Heat leaving a house as hot water Most would agree that there is a current need for us to reduce energy consumption to reduce our carbon footprint.
There have been a lot of articles on the news here recently about the need for insulation, possibly swapping gas boilers for heat pumps etc...
So something came to mind, (like Archimedes) whilst lying in the bath.
When the plug is pulled out, many litres of hot water leave the house - straight down the drain.
Surely this is a waste.  There's no point (for the environment) having well insulated houses, or using heat pupms, if lots is wasted this way.
Here is a rough estimate...
The heating bill for June - September showed 2000 kW hrs.  Including winter the estimate is 12000 kW hrs per year, or $$4.3 \times 10 ^{10} J$$
For 3 people having a bath every two days (and we could add showers and hot water from washing dishes etc...) means over 500 bathfuls of water that has been heated from 10C to 50C.  Each bath is approximately 0.2 cubic metres (200kg)
From the heat capacity equation $$E=mc\Delta \theta$$
$$500\times 200 \times 4200\times 40 = 1.7 \times 10^{10} J$$
Is it really true that a typical house loses about 40% of its heat as hot water, going down the drain?
Is so have there been any inventions/methods for (maybe new) houses to divert hot water to circulate around, perhaps under the floor, until cool enough to join the drain systems.
Is the estimate accurate and what ideas or attempts have there been to reuse this heat energy?
 A: Ignoring the implied need to take 3 full baths a week, this is correct : a lot of energy is spent heating water, and is lost when said water is replaced with cold water.
If you can use solar energy to heat the water, then since that energy falling on your roof is lost anyway, using it to heat the incoming cold water is just as good as reclaiming the lost energy from the drain.
More directly though, I have seen some attempts at heat exchangers to pre-heat incoming water with outgoing water. That nice idea comes with various difficulties :

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*Heat exchangers are of limited efficiency due to the combination of a large thermal capacity of water, and limited thermal conductivity of solids.


*Heat exchangers need a large surface area, so splitting the flow into narrow channels is a necessity ; however the outgoing flow contains various dirt, skin, hair etc. which would make the exchanger unacceptably prone to clogging.


*Swapping heat between water going out and fresh water coming in only works if both happen at the same time; this is typically the case for a shower but not for a bath or washing machine.
In the case you want to heat the house instead of the incoming water, the last point doesn't apply ; however the clogging problem is compounded with the need for pumping (for forced circulation).
Obviously, health regulations could possibly prevent some/all of the above.
A: Heat recovery systems have been around for a while.  Here is one for sale at Home Depot.  They are widely used in commercial gyms where lots of people are having showers.
If you want to be green, then just divert your grey shower and sink water into your toilet and your garden.  It is your toilet that uses the most water in your home.
