One of the joys of a tankless water heater is having hot water on demand. It's frustrating when the heater doesn't deliver as promised and the water fluctuates between hot and cold. A common cause of this problem is something referred to as the "cold water sandwich."
What Is a Cold Water Sandwich?
The term "cold water sandwich" is used to describe the phenomenon when you turn on your faucet and hot water begins to flow, to only be replaced by a moment of cold water before the water is hot again. Although this phenomenon can occur with any type of water heater, it is an especially common problem with tankless water heaters.
It's important to note that the cold water sandwich problem is very different from other types of heat fluctuations. If your tankless water heater is slowly losing heat as it runs, or if it is spiking between hot and superheated, then you have a different issue that will require further inspections and troubleshooting so that the cause can be determined.
The good news is that a cold water sandwich doesn't mean that there is something wrong with the heater. It is a natural byproduct of having hot water on demand. It's important to note that the water lines that connect the heater to the faucets and taps throughout your home do not run completely dry when you turn off the tap. Some of the heated water remains in the water lines.
The issue occurs when someone runs hot water. A small amount of hot water is present in the tap, but the water remaining in the supply line cools quickly. When you turn on the tap, you get the initial burst of hot water, followed by the cooling water trapped in the line, before the newly heated water begins to flow again.
There are a few solutions, beyond allowing the water to run until the cold water flushes from the line. The most effective solution is to install a recirculating system on the heater. An extra line is installed on the hot water supply lines. This line loops back to the heater. When the tap is turned off, any water remaining in the line is flushed into the recirculating line so it isn't left in the supply to grow cold.
If the cold water is only an issue on a single tap, such as the shower, a more efficient solution is to install a small storage tank heater. This device holds the cold water stuck in the line and reheats it briefly before it runs out of the faucet.
Contact a plumber to learn more about tankless water heaters.