Hard water, scale, and what it does to each heater
Grains per gallon, why heat makes calcium fall out, how softeners eat magnesium anodes, and which maintenance each heater actually needs.
Hard water is water that carries dissolved calcium and magnesium. Heat drives those minerals out of solution. They land on the hottest surface available: the bottom of a gas tank, the sheath of an electric element, the passages of a tankless exchanger, the condenser of a heat pump water heater.
The result is insulation where you wanted heat transfer, noise, slow recovery, and eventually a failed part or a lockout. Softening removes or exchanges those minerals and creates a different problem, because the sodium-softened water is hungrier for a magnesium anode.
This page is the water. The hose procedure is flushing a tank. The pump procedure is descaling a tankless heater. The rod choice is anode rods.
A number you can get without a lab
Hardness is measured in grains per gallon or in milligrams per liter. One grain per gallon is 17.1 milligrams per liter. A test strip or a drop-count kit on the cold inlet, before any softener, is accurate enough to choose a maintenance interval.
Water suppliers publish average hardness and will tell you if your zone differs. Wells vary by season and by which pump is running. Test the well, not the neighbor’s city water.
As a working language, water under about 3 grains is relatively kind to heaters, water around 7 to 10 grains will scale a tankless exchanger on a noticeable schedule, and water above about 10 or 15 grains will punish an undescaled tankless unit and will blanket a tank bottom.
Those bands are maintenance planning, not a regulatory standard. Your heater’s manual may state a maximum hardness for warranty coverage. That number is a contract term.
Exceed it and skip the descale, and the heat-exchanger warranty is the manufacturer’s to deny. Read the warranty before you argue. The structure of those terms is summarized in how long heaters last.
Why the set point matters
Calcium carbonate is less soluble as the temperature rises. A tank or a tankless unit set at 140°F scales faster than the same machine at 120°F, in the same water. The Consumer Product Safety Commission already wants the tap near 120°F because of scalding.
Hard water is a second reason not to “turn it up so the shower lasts.” If you need stored water hotter for a particular reason, a mixing valve holds the tap temperature down while the tank is hotter, and you accept faster scale as a cost you are choosing.
The temperature page is scalds and Legionella. Do not pick 140°F as a scale strategy. It is the opposite.
Scale is not the same as sediment from a dirty well, and it is not the same as rust. Scale is white or tan and brittle. Rust is brown and means steel.
If the flush water is full of rust, you are on discolored hot water, not on a hardness regimen. If the flush water is full of sand, you need a sediment filter ahead of the heater and a well contractor.
A filter does not remove dissolved hardness. It removes particles. People buy a cartridge filter, feel the water is “treated,” and still destroy a tankless exchanger. Dissolved minerals pass through a sediment cartridge as if it were not there.
What each heater does with the same water
A gas storage tank drops scale on the bottom, over the burner plate. Heat has to cross the pile. You hear popping, recovery slows, and the steel at the bottom runs hotter than the designer intended.
An annual flush while the pile is still soft is the answer. A tank that has not been flushed in a decade on 15-grain water may already have a cake the drain valve cannot move.
Plan on inspection, not on a miracle flush. The noise is explained in water heater noises.
An electric element runs extremely hot at the sheath. Scale bakes onto it, the local water boils, and the element sings or fails. Flushing helps the tank bottom and does less for a sheath that is already encased.
Replacing the element without changing the water buys a repeat. Sometimes a heater with a known hard-water kit, or a different element style the manufacturer offers, lasts longer. The electrical limits on element replacement are in electrical safety.
A tankless exchanger has little water volume and a lot of surface area.
Scale closes the passages. The symptom is often seasonal, because winter rise was already using the whole burner, and now part of the surface is insulated. Descale on the manual’s interval, shorter when the strip says the water is hard.
Isolation valves are what make that possible. A softener ahead of a tankless unit is a common and reasonable design when hardness is high, if the softener is maintained and if the anode or the warranty conditions of any remaining tank in the house are adjusted.
Some tankless warranties require a softener above a stated hardness. That is not a sales pitch. It is the condition for keeping the exchanger coverage.
A heat pump water heater still has a tank, so it still collects scale in the tank and on the backup elements, and the condenser that heats the water can foul.
Follow that model’s maintenance sheet. The air filter is a separate chore and does not remove hardness.
People maintain the filter and forget the water. Both matter. The machine is described in heat pump water heaters.
Softeners and the anode
A home ion-exchange softener trades calcium and magnesium for sodium, or sometimes potassium. The water that leaves is softer and more conductive in a way that accelerates corrosion of a magnesium anode.
The practical response is to inspect the rod early and to use the aluminum or aluminum-zinc rod the heater manufacturer recommends for softened water, or a powered anode. Do not disconnect the softener forever because the anode article made you nervous.
Hard water and a missing softener will scale the heater. Softened water and a neglected anode will rust the heater. You pick the maintenance that matches the water you actually have, and you write the date down.
A softener that is undersized or out of salt will let hard water through in slugs, usually when the laundry runs. The heater sees hardness the average test, taken at noon, does not show. If scale is heavy and the softener “should have prevented it,” have the softener capacity checked against the household’s gallons and the water’s grains.
That is a water-treatment calculation, not a heater defect. Resin that has aged or been attacked by chlorine can also stop exchanging. Beads in the piping are a failed softener, and they are mentioned in the discoloration guide because they are visible.
| What you have | Tank | Tankless | Anode note |
|---|---|---|---|
| Soft city water, under about 3 grains | Flush yearly | Descale on the long end of the manual’s range | Factory rod often fine |
| Moderately hard, roughly 5 to 10 grains | Flush yearly or twice a year | Descale at the manual’s shorter interval | Inspect on schedule |
| Very hard, above about 10 grains | Flush often; expect sediment noise if you skip | Service valves and a softener are part of the install, not extras | Check warranty hardness limits |
| Softened water | Still flush; scale risk drops | Often the right pretreatment | Magnesium disappears faster; use the recommended alloy or a powered anode |
| Sand or rust in the flush | Sediment filter or pipe replacement | Screen will clog | Color guide, not just this page |
What not to add to the water
Do not dump acid down a storage tank. Do not hang a magnetic gadget on the pipe and cancel the flush. Independent expectations for those gadgets should be a measured drop in grains, not a testimonial.
If a device does not change a titration test, it did not change the maintenance interval. Do not remove the softener and also stop flushing.
Do not set the softener to a hardness residual “for taste” without telling the person who warranties the tankless exchanger. A residual of several grains is still scale.
If you are buying a heater into known hard water, say the number to the installer. A tankless quote that omits service valves and omits a softener on a 15-grain well is not a complete quote.
The cost guide is where those omitted lines show up as callbacks. A tank quote that omits an anode inspection on a house that already has a softener is the same kind of incomplete job. The heater can be perfect and still be the wrong maintenance plan.
Sources
Manufacturer warranty and maintenance statements on maximum hardness and descaling intervals (Rinnai, rinnai.us; Navien, navieninc.com; A. O. Smith, hotwater.com; Rheem, rheem.com). U.S. Department of Energy, Energy Saver, storage and tankless descriptions, for why scale attacks the two machines differently. U.S. Consumer Product Safety Commission publication 5098, for keeping the set point out of the scald range instead of using temperature as a cleaning method. Local water-utility hardness reports and a home titration kit are the right measurements; a sediment filter specification sheet will show it does not remove dissolved minerals.