Hard water, scale and your tankless water heater
A tankless water heater heats water as it flows through a narrow heat exchanger. In very hard Valley water, that's exactly where scale likes to build up.
Why tankless heaters are sensitive to scale
Heat drives calcium carbonate out of hard water. In a storage tank heater it settles at the bottom of a large tank. In a tankless heater it coats a compact heat exchanger with small passages, so even a thin layer affects how well it heats and how much water gets through.
Signs of scale build-up
- Hot water that runs lukewarm or fluctuates
- Lower hot water flow than before
- Error codes the manual links to flow or overheating
- Noise from the unit while heating
Flushing (descaling)
Most tankless heaters have isolation valves with service ports for flushing. The usual method: shut off the gas or power and the water, connect a small pump and hoses, and circulate a descaling solution through the heat exchanger for the time the manual gives, then rinse. The manual sets how often; harder water means more often.
Where a softener fits
A softener on the main line, ahead of the heater, removes the minerals that form scale, so new scale largely stops. Existing scale still needs flushing out. Keep following the maker’s maintenance schedule either way: it protects the warranty.
Check the warranty
Some tankless makers state a maximum water hardness, or exclude scale damage from cover in hard water. With most Valley figures between about 11 and 17 grains per gallon, it is worth reading the warranty before deciding against treatment.
Common questions
- Do I need a water softener for a tankless water heater?
- Many tankless makers recommend treating hard water, and some limit warranty cover for scale damage. Check your model's manual and warranty for the hardness it allows.
- How do you descale a tankless water heater?
- Most are flushed by circulating a descaling solution (often white vinegar or a maker's product) through service valves with a small pump, following the steps in the manual.
- Does a salt-free conditioner protect a tankless heater?
- Some salt-free products are sold for this. Ask for independent test results and check whether your heater's warranty accepts them.