Is there a way to keep bathwater from cooling?
February 5, 2014 2:44 PM   Subscribe

I thought for sure there would be a device that would keep bath water warm while you're soaking in it since it seems like such a simple thing. All I can find so far however are little warmers for aquariums to keep the fish swimming in nice warm water. Why not something like this for humans? Am I just not using the right search terms? Even just a heating mat that can go under water would be fine. I could just lay down on it to keep it from floating up.
posted by manderin to Home & Garden (40 answers total) 15 users marked this as a favorite
Hmmm. What's your budget here? You can buy a heated bathtub.
posted by Lutoslawski at 2:45 PM on February 5, 2014

Can you just keep hot water trickling?
posted by rabbitrabbit at 2:46 PM on February 5, 2014 [1 favorite]

My answer to this has been to let a little lukewarm water drain and replace it with new hot water. Not the most environmentally friendly approach, I suppose, but no worse than buying some gadget.
posted by Sara C. at 2:49 PM on February 5, 2014 [2 favorites]

I think that the traditional method is to add more hot water. A heating mat (presumably with an electric cord?) would seem to be fairly problematic if something went wrong and electrocuted the bather.
posted by ActingTheGoat at 2:50 PM on February 5, 2014

My problem with using hot water from the tap (I haven't tried keeping it trickling yet) is that there's always cold water to get through before the hot water kicks in, so I end up with a net colder tub. I use my electric kettle to heat more water but this isn't really optimal.

(If I were to ask this question I'd add the criteria cheapass and rental-bathtub-compatible.)
posted by asperity at 2:54 PM on February 5, 2014 [4 favorites]

I use Sara C.'s technique. If you really must conserve water but not electricity, you could use an immersion heater to get the water back up to temperature. But not while you're in the tub!
posted by payoto at 2:55 PM on February 5, 2014

I have utterly no experience with this, but this UK company offers a solution:
Also, not sure where you could purchase it or how difficult installation would be.
posted by piyushnz at 2:56 PM on February 5, 2014

A layer of ping-pong balls would keep water from evaporating. I wonder if it would insulate a bit as well.
posted by supercres at 2:59 PM on February 5, 2014 [15 favorites]

asperity - if you have a shower attachment you can use it to channel the cold water directly to the plughole until it starts coming through hot.

Also, using the shower to warm up the bath means you get better redistribution of warm water than using the tap - you can direct where it goes.
posted by Skaramoosh at 3:04 PM on February 5, 2014 [4 favorites]

Bubble bath! The bubbles have a slight insulating effect that slows down the cooling.
posted by His thoughts were red thoughts at 3:07 PM on February 5, 2014 [5 favorites]

Response by poster: I never quite thought of bathing in ping pongs. Sounds like fun :) but I don't think it's worth cleaning up ping pongs afterwards.

The problem with the 'adding more hot water' technique is that it requires that I either get out of the tub or at the very least get some grand flexibility in my toes to turn the faucet on and off at the other end. Not very relaxing. And in the winter I feel that the water cools down so quickly that it's barely worth doing the bath.

It's hard for me to believe that people have invented things to keep fish warm in the water, but not humans. I wonder if I should just buy a couple of aquarium warmers and sit in the bath with them. If the fish don't die I don't see why I should.
posted by manderin at 3:12 PM on February 5, 2014 [1 favorite]

It's not particularly cheap but there are a new generation of immersion circulators that would work nicely for this and are in the $200 range. They should be safe around water if they have their proper listings. It would make more financial sense if you also got into sous vide cooking at the same time.

A crappy little CVS immersion heater would also heat the water but with no temperature control and with much less safety. In the middle ground, people on Amazon are talking about this guy for bath water:

The aquarium warmers have thermostats and are probably generally not designed to get much over 80 or 90.

These items are designed to be moderately safe around water but I am not a safety engineer and I am not your safety engineer. One failure mode I can think of right off the bat is if you fall asleep and the tub is overheated and burns you badly and I'm sure there are others.
posted by ftm at 3:13 PM on February 5, 2014 [2 favorites]

These Warm On pebbles are only prototypes, but they may be for sale in the future.

For the analog version, possible to heat some large stones and add them to the bath? I would also consider covering the tub with some sort of insulator, like maybe a foil emergency blanket or a blow-up mattress resting on the water.
posted by mochapickle at 3:21 PM on February 5, 2014 [1 favorite]

Another "lifehack" to make winter baths nicer: use a robe before and after.

If there's a heating grate or hot pipe in your bathroom, hang the robe there, and you have a nice warm robe to put on after your bath.
posted by Sara C. at 3:22 PM on February 5, 2014 [2 favorites]

This should do it. We had them in the bathtub in Japan.
posted by mukade at 3:30 PM on February 5, 2014 [2 favorites]

I found this "portable bath heater" online -Eco Poka Bath -which makes me think, wouldn't a hot water bottle or similar have the same effect?

I believe for much more money you can buy a heater that can be installed in your home, if that's an option.
posted by sm1tten at 3:33 PM on February 5, 2014

Not exactly a water heating solution, but if the bath is cast iron, or enamel, it will cool down a lot faster than a plastic tub. A trick is to swish the hottest water from the tap around the bath first and then fill it up. If the bath itself is hot to start with, it will keep the water warmer for longer.
posted by Youremyworld at 3:34 PM on February 5, 2014 [3 favorites]

I'm seconding bubbles. Keeps the bath warm waaay longer.
posted by asavage at 3:39 PM on February 5, 2014 [1 favorite]

What you need is an immersion heater. This looks like it would do the trick: A "Bucket Heater With Guard, 1000-Watts" First reviewer used it for a home made hot-tub.
posted by cosmac at 3:43 PM on February 5, 2014

If you want to make the tub quieter and retain some heat stuff blow insulation in around the base. My neighbor who is a plumber tells me it is lovely.
posted by munchingzombie at 3:44 PM on February 5, 2014 [1 favorite]

If your tub is not plastic, a workaround is to boil a kettle of water (yes, boil!) and put that in the tub with your hot water. It's often enough to bring the temperature up high enough that it doesn't cool off as fast. (Just one kettle, be careful, don't burn yourself, etc.)

But I have to say, the idea of a bath that doesn't cool down or have to start off ridiculously hot is really alluring. Watching this thread with interest.
posted by purple_bird at 3:47 PM on February 5, 2014 [1 favorite]

This should do it. We had them in the bathtub in Japan.

There's a lot of stuff on the numerous webstores at, e.g.

For Japanese baths, you wash off the grime using a showerhose and then soak in cleanish water that more than one family member might have used that day. You may want to install a Japanese-style bath when refurbishing a home.
posted by sebastienbailard at 3:59 PM on February 5, 2014

I can tell you from personal experience that immersion heaters all fail eventually and start shocking you. Generally mildly at first, luckily. Used them for years in livestock tanks.

If you ever see a bunch of horses or cattle clustered around their water tank looking alarmed and not drinking that's probably why. It perturbs them greatly when the water starts to tingle!
posted by fshgrl at 4:09 PM on February 5, 2014 [10 favorites]

at the very least get some grand flexibility in my toes to turn the faucet on and off at the other end.

This is called being aquadextrous, and it's a crucial skill for any bather.
posted by rhizome at 4:15 PM on February 5, 2014 [7 favorites]

I have one of these. Used it for years. Works like a champ.

On preview, I'm not sure that this is the model I own, but the one I have heats up the water to the point where it gets uncomfortable.
posted by nedpwolf at 4:44 PM on February 5, 2014 [1 favorite]

Another vote for pre-heating the tub with boiling water, like you were making people tea, and then a cartoonish layer of bubbles for primo insulating properties.
posted by Iteki at 5:06 PM on February 5, 2014 [2 favorites]

I used to know someone who didn't like showers. She was from the deep South, so winter-cold tubs were a new thing to her. Add in small apartment water heaters, and my drinking buddy had a problem on her hands.

The best solution I came up with was to figure out how long it took to recharge ten gallons in her water heater (based on BTU), and applying ten gallons of straight-hot water to the tub that much time in advance. The heat of that ten gallons goes into the tub itself, mostly. Once you wait the required time for the water heater to recharge, you can fill the tub with no fear of the cast-iron tub itself robbing you of precious warmth.
posted by notsnot at 5:10 PM on February 5, 2014 [1 favorite]

I have an inadequate water heater, a porcelain tub on the outside wall of my uninsulated house, and on the other side of that wall air that is currently 1 fucking degree. I start by putting about two inches of pure hot water in the tub and letting that sit for about twenty minutes to get the tub itself warm and give the water heater time to recoup the hot water. Then fill it up, and when it starts cooling off, take the shower wand, turn it on hot--and here's the brilliant part--run the cold water that comes out first straight into the toilet. Ha.
posted by HotToddy at 5:15 PM on February 5, 2014 [5 favorites]

My understanding is that Epsom salts are used for soaking sore muscles because they keep the water warmer longer.
posted by jgirl at 5:50 PM on February 5, 2014 [1 favorite]

I have found that if you have access to the tub from the bottom a little insulation goes a long way.

Not all tubs are set up so you can get at the area around and under them, though - and that void under there is usually not heated directly, so it's cooler than the bathroom air.
posted by Pogo_Fuzzybutt at 5:56 PM on February 5, 2014

tea light candle under the tub
posted by hortense at 6:12 PM on February 5, 2014

Buy or make a Japanese furo tub that fits in your existing tub. It has less surface area and retains heat longer:

Build your own furo tub
posted by mjklin at 6:35 PM on February 5, 2014 [1 favorite]

As pointed out, nothing electrical except hot tub heaters are going to be safe for immersion use.

What you can do is heat the outside of the tub. Pipe heating tape could work. Or a couple electric heaters blowing on the sides?

Make sure you have RCD/GFCI outlets before you plug anything electrical into the bathroom!

Another idea is to insulate the tub really well and put something insulating over the top, like hot tub covers, with enough space for your head to stick out.
posted by flimflam at 6:38 PM on February 5, 2014

1. Insulate from underneath if you have access.

2. Attach bubble-foil insulation to the outside (like the stuff used in attics).

3. Cut a sheet of thick blue styrofoam like that used in construction to cover the top of the bath, leaving just an opening for your head.

I had an outdoor metal tub and used styrofoam board underneath and on top and wrapped the sides with bubble-wrap. Not pretty but toasty!
posted by ceiba at 6:58 PM on February 5, 2014

Not sure how major you want to get with this, but here's my story of the warmer bathtub:

Our tub sits on the outside wall and was always colder than brass monkeys. DH pulled the siding off the outside of the house, and replaced the old fiberglass insulation with heavy duty insulating sheets. He then drilled through the basement flooring and foamed in all the way around the bottom of the tub. Absolutely amazing change in temperature. If you have a cast iron tub, you can build a nice looking box around it with insulation inside.

My grandmother had an old tub stored up in her barn that was 3/4 covered over the top--apparently that was to hold the warmth in as well as for modesty. So covering might work.

I agree with the poster above--if you have a heavy cast iron tub, you can pre-heat it. Fiberglass, not so much.
posted by BlueHorse at 7:37 PM on February 5, 2014

This is likely just not going to work for you but I thought I'd throw it out there. At my mom's house, she has radiant flooring in her daylight basement. There's a nice, big tub down there and the radiant flooring goes right underneath which means the bottom of the tub is always nice and warm and it pretty much stays that way. It might be insulated, too, I don't know. The cat loved to sleep in there.
posted by amanda at 10:03 PM on February 5, 2014

Same technique as for keeping coffee in the mug hot. Heat the container first by adding a couple inches of the hottest water. When that's cooled (i.e. transferred its heat to the tub) then drain, add your hot bathwater.
posted by fingersandtoes at 10:28 PM on February 5, 2014

I would go with a lab grade immersion circulating water bath heater like this one. It wouldn't be cheap and while it could warm yourbath from cold to warm it would take quite a while. It is designed for keeping any bath at a set temperature with about a 5C window of temperature. Essentially you draw your bath as normal and then put the heater in and turn it on. I would use a GFI to be extra safe about electrocution, but these are pretty robust and should survive immersion. Then you can set it to whatever temperature you want it to stay and it should keep it at that temperature. It is quite expensive, but it should work and also be safe.


My understanding is that Epsom salts are used for soaking sore muscles because they keep the water warmer longer.

I have no idea how that could possibly work, other than raising the bioling point (aka vapour pressure due to dissolved solute) but in order for that to have an appreciable effect you would need kilograms of the salts per bath.
posted by koolkat at 5:45 AM on February 6, 2014

A (bathroom rated) space heater in the bathroom makes a significant difference.

A hot toddy of some sort or a spiked hot coco also works wonders.

Good luck. It is very sad that we can put a rover on Mars and yet we have to freeze in our tubs like a common animal!
posted by Lesser Shrew at 7:03 AM on February 6, 2014 [3 favorites]

It's hard for me to believe that people have invented things to keep fish warm in the water, but not humans. I wonder if I should just buy a couple of aquarium warmers and sit in the bath with them. If the fish don't die I don't see why I should.

Please don't do this. If there is a failure, it could kill you. The risk may be small, but dying is probably a more significant event than the loss of some fish.

There are very good reasons for the lack of electrical devices for keeping human-occupied water warm.
posted by HiroProtagonist at 4:58 PM on February 6, 2014

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