Residential single-family building practice currently ignores the losses of energy and water caused by the poor design of hot water systems. These losses include the waste of water while waiting for hot water to get to the point of use, the wasted heat as water cools down in the distribution system after a draw, and the energy needed to reheat water that was already heated once before.

Average losses of water are estimated to be 6.35 gal (24.0 L) per day. (This is water that is run down the drain without being used while waiting for hot water.) The amount of wasted hot water has been calculated to be 10.9 gal (41.3 L) per day. (This is water that was heated but either is not used or is used after it has cooled off.) A check on the reasonableness of this estimate is made by showing that total residential hot water use averages about 52.6 gal (199 L) per day. This indicates that, on average, about 20% of hot water is wasted daily.

Units: Dual