When I turn the shower on, the water is cold. The water in the hot water pipe has been sitting there and is at the same temperature as the cold water pipe. I turn the water mixer to hot. After a while, hot water from the hot water loop in the cellar reaches the shower upstairs, and the shower turns hot. Too hot. I turn the mixer colder. After a while, the water coming in from the cold water pipe is no longer room temperature water that’s been sitting inside the house, but coming from the municipal pipe outside and really cold. The shower turns too cold. I turn the mixer hotter. Soon after this, the hot water from the hot water loop has been exhausted and the water heater hasn’t adjusted to make more hot water yet. The shower turns cold. I turn the mixer even hotter. After a while, the heat exchanger has started producing hot water and the shower is hot again. I make a final adjustment to the mixer, which is now receiving consistently hot and cold water. Having depleted its destabilising reserves, the system has reached a flow equilibrium. But the shower’s almost over.