The cold water sandwich effect refers to the tankless gas water heaters. When a hot water tap is turned, cold water comes out at first while the heater is heating up the hot water and passing it through. The cold water will eventually turn to hot after a few minutes. However, when the tap is turned off, so too is the hot water supply. When turned on again, the water will be cold until the hot water is arrives. This is because the water heater shuts off immediately when the tap is closed and because there is no tank to store the water, hot water supply is not readily available. There are ways to prevent the cold water sandwich effect.
Install a tankless water heater that does not shut off when the tap is closed. Some manufacturers, such as Rinnai, have developed tankless water heaters hold onto the hot water supply for an additional 60 seconds. Although cold water will still arrive when the tank is first opened, it will remain for a minute after it is closed, in case it is turned on again immediately after.
Keep the tap on to ensure there is a steady flow of water. This is an ideal option for activities such as washing the dishes, having a shower or running a bath, where leaving the hot water tap turned on will result in continuous flow of hot water.
Keep the tankless water heater and inlet water filter clean and free of debris. Some water supplies contain high amounts of iron, which could affect the water pressure and cause the hot water to temporarily cut out. You can also remove the showerhead and fixtures and clean those.
Replace the showerhead and fixtures with ones that provide more pressure. The tankless water heater needs to sustain pressure in order to keep the hot water flowing. Some manufacturers require 0.6 gallons per minute in order for the hot water to continue flowing.