Hmm, Toilet water that has been standing in the tank for more than 1 hour is warmer, in most all cases, than the cold water inlet. Inside air temps are almost always warmer than ground water temps, so the water in the tank is slowly heating up as it sits there.
This is an argument for the balanced flow model in GFX. That is, hook GFX output to cold water supply EVERYWHERE, in EVERY sink, EVERY toilet. This optimizes heat recovery from the waste water.
There is an exception to this rule however. In areas that have average ground water temps of 60 degrees or above, the sinks and perhaps washing machines will not get GFX heated water. Even dishwasher supply can be GFX connected. This can be retrofitted in a house with a basement as you only need to separate sink and washing machine supply lines from the main supply and route them to a new feed line that originates on the cold side of the GFX.
Canada and the US states that border Canada can easily heat EVERYTHING as average year round ground water temps are at least 15F or more lower than US Gulf Coast states water supplies.
This GREATLY simplifies the hookup and significantly increases the heat recovery of the unit when there is a balanced flow rate, i.e. fresh water in == waste water out. The closer we come to that ideal, the better the efficiency of the unit.