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Apologies in advance if this has been addressed before... it's something if a daily puzzle during my morning ablutions and I haven't gotten a handle on it yet.

Consider the hot water pipe from my basement to second floor sink. Between uses this cools to ambient temperature; call it 15 degrees C.

When I turn on the hot water tap in this state, I am both flushing that cooled water out of the pipe and losing heat until the pipe comes up to (approximately, ignoring gradient and ongoing losses to the surroundings) the temperature of the incoming hot water (let's say 60Cish for simplicity?), at which point I get a steady stream of as-hot-as-it's-going-to-get water until I turn off the tap and the system starts cooling again.

The question is, do I waste less water and/or energy before that steady state is reached by running the tap at a lower flow rate, a higher flow rate, something in the middle, undeterminable without a complete model of the system, or does it actually not make much difference?

Currently I'm leaning towards "too many unknown variables, just try to measure it." But I'd be delighted to be wrong!

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  • $\begingroup$ Grant the point that it's engineering rather than pure science. $\endgroup$
    – keshlam
    Commented Feb 19 at 5:05

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Imagine running the water at a very low flow rate, so low that the heated water reaches ambient temperature by the time it comes out of the tap. In this scenario, water coming out of the tap will never get hot however long you wait, you are just wasting water and energy.

What you want is to make the flow rate as high as possible to flush out the cold water as quickly possible, so that water from the heater loses as little heat as possible as it makes its way through the pipe.

The one exception I can think of where you might want to reduce the flow rate somewhat is to give cold water going into the heater enough time to heat up. If your heater can't quite keep up, the steady-state tap water temperature could be reduced at high flow rates. Whether this matters at all for your setup depends on the maximum flow rate, the power output of the heater, and the size of the heater's tank.

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  • $\begingroup$ Hm. That certainly comes up to temperature fastest; wasn't sure it also did so in least volume. There's both heat in the pipe from its resting state and offsetting ongoing losses -- and losses are higher when the pipe is warmer. But I'm very willing to believe I overcomplicated this. $\endgroup$
    – keshlam
    Commented Feb 18 at 17:39
  • $\begingroup$ @keshlam It doesn't necessarily take less volume to flush out the cold water this way, here you are just minimizing the heat that water loses along the way so it is hotter when it comes out of the tap. I would imagine how much volume it takes would be approximately independent of flow rate. The rate at which water loses heat would actually be lower if the pipe is warmer, so the tap water will likely get hotter over time as the pipe heats up. If you do end up making any measurements, feel free to post them as an answer. $\endgroup$
    – Puk
    Commented Feb 18 at 17:48

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