You probably got a tankless hot water heater so you could have a continuous supply of hot water. So, when the water starts flowing cold, you're right to be alarmed! The good news is, many issues with tankless hot water heaters are easier to solve than you might imagine. Here's a look at the four most common reasons tankless water heaters stop heating.
A Tripped Breaker
If you have an electric tankless heater, check the breaker box to make sure the circuit to which the water heater is connected has not tripped. This can happen if there is a power surge or there are too many appliances connected to one line. If the circuit has flipped, flip it back into the "on" position -- and your water heater should start working again. If the circuit breaker keeps tripping repeatedly, you may need to have an electrician come look at your system and move other appliances over to another circuit to avoid overloading.
If you have hard water, there is a good chance that minerals are depositing themselves inside the tankless hot water heater. They might deposit directly on the heating elements, which keeps the elements from effectively transferring heat to the water. Usually, heat loss brought on by mineral deposits comes on slowly. Your heater may slowly stop warming the water, so the temperature of the water decreases over time.
In the short-term, your plumber or appliance repair expert can address this problem by replacing the heating element or cleaning the mineral deposits out of the water heater. In the long-term, you will want to have a water softener installed so that the minerals are removed from your water before it reaches the water heater. This will prevent you from having to deal with the same problem again.
A Closed Tap
If there is not enough water flow into the tankless hot water heater, it will not heat properly. Check the inflow valve to make sure it is completely open. If it's not, open it all of the way. Sometimes, a valve might get bumped when someone is moving something, resulting in it being closed accidentally. You may need to apply a little lubricant to the valve to loosen it.
If your tankless hot water heater stops heating, do not panic. In most cases, the issue can be easily repaired and you won't have to replace the water heater.