Comment by sideshowb | Hacker News Reader

Comment by sideshowb | Hacker News Reader

We are increasingly encouraged to take showers for environmental reasons (less water and less heating) but I don't find them at all relaxing like a bath. This leads me to ponder environmental alternatives. I don't know which has the greater energy cost, purifying bath water and piping it to and from my house, or heating it, but on the financial front I was surprised to calculate that the water itself costs more than the heating.

Hot tubs have the advantage of not continuously requiring more water, but disadvantage of needing continuous heat. It seems pretty daft to me given the upfront cost of a tub that the default is to heat them electrically, surely heat pumps would be a better option, or even just plumbing them into the house radiator system? Either way, though nice to have, still they're likely to be even less eco friendly then a bath. (I'm excluding wood fired options here for lack of convenience plus the fact that they are usually refilled each use).

Saunas, with little water to heat, are presumably much more efficient, and even have some medically proven health benefits. For me they don't quite beat soaking in water but they're a close second.

> Hot tubs have the advantage of not continuously requiring more water, but disadvantage of needing continuous heat.

Hot tubs also chew through chemicals. If people are in them every day you have to be constantly managing the purification mix. Pro tip: If you are on holiday and there is a hot-tub, if it doesn't look ultra-clean, don't use it - warm water and endless turns of dirty skin turn the whole thing into a giant petri dish.

As an aside, I've just got out of a long 2-hour Epsom Salts bath. I feel wonderful, relaxed, and clean. A shower can't do that for you. You also can't read a good book in a shower.

It's common to reuse bathwater in Japan:

https://whatjapanthinks.com/2006/11/28/over-half-the-japanes...

I've seen foam bath covers which are used to keep the water warm between uses.

AIUI, Japanese people shower first to get clean, then sit in the bath to relax. They don't use soap or shampoo in the bath, but may use bath salts. Therefore, the water in the bath stays reasonably clean. The bath is essentially a tiny, well-kept swimming pool.

I don't know how many times they will reuse the water, or if there is any process to clean it. You could imagine a sort of solera system, where you drain away some fraction of the water every time, then refill the bath with fresh water.

Generally the bath is just used for one night, the systems that I am familiar with will add additional hot water to to tub but do not reheat the existing water. The grey water is often reused to wash clothes.

Shower flow rates are somewhere of the order of 10L/min -- potentially rather more if you're talking about power showers -- so the showers-are-more-efficient-than-baths thing is only really clear cut if you take relatively brief showers. I think some of the comparisons I've seen in the past have pointed to 2 minute showers, which seems pretty rushed, especially if you have hair to wash.

If you have a family, can you share a bath (taking turns)? That's a substantial and fairly easily-achieved saving.

If you've got a garden, adding a gray-water collection system also makes things that little bit less wasteful, at least in the summer (but I suppose that would also work with a shower too...)

>* I think some of the comparisons I've seen in the past have pointed to 2 minute showers*

Wait?

Wouldn't that mean that you bathe in 20L of water? I know it's only the eyeball test, but most of the bath tubs I've seen seem to be filled with more than 20L when in use. Particularly if people are soaking, which would be a fair comparison to a long shower.

I think maybe we need some kind of way to recycle water for more than one night, and reheat it efficiently. Then baths start being better for the environment.

I have no idea how to do that? (Or even if you can do that?) But it seems a good way forward if some company made tubs like that.

Sorry if I wasn't clear, I meant that most people I know seem to shower for >>2 minutes, so I doubted the credibility of the comparison.

Clearly, if you are fairly consistently sticking to 2-minutes showers, that's going to be more economical than baths -- even with some degree of sharing.

It's trivial to test: put a stopper in the tub before you take your shower, and see how close you were to a bath.

Only if you have a combined bath/shower. Mine are separate

It would be nice if all the heat you put in the water wasn't lost, but at least partially recovered when you're done bathing. Today it just heats up the sewers.

Agreed. I suspect one barrier to that is making a good heat exchanger that doesn't get clogged with hair etc. Another is that whatever you use it for, you will likely want a higher temperature than waste bathwater so it only works as pre-heat for something ... which means you need something running that needs it.

Drain line heat exchangers are built for this. They preheat the water heater supply line using waste heat from drain water.

Renewability Power-Pipe [1] was the first one I encountered, but there are several in the NA market.

[1] http://renewability.com/

> Water falling down a vertical drain stack does not run down the middle of the stack, but instead clings to the inside wall of the pipe. In a Drain Water Heat Recovery (DWHR) application, falling drain water forms a thin film that rapidly imparts its heat to the colder pipe wall. The Power-Pipe® captures much of this heat, then "recycles" it to raise the temperature of incoming cold water.

That's brilliant. And it means that if you want to increase the surface area for heat exchange, you make the waste pipe bigger, which reduces, rather than increases, the chances of clogging!

>which reduces, rather than increases, the chances of clogging!

Yes and no. A bigger pipe will have slower flow (it's not linear based on area since the pipe is not full at all times) which lets things settle.

The point is that the pipe is never full, because the waste water runs down the walls. Nothing's going to settle in a vertical pipe.

When the pipe leaves the heat exchanger and goes back to wiggling around the building on the way to the sewer, then sure, go back to sizing pipes based on thinking about settlement and other things.

These are required by building codes in my area, and I've wondered how effective they really are. It never occurred to me that water would cling to the side of the pipe. Some of the other trades suggested that it would make more sense to have them on horizontal runs, but that would probably be less effective.

If you're showering, then you have a need to pre-heat cold water at a rate perfectly matching the rate at which you discard warm water!

If you don't mind the humidity, you can simply leave the water in the tub until it gets cold. The water in the tub becomes cold by exchanging its heat with the ambiant air.

That's of course assuming you're in a climate where you want to heat the air.

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