wild temperature swings will damage the finish quite quickly.
posted by flabdablet at 5:47 PM on March 9, 2013 [2 favorites]

You probably have poor-to-no insulation around your bathtub. Since it's an apartment, there's probably nothing you can really do about it, either; if it were your own place, you could remove the "wall" covering the front of the tub and stuff insulation all around and under the tub and reinstall the "wall." I did this in a house I owned many years ago and it made a world of difference. If your bathtub is on an outside wall, that also makes it more difficult to keep water hot. It also might help if you make sure your hot water tank is wrapped in a hot-water-tank blanket (insulation again). Hope you get a nice, hot bath soon - it's hard to relax in chilly water.
posted by aryma at 5:59 PM on March 9, 2013 [4 favorites]

flabdablet: "Tanks with two heaters typically have the bottom one wired via a timer and the top one wired directly to the mains. The bottom heater runs overnight, to heat the entire tank using cheap off-peak electricity. The top heater only comes on when the boundary layer between hot and cold water rises above it, typically meaning that well over half the tank's hot water has been drawn off."

YMMV. Here we don't have differential electrical costs and I've never seen heating elements on a timer. Both elements come on whenever the thermostat calls for heat and the reason for two elements is for faster recovery.
posted by Mitheral at 6:35 PM on March 9, 2013 [2 favorites]

In Australia, where "renewables can't do baseload" is still considered a respectable argument for the perpetuation of the demand-side price incentives that keep our brown-coal power stations running overnight, the timer is typically installed in the meter box or, if the meter is one of the newer "smart" electronic types, incorporated in the meter itself. I've never seen a water heater with its own timer.
posted by flabdablet at 7:08 PM on March 9, 2013

Many tub/shower fixtures made in the last two decades have scald guards built in. The scald guard (also known as a limit stop) prevents the hot water faucet from turning all the way open. Most of these type of faucets also have user-adjustable settings, such as this video on how to adjust a Delta faucet. Try identifying your tub's fixture brand (it's usually stickered or stamped on there somewhere) and googling for a how-to for that specific brand.
posted by jamaro at 8:10 PM on March 9, 2013

Something else you'll find with relatively new plumbing is a tempering valve. This is a thermostatically controlled mixer placed just downstream from the hot water tank outlet, which pre-mixes the scalding-hot water (could be 65°C or more) from the heater with some cold water so that what comes out of the bathroom hot tap is never hotter than (typically) 50°C.

Lowering the thermostat on the tempering valve decreases the draw rate from the hot water tank for any given flow rate from the bathroom hot tap, which actually increases the apparent capacity of the heater. So if your bathwater is never really hot enough to begin with, cools down to lukewarm after ten minutes, and can't be made warm enough again by adding more hot water - as opposed to the water from the tap cooling to lukewarm after ten minutes - there might be nothing wrong with your heater; perhaps the tempering valve is just set too low.
posted by flabdablet at 9:52 PM on March 9, 2013

By the way, if electricity is cheap where you live, or if you have access to piped gas, and if the existing hot water service is on its last legs and due for replacement, perhaps you could sell your landlord on replacing it with a tankless type that heats water on demand. These used to be awesomely expensive to install and run, but efficient modern designs usually consume less energy for a given amount of hot water demand, depending on demand patterns. And you never, never run out of hot water.

Adding a tankless heater to the outlet of a solar-heated storage tank also works well: the tankless heater won't cut in at all as long as there is still hot water coming from the storage tank.
posted by flabdablet at 10:01 PM on March 9, 2013 [2 favorites]

Outside wall and no insulation were two things we fought until we remodeled. Prior, we'd get the bathroom good and warm with a space heater, boil a couple pots of water to get the water to lobste-boil temps, put a towel in the bottom of the tub, and then close the shower curtain. Left the space heater running until done (well away from all water, of course!) Then we had to open the window and keep the space heater on to dissipate the tropical moisture.
posted by BlueHorse at 1:23 AM on March 10, 2013

I run pure hot water until the tub is about two-thirds full, then run pure cold water and carefully stir it around. Seems to work better and last longer than just running the temperature I wanted. Also, put something metal (metal massage balls are a good size), brick or stone in the tub before you start running the hot water. Leave 'em in there throughout your bath. If you think you'll be in there a really long time, keep a bucket or pot of boiled water with stones or metal balls or whatever to drop in later.
posted by windykites at 3:28 AM on March 10, 2013 [1 favorite]

put something metal (metal massage balls are a good size), brick or stone in the tub before you start running the hot water. Leave 'em in there throughout your bath.

This will probably be counterproductive unless you preheat them in your oven, as the volumetric heat capacity of the brick or stone is lower than that of the water it displaces (see the volumetric column in this table of specific heat capacities).
posted by flabdablet at 5:41 AM on March 10, 2013

A simple solution: bubble bath. Swirl the water vigorously as the water enters the bathtub. You will create a thick layer of bubbles on the surface of the water, which will insulate the water from the cold air, helping the water retain its heat.
posted by TristanPK at 10:29 AM on March 10, 2013 [3 favorites]

Response by poster: To all - thank you for your responses.

I eventually purchased a bucket immersion heater and it works very well. I have to wait two hours to get in the tub, but the water is very hot!
posted by squirbel at 3:37 PM on March 25, 2013 [1 favorite]

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