Tankless waterheaters can provide you with reduced energy usage and a limitless supply of hot water. There are a few significant differences in the way tankless water heaters work and how standard storage water heaters work.
A gas water heater may need a new thermocouple because the pilot light won’t stay lit, or the high limit switch may need replacement in an electric water heater, other than that and similar minor problems they are pretty darned reliable appliances. You set the temperature dial to warm hot or hottest and forget about it (or the upper thermostat on electric units).
With tankless waterheaters there are some important differences you should be aware of. Tankless units have both a minimum flow rate and a maximum flow rate to be able to maintain a constant outlet temperature. If the water flows too slowly through the heater, the heat exchanger can overheat and become damaged.
Tankless water heaters use a flow sensor to turn on the water heater only if the minimum flow rate is reached. Tankless heaters require ½ to ¾ gallons of water per minute to initiate operation depending on the brand and model. This means you can’t use “just a trickle” of hot water, you need to run at least a half to three quarter gallons per minute to keep the waterheater operating.
Since you will need to mix hot and cold water at the fixture to get a usable water temperature from the faucet, and you must maintain the hot water part to at least ½ gallon per minute or ¾ gallons per minute for larger models, then you could end up needing to use at least a gallon per minute for the right temperature water.
Tankless water heaters also have a maximum flow rate for which they can heat water to the temperature that has been set for the output. Higher flow rates will cause the outlet water temperature to drop.
Some tankless models require periodic maintenance like de-scaling the heat exchanger in hard water areas. Scale buildup from hard water can be a serious problem. The owner’s manual will describe how to clean the scale out if it is needed.
The installation of a tankless waterheater should not be done by someone not trained in installing the brand that is purchased. Improper installation can lead to things like carbon monoxide poisoning, a fire hazard, or unstable water heater operation.
Tankless water heaters take time to heat water to full temperature, so they take a little longer to deliver hot water to the fixtures, typically 10 to 20 seconds longer. That means more water being run down the drain while you wait for hot water.
If you have a need for large volumes of hot water tankless waterheaters are the way to go. Just make sure you size it correctly and have it installed correctly. And if applicable add a hot water demand system as well.
Derived from my article: Tankless Waterheaters
