They exist, but are really only good in the southern part of the US. In the north the incoming water is cold which really slows down the hot water creation (re

They exist, but are really only good in the southern part of the US. In the north the incoming water is cold which really slows down the hot water creation (recovery) rate, and that cold exhaust then needs to be warmed up by the heat. If you have low usage (i.e. less than 40 gallon/day of hot water) you could get away with it, but that's two showers - so if your usage is more than 2 showers per day I would avoid them. But in the south they are a much better choice, since the recovery rate is much better and the cold air helps with A/C.