Hard Water and Tankless Water Heaters: A Costly Combination
Why tankless water heaters are especially vulnerable to hard water damage and scale.
Tankless heaters concentrate the problem
A tank water heater has a large volume of water and scale settles to the bottom. A tankless heater forces water through a narrow heat exchanger at high temperatures, which is the worst-case scenario for scale formation. Calcium precipitates out of solution rapidly, coating the heat exchanger and reducing flow.
Warranty implications
Most tankless manufacturers (Rinnai, Navien, Noritz, Rheem) state in their warranty terms that water hardness should be below 7-11 GPG (120-188 PPM). Operating above this without a softener or conditioner may void your warranty. Given that a tankless unit costs $1,000-3,000 installed, this is a significant financial risk.
What to do
If you have a tankless water heater and your city's hardness exceeds 120 PPM, install at minimum a salt-free conditioner (which prevents scale without removing minerals) or ideally a salt-based softener (which removes minerals entirely). Annual vinegar flushing is also recommended regardless of water treatment.