This is a term used to describe what can happen with a tankless water heater when you turn the hot water on and off frequently. As we learned earlier, it takes about 3-4 seconds from the time you turn on a faucet before the tankless heater is actually on and sending hot water into the water lines. This is due to the sequence of ignition, and then the time it takes to send the water through the heat exchanger and out into the pipes. Now with this knowledge, let’s take a look at a scenario where you may experience the CWS. Let’s say you are in the shower enjoying the endless hot water. For whatever reason, you decide to turn off the hot water (maybe you have “scamper” out and grab a bar of soap…?) Anyway, the water is off for 15 seconds and then you turn it back on and resume your shower. Just like when you started your shower, the water heater has to go through the 3-4 second ignition and startup again, which introduced 3-4 seconds of cold water in between the previously heated water, and the current hot water flow that is heading your way. Depending on how far the water heater is to the shower, at some point you will feel that 3-4 second cool down until it clears the line. Although you will definitely feel a cool down in water temperature, it doesn’t go completely ice cold as some would have you believe (like tank manufacturers…). In reality, it is a small price to pay on occasion for the huge benefits of tankless.