I have a tankless water heater. After I finish to use hot water and I close the faucet, the water heater starts a fan and make a typical noise, it's very typical and expected. I guess it blows off excess moisture or similar.
I noticed however, that when I flush a toilet (only the one of the three toilets I have), the moment toilet valve stops the stream of cold water, my tankless heater starts the same noise as when you shut-off hot water. This leads me to believe that some hot water is pulled in when flushing a toilet. I closed the output hot water valve on the heater and – obviously – it does not make the noise when I flush that toilet. Also, the "in use" light of the water heater does NOT turn on when the toilet is being flushed, unlike when you open hot water "explicitly".
I also made another experiment and if I open the faucet of the tubs, fully on cold, when I shut it I also get the same reaction from the water heater.
I suspect I'm literally draining energy down the toilet every time I flush, and I'd like to understand what is going on.
Best Answer
Do you have a mixing valve somewhere in the supply to the toilet ?
In cold climates it is common to mix some hot water in with the cold so that the water supplied to the tank does not cause condensation on the outside of the tank do to a temperature difference between the water and the ambient room temp.
Thus if your toilet is calling for water it will use some hot water from the hot water heater.