A tankless water heater provides continuous flow of water which means that a properly sized tankless water heater may fill the requirement to fill a tub. The heater must be having a capability to maintain the flow of water. The tankless water heater continues to provide hot water till the hot water tap is not shut off.
A tankless water heater heats the water continuously, when the cold water from water supply pipe goes through the heater it becomes hot instantly. A tankless water heater does not have a tank, so the quantity of water passing through the tub is not fixed.
It has a flow meter and a heating element or burner installed with heat exchanger. There is an electronic meter installed in tankless water heaters which turns on when the water starts to flow and keeps the temperature of water near to the desired one.
What if we want to fill the bath tub quickly?
A tankless water heater can fill a bath tub with hot water but we must be having flow rate of our bath tub matching with the flow rate of the water heater. If we have a high flow rate in bath tub (to fill the bath tub quickly) a heater with a very high heating capacity would be required. If we have a small tankless water heater and we want to fill a bath tub with high flow rate, the hot water supply will be inadequate. A storage water heater may be a better choice to use with a bath tub anyway.
There is one more issue. A tankless water heater can provide continuous water supply but to fill the bath tub with hot water we must maintain the flow rate because if the flow rate exceeds the capability of water heater, the temperature of the water will start to fluctuate and heater would not be able to heat water properly.
But if the water flow is kept low, it will take a very long time to fill the bath tub and indeed the tankless water heater cannot heat the water inside the tub, it will start to cool down. This will result in dropping temperature of water before you get to take a bath in your tub.
A tankless water heater takes a couple of seconds to detect the flow of water. So, the cold water starts flowing from tankless water heater when you are using hot water and shut it off and back on. This may be termed as a “Cold Water Sandwich”. The tankless water heater takes couple of seconds to activate it.
This delay will cause the temperature of the water in the tub to fall and will become a discomfort for the one taking the bath. If the hot water is not shut off, it may result in high temperature inside the tub which may harm the person in.
As the water flow must be kept slow to get the desired temperature, when the water faucet is shut off, the water inside the tub will become cooler. This is the reason storage heaters are suggested to be used with bath tubs because hot water may start to flow instantly and at any flow rate to fill the bath tub.
Some people do not have enough space to install a storage heater.
If you want to use a tankless water heater, you can use it. All you have to do is to get some professional plumber to calculate flow rate of your bath tub in which it can be filled easily and timely. Now look for the tankless water heater, which has the capability to heat the water to your desired temperature at the same flow rate.