A cold water sandwich is just as unpleasant as it sounds! And avoiding it is a smart idea – just as avoiding the tankless water heaters that cause them can be for commercial applications.
Tankless water heaters are great for some residential applications, particularly in small spaces with a relatively low demand for hot water. But in a commercial application, the volume of hot water required is generally much, much higher. (Think about how many of your hotel guests or apartment complex tenants want to shower or wash dishes at a similar time.)
Because the tankless water heater has no storage capacity and the operation is measured in gallons per minute, it can only provide hot water as fast as it can heat the water. Therefor, it may not be equipped to deal with three faucets running at once – let alone 20 or 30!
The cold water sandwich occurs because the tankless heater works on flow, so the water can be coming out hot but if the sensor turns off the cold water comes back briefly before the sensor turns on again. This results in a hot-cold-hot scenario. Best to be avoided in the shower!