Comment by schoen | Hacker News Reader

Comment by schoen | Hacker News Reader

I was just thinking about this earlier today when washing my hands in the sink.

When you first turn on the hot water tap in most homes, the water that comes out is cold. After some period of time, hotter water starts to come out. My mom used to describe this as "waiting for the water to warm up".

For decades, I didn't consider the mechanism behind this. That is that there is water in the pipes between the home water heater and the tap. That water can't retain its heat without any further heat input, and gradually loses heat and comes into near-equilibrium with the temperature of the rest of the house. The hot water inside the water heater tank, on the other hand, is constantly being reheated as necessary by a heating element.

When you turn on the tap, after not having used it for some time, you're waiting for cold water from the pipes to be flushed out through the tap and be replaced with freshly heated water from the tank. Once this happens, the water coming out of the tap will be hot because it's been heated recently enough.

I probably didn't realize this until I was about 30 years old, and then I thought of Feynman's anecdote of his students not connecting their theoretical knowledge to understand the mechanism of a real physical situation. It seems I wasn't curious enough as a child to apply my own knowledge to the mechanism of the hot water tap!

One of the best ways to learn to think Scientifically i.e. in terms of Physics/Chemistry/Mathematics/Biology is to start with the classic Soviet era books by Yakov Perelman - https://mirtitles.org/?s=yakov+perelman

Specifically; start with the 2-vol Physics for Entertainment (the breadth of coverage is really good) followed by Mathematics can be Fun, Algebra can be Fun etc.

Note: Do not be misled by the words "Entertainment"/"Fun" etc. in the title. As is typical of Soviet era books these are serious works with precise, succinct and focused topics quite unlike most current "pop science" books.

Weirdly enough, this is not the case in the former Soviet Union. Because there's always a reverse pipe ("obratka") that has hot water constantly circulate and the only part that cools down is the small portion of pipe between "obratka" connection and the faucet, so "heating up" takes a second tops.

It's puzzling why no one in the civilised world adopted this idea :/

Many things are told about emigres being constantly homesick etc., and i believe this is largely bullshit, but this is the only thing i really miss from my Soviet past.

It’s a thing in the US too, but not common. I always understood them to be expensive to use, as you always have a pump running and hot water cooling down and needing reheating.

Hot water recirculating pump. https://www.familyhandyman.com/article/hot-water-recirculati...

Yes, in the Soviet Union, hot water was a byproduct of production of electricity - using combined heat and power, it was a waste heat. And it used lots of electricity to make weapons to advance dictatorship of proletariat, so there was plenty of free heat, too.

My house originally had a recirculating pump for the hot water but it burned out. Somehow though, it still (mostly) worked and had instant _warm_ water.

I think it was through natural convection/circulation - the hot water expanded in the tank and pushed it through the recirculating loop?

So maybe there's a good-enough solution that doesn't require a pump, just a return loop.

Now I have an on-demand water heater with a built-in recirculating pump, so it's instantly hot :)

This is how rooftop solar water heaters circulate water as well. Though these days you'd be better off with PEV plus a small electric heater.

You can install an immediate hot water heater at the sink if you’d like.

Hot water recirculation is a thing I have heard of done in the United States. I don't know how common it is, but a simple Home Depot search brought up a bunch of results for options.

https://www.plumbingsupply.com/recirculating-systems-explain...

https://www.homedepot.com/s/hot%20recirculation?NCNI-5

This idea comes for free if you're also using hot water in radiators as a way to heat your home at the same time. Which the Soviets did. And in the Soviet era, they also were generally heating that water at the district level, then circulating it to all of the homes. This can only be workable if you're pumping the water continuously. So the cost of the pumping is just part of the overall system.

Places where this was built up, still generally use it today.

In the USA, nobody ever built the district wide heaters. Nor would they be viable in the suburbs that many of us live in. We generally use central air instead of radiators to heat our houses. And the result is that constantly circulating hot water is significantly more expensive for us.

Does that answer your question?

the places i lived in germany and china did have district wide heaters, but only for the water that circulated the heaters, not the water that came out of the tap. i believe the reason being that tap water needs to be potable (at least in germany, in china you still need to at least boil the tap water), whereas water for heating doesn't. you don't actually want your tap water go through the heating system. fun story: in china i once managed to open the heating pipe. the water that came out of that was black. probably rust and other stuff from the pipes. i wonder how the soviets managed to keep the water clean.

They did it this way: from the powerplant, it was hot steam under pressure, not even water, that got to the block-sized heat exchanger (serving maybe 500-1000 people, 200-300 flats - one large soviet-style block or 2-3 smaller ones). There, heat exchanger warmed the technical (indeed totally non-potable, almost poisonous, and constantly circulating) water for the heaters, that was only there during the heating season, and separately, tap water, these were two completely different, non-mixing circuits (and both of them short without much insulation - maybe around 100 meters each way outside, plus within the buildings). And sure the tap water wasn't potable, but it had nothing to do with the way it was heated - there were two completely different circuits for hot and cold water anyway, cold water never entered the heat exchanger building. It was always a separate building by the way, i think because a potential accident with high pressure steam could kill a lot of people if it happened right within the residential building itself.

We were told that hot tap water was totally dangerous to drink and no one tried to. Cold water was unsafe to drink but some people either had reverse osmosis filters, which i know aren't good because they deprive water of a lot of useful stuff too, but no one knew it back then, or they boiled it, but most just drank it as it was. Shorter lifespans and much lower average age, and plenty of other life dangers like mass alcoholism, made water quality was a lot less important than it is today in the West - it won't be water that kills you anyway, it's vodka, lifestyle, of the Party itself.

I've seen this in a few places, but it's rare because of all the energy (heat loss and energy required to drive the circulation) that gets used up when the water is recirculating but nobody is using it--which is most of the time.

I was recently in Iceland, and since a lot of heat is geothermal, recirc would probably make sense, but I can't remember having it. Maybe it's the pumping cost? Although natural convection driven by the difference in density between hot and cold water might make up for at least part of that.

Same with cold water.

I don't think I have anything in my house (other than an ice cube tray?) that's meant to deliberately make cold water colder than the ambient temperature of the house! Maybe some people do, though, especially in hotter parts of the world.

Water in mains pipe is typically colder than your house walls (where the pipes go). So you need to flush some water from your pipes to get that colder mains water.

This is a good point I think is comparatively less noticeable where I live. But on reflection, I've sometimes noticed it.

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