I want to build another shower in an already existing shower. The essence of this work is to be able to stay in warm water as long as possible, and not use a lot of water and energy. To use this shower, you need to close the main drain and instead open the drain to the water tank.

enter image description here

The capacity of the water container will be ~10 liters, and then this water will go in a circle like in a fountain. And the heating element will only reheat water that has cooled down a little.

I think that such a system should be quite effective. But I can't figure out what to do about water pollution. I will not be able to use soap or shampoo, because then replacement of filters will be needed very quickly, otherwise the water pump may break.

Can you suggest ways how to improve this system to get clean water, prevent damage water pump, and avoid replacement of filters too often?

Edit #1.

I made some improvements:

  • placed a drain under the tank, as @Ecnerwal advised, and instead of the top drain will be sink grid to make it convenient to open it and wash the tank
  • moved water heater into shower head

enter image description here

It looks so much better! It remains to figure out how to reliably protect the water pump from contamination in such a way that the filter can be easily cleaned, it would be ideal.

Are there any ideas what can be easy-to-clean filter?

  • This is a hypothetical question, that product does not exist. As you said the way it is designed it will automatically lead to high level of contamination. They make point of use shower heaters, that are mounted in the shower head
    – Traveler
    Commented Jan 24, 2023 at 19:02
  • 1
    This question is unanswerable in its current form. Obviously you would have some kind of filter, at least a screen, in the floor drain to the tank. That will catch large debris, hopefully most hair, jewelry , etc. And you will clean that as often as it becomes dirty. There is no way around that. Cut your hair and don't wear jewelry in the shower? Other grime and dirt will get recirculated. You need to clean and flush the system sometimes or it will get moldy. This is all standard stuff, buy and use components and cleaning agents for jetted tubs.
    – jay613
    Commented Jan 24, 2023 at 19:48
  • The only difference to a jetted tub is the heater will have to be much bigger. A tub reheater won't be able to keep up with the cooling effect of the shower spray. You'll need something closer to a point-of-use shower heater. It just won't work as hard as it would with fresh cold water.
    – jay613
    Commented Jan 24, 2023 at 20:41
  • Could take a look at the showerloop instructable (instructables.com/Showerloop) and see if any ideas from there might fit. Not exactly the same idea, but uses similar filters.
    – BowlOfRed
    Commented Jan 25, 2023 at 0:46

4 Answers 4


You would never do such a thing for sanitary reasons.

You would use a heat exchanger instead, by which cold water for the shower is pre-heated by water headed down the drain. Thus the remaining heat in the shower water is extracted into the sanitary and fresh feed water.

This system will not require a storage tank, nor a pump (unless the heat exchanger is above the shower drain, nor a second shower head. Nor will it require any "gear shifts" from one mode to another, as there will only be one mode at all times. With shower drain water always going through the heat exchanger, and supply cold water doing the same.

Water is the cheap part of a shower. Heat is the expensive part. If you're keen on saving water, look into grey water systems *and for Pete's sake move the water heater closer to the point of use. Britain has it figured out with their electric showers!

How to blend, though.

If the shower has an "electric shower" or tankless on-demand heating right before the shower head, then this is a simple matter. The heat exchanger interchanges with the cold feed water entering the heater, warming it and reducing heating requirement.

However if this shower is fed by a water heater elsewhere, the plan still works, but it will require a thermostatic, automatically adjusting shower valve. This already blends hot and cold to reach your target temperature. In this scenario we interchange heat with the cold water. As the cold water into the thermostatic valve becomes warmer, it will automatically reduce its use of hot water.

Note that if you keep your water heater at a tepid temperature, say 40°C, then you may not be blending in enough cold water for this plan to work. However, that is the least of your problems. Science shows that tanked heaters kept that cool are breeding grounds for bacteria. Water heater temperature should be revisited, as well as anti-scald protection (make sure to use a shower valve which provides anti-scald).

But don't get your hopes up too high.

While you have a cute back-of-napkin idea, the water at the bottom of a shower is cooler than you might expect. You may not be figuring on the thermal losses due to the high humidity caused by the shower. In a water spray, of course humidity will go toward 100%. All that new water vapor needs to absorb its latent heat of vaporization, which is taken from the shower water as it falls. I strongly advise you to do some measurements and do the science before investing too much money.

  • +1. I built a heat exchanger for this. While it worked great and reduced hot water use tremendously, it was still not enough to justify the pain of having to clean a heat exchanger full of hairs and sludge a lot more often than I thought I would have to when I built it.
    – bobflux
    Commented Jan 24, 2023 at 21:37
  • 1
    There are heat exchangers which wrap around or replace a section of drain pipe, using the captured heat to preheat water going into you water heater(s). They aren't as efficient as a system which interleaves the two streams more thoroughly, but they're full-width drain pipes and won't clog any more than other drains do. If I used more hot water and more of that ran through a single common pipe (before hitting the cast-iron in the basement), I'd consider it.
    – keshlam
    Commented Jan 24, 2023 at 21:48
  • @bobflux yeah, the real trick is going to be designing the dirty-water passages and/or service plugs for low trouble and ease of maintenance. Commented Jan 24, 2023 at 23:42
  • Yeah I don't have a basement so couldn't use the option suggested by @keshlam
    – bobflux
    Commented Jan 25, 2023 at 0:02
  • @harper-reinstate-monica I made some improvments now, please see post again with new edition. My idea is to put a filter in front of the pump, so when it stops working, all the dirt falls to the bottom and goes down into the drain. In this case, do you see any sanitary violations? Commented Jan 25, 2023 at 0:41

Seems that you could treat this a little like a swimming pool/spa, or a lot like one.

  1. Bathe yourself with fresh water; send that water along with soaps and other contaminants down the regular drain.
  2. Switch the drains, fill the reservoir, linger and enjoy the warm recirculated fountain water.
  3. If you'll be treating this "a lot" like a spa then make regular use of a water chemistry test and add chlorine or bromine chemicals for sanitizer, adjust pH, etc.
  4. Periodically empty the reservoir and refill with fresh water. If you're treating this "a little" like a spa and not doing the chemistry in step 3 then empty the reservoir after each use.

This is different from a spa because the parts to not remain continuously wet and recirculating with sanitized water. Especially in the case that you drain the water after each use, I really have no idea whether you'll have any trouble with microbial growth in the system.

  • If emptied after each use, just follow procedures and use cleaning compounds as for a jetted tub. Rather than get the chemistry "right" as for a pool, which is a lot harder, you flush the system with a higher, non-precise, concentration of sanitizing agent after use, leave that in, and flush that out before next use. So the components that are permanently wet are left soaking in compound between uses but not during use. Then the chemistry can be less precise.
    – jay613
    Commented Jan 24, 2023 at 20:46


Put the "regular drain" in the bottom of the reservoir, with a plug you can operate easily from above.

Clean yourself off with the plug open, then plug it and fill, then switch showerheads. At the end, open the plug and drain the reservoir.

While you are at it, put a cold waterline heat exchanger on the regular drain pipe. That prewarms the cold water with waste heat from the drain water.

  • Thank you very much for your ideas! Its awesome! I made some improvments now, please see post again with new edition. Maybe you can tell me how to make an easy-to-clean filter? I think this is very important to protect the water pump. Commented Jan 25, 2023 at 0:23

In swimming pools this is dealt with by backwashing the filters periodically to clear them, and by adding sanitizing chemicals (chlorine or bromine, traditionally, sometimes ozone these days) to break down the things that a filter can't catch (and managing other aspects of water chemistry to trade off optimizing that and preserving the materials of the pool).

Of course that is also why swimming pool water has less than ideal effects on hair and skin and organic-fiber cloth. Which is why pools and hot tubs ask you not to wear cotton into the pool; some of the chemicals get wasted in trying to bleach the fabric and its dyes. It's not just a plot to get you to strip down. (Though for some hot tub owners that may be another motivation.) It's also why the want you to shower before entering the pool, again to minimize how hard they have to work to maintain the chemistry.

I suppose you could get something close to what you're describing by running a hot tub with one jet hooked to a shower head, but as noted even if that coilf be reasonably close to sanitary if you're willing to throw the chemicals in, it doesn't address wanting to use soap and shampoo and such, which are not handled well by standard chemistry and filters. There might be a way to manage that, or might not. In any case, note that this has additional maintenance obligations.

Or perhaps separate the issues; take short disposable showers followed by long recirculating soaks, hot tub style.

Or just restrict long showers to one day a week per person, and work hard to minimize hot water use the rest of the week. Which might be more net gain.

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