A tankless water heater is a sophisticated appliance, but there are a number of things that can go wrong. Most common among such faults are…
Failure of the ignition or flame
Problems with the gas supply
Blockage to the air supply or exhaust
Buildup of minerals, often due to hard water, or of dirt on the burners
It’s also possible that the system can overload. This can be a problem with a near constant supply of hot water – and everybody in your home then wanting to make best use of it at the same time, perhaps when arriving home on a bitter-cold winter’s day!