The Cold water Sandwich
What is the cold water sandwich?
This sandwich happens when there is warm water for a few minutes then a burst of cold and then rapidly back to warm. This occurs in homes with tankless water heaters as the residual water in the pipes going to a faucet or shower can still be hot. The user in the household will then turn the tap on and feel warm water almost instantly. A tankless water heater takes a few seconds to heat up the heat exchanger and start transferring that heat to the water. The result is that a small amount of cold water can make it through the heat exchanger without warming up. When this cold water reaches the end user it may not even be noticed or if the person is in the shower it may be quite evident. After a second or two the heat exchanger is working and hot water is being produced and sent though the hot water pipes bring the desired water temperature again. This does occur more frequently in electric tankless water heaters over gas powered tankless systems. Gas is at full temperature almost immediately where an electric element can take a minute to fully reach its operating temperature.
Now you know what a hot water sandwich is in reference to plumbing and tankless hot water heaters, ask your contractor or installer as there are ways to mitigate this sandwich.