Lack of Hot Water
This is of course the most apparent red flag that something is amiss with your tankless water heater. If the water is cold or even lukewarm, then there’s most likely a problem with the heating array. Or it can be trouble with the fuel source—gas or electric—such as a clogged line or a loose wire. Whatever the cause of the issue, it needs a professional touch to resolve it.
Leaks
Because of their compact nature, when a tankless water heater suffers a leak it can be a big deal. A water leak can easily reach other components in the system and damage them. However, this type of problem is relatively simple for a trained technician to repair. Check for puddles around the base of your tankless system, or a drop in water pressure in the more extreme cases.
Odd Noises
No matter the appliance or system, odd and/or unfamiliar noises are never a good sign. You may notice a gurgling or bubbling, or even a groaning or hissing sound. Anything that can’t be associated with the regular functioning of the system is something that warrants a service call. If you notice a strange sound, shut down your water heater and call for repairs right away.