The so-called cold water sandwich effect is most pronounced with tankless water heaters. Since there is no reservoir of hot water (because water is heated on demand), the water in the pipes between your tankless water heater and your shower may still be warm from a previous use; whereas water coming into the water heater is quite cold.
So if you start another shower, you will initially enjoy the warm water still in the pipes which might suddenly turn cold as the new cold water begins flowing into the water heater. This effect seems more pronounced with electric tankless water heaters because the heating element takes a bit to get hot after water starts flowing.
Reducing or eliminating the cold water sandwich
There are several things you can do to mitigate the effects of the cold water sandwich.
- A gas water heater heats up faster than an electric water heater, so you can minimize the cold water sandwich problem.
- Choose a tankless water heater that has a delayed shut-off feature.
- Install a small reserve tank (although this somewhat defeats the purpose of a tankless system)
- Use multiple point-of-use tankless systems to reduce the time lag until you have freshly heated water
- Simply decide to wait a minute after your shower is turned on before you actually hop in.