Hard water, scale, and flushing a tankless unit in Utah County

Hard water carries dissolved calcium and magnesium. Heat a stream of it inside a tankless unit's heat exchanger and some of those minerals drop out as scale.

Published hardness by city

Orem's 2025 report says about 60% of its water is treated surface water and about 40% comes from its own springs and wells, so the hardness at your tap depends on the mix. Lehi, Spanish Fork, and Springville did not include a hardness figure in their latest water reports. If your city isn't on the list, look for its annual water quality report, or test a sample with an inexpensive hardness kit.

To convert, one grain per gallon equals about 17.1 mg/L.

Scale and the heat exchanger

The U.S. Department of Energy's tankless guide says scale "can restrict the flow of water through the heat exchanger and diminish the life of the unit." It also says manufacturers may require a water softener where hardness exceeds 8 to 14 grains per gallon (136 to 239 ppm).

Manufacturers set their own limits. Rinnai's Value Series manual lists a total hardness limit of 200 mg/L, about 11.7 grains per gallon. The upper half of Provo's published range sits above it, and so does American Fork's softener setting.

Rinnai's documents address scale too. Rinnai's Value Series manual says damage caused by lime build-up is not covered by the unit's warranty. Rinnai's SENSEI residential warranty excludes improper maintenance, naming scale build-up, and any damage caused by poor water quality. Other brands write their own terms, so read the document for the model you're buying.

Your treatment options

• Whole-house softener. Exchanges hardness minerals for sodium or potassium across the whole house. It protects the water heater and fixtures, and it needs salt refills and a drain.
• Scale filter or inhibitor at the heater. A cartridge or media unit on the cold line into the tankless unit. It protects only the heater. Before you rely on one, confirm the manufacturer accepts it for its water-quality requirement.
• No treatment. Possible, but plan on the short end of the flushing range and keep records.

How often to flush

DOE's guide says to flush according to the manufacturer's instructions and puts the interval anywhere from twice a year to once every three years, depending on water quality. Harder water calls for more frequent flushing. Rinnai's Value Series units show an LC code, LC0 through LC9, when scale has built up and the heat exchanger needs flushing.

How a flush works

A flush circulates a descaling solution through the heat exchanger to dissolve scale. Rinnai's manual uses white vinegar. The general steps:

1. Turn off power to the unit and close the gas valve.
2. Close the hot and cold isolation valves.
3. Connect one hose from a small submersible pump in a bucket to the cold-side service port, and a second hose from the hot-side service port back into the bucket.
4. Fill the bucket with the descaler the manufacturer specifies.
5. Open the service ports and run the pump for the time the manual gives.
6. Drain the solution, flush with clean water, clean the inlet filter screen, and restore valves, gas, and power.

Follow your own unit's manual, which may differ in steps, solution, and run time. If your unit lacks isolation valves with service ports, each flush becomes a plumbing job, so ask for them on any new install.

Questions to ask a contractor

• What hardness limit does this model's manual list?
• Do you recommend a softener or a scale filter, and why?
• Does the install include isolation valves with service ports?
• How often does the manufacturer want the unit flushed at my hardness?
• Where will the service ports sit, and is there room to set a bucket under them?
• Which descaling solution does the manual call for, and where can I buy it?
• If I add a softener later, does anything about the install need to change?
• Can you show me how to close the isolation valves before you leave?

Whichever treatment you choose, write it into the request so every contractor prices the same setup. A bid with a softener and a bid without one aren't comparable. Note each contractor's answers on the bid itself, so you can see where their plans differ and ask each one about the other's approach.