The time it takes for hot water to arrive at the tap is the same on both types. The best thing about the hot water that comes from a tankless model is that you will receive an endless supply of hot water for as long as you open a faucet. With traditional tank type water heaters, you have hot water until the hot water tank’s supply is depleted. Then you must wait for the storage tank of cold water to re-heat before you again have hot water.
If you are running out of hot water with your current tank type heater, and are thinking of purchasing a tankless model, you may wish to conduct a simple test by keeping track of the amount of time from when you turn on the hot water to the moment the hot water runs out. Take the total capacity of your tank-type heater. (This information can usually be found on the information stickers on the side of the tank.) Divide the capacity of the tank type heater (in gallons) by the number of minutes that it takes to run out of hot water. This is your peak usage of hot water and this calculation should be the minimum GPM that you need when selecting a tankless heater.
To perform this test, the home owner must decide which hot water devices to run for this test as the more valves open the higher the total GPM will be. So, if you have two showers and sometimes they are in use when the hot water runs out, then this would be the ones to use for the test.
If your test results in your 40 gallon heater running out of hot water in 12.5 minutes. Use the equation, G ÷ M = GPM, where G is equal to the total Number of Gallons of your tanked water heater and M is equal to the total time the water was running before it went cold. To be accurate in your calculation it is recommended that you wait an hour before running your hot water, only to make sure that your hot water heater is full of hot water.
40 gallon water capacity divided by 12.5 minutes = a flow rate of 3.2 gallons per minute.