Scale in a tankless heater: prevention and descaling

Scale in a tankless heater forms inside the heat exchanger, where the water gets hottest and the passages are narrowest. It reduces flow, makes hot water output erratic, and eventually triggers error codes. Descaling removes it; a softener prevents it. The order matters, because descaling a scaled unit every year is a maintenance bill that never ends.

Why Tankless Units Scale Faster Than Tanks

A tank heater has a large volume of water and a heating surface that scale can coat, but the passages are wide. A tankless unit forces water through a copper or stainless exchanger with channels measured in millimetres. Heat transfer is intense and the water is at its hottest right at the metal.

That combination is exactly what precipitation needs: high temperature plus a surface. Calcium carbonate comes out of solution and adheres. The layer grows inward, narrowing the channel. Flow drops, the unit senses a lower flow rate, and it may derate or shut down to protect itself.

The hardness bands tell you the risk: 0 to 3 GPG soft, 3 to 7 moderate, 7 to 10 hard, above 10 very hard. In the very hard band, annual descaling is not optional maintenance; it is what keeps the unit running.

The Symptoms

Scale does not announce itself. The early signs are easy to attribute to something else:

• Hot water flow at a tap feels weaker than it used to
• The shower starts hot and cools after a few minutes
• Temperature fluctuates when two fixtures run
• The unit displays a flow or overheat error
• The unit runs longer to reach the set temperature

Any of those on a unit more than a year old in hard water is a reason to descale before calling for service. A descale is cheap; a heat exchanger replacement is not.

The Descaling Process

Most tankless units have service valves on the cold inlet and hot outlet specifically for this. The procedure takes about an hour.

1. Shut off the gas or power and close the isolation valves.
2. Connect the pump to the cold service port and a return hose from the hot port into a bucket.
3. Mix the descaler in the bucket per the manufacturer’s ratio; white vinegar works for light scale where the manual allows it.
4. Circulate for 30 to 60 minutes, then flush with clean water until the return runs clear.
5. Restore the valves, reopen water, and run hot to confirm flow and temperature.

Prevention Beats Descaling

The upstream fix is removing hardness before the water enters the heater. A softener does that by ion exchange, and it is the only approach that eliminates the calcium and magnesium rather than altering how they behave.

Installed cost runs $1,200 to $4,500, with salt at $120 to $400 per year. Against that, a plumber descale at $200 every year for ten years is $2,000, and it does not prevent the gradual loss of heat transfer between services. The softener also protects the dishwasher, the washing machine, and the fixtures, which is where the rest of the value sits.

Salt-free conditioners are a different product. They alter crystallization so scale adheres less, which may reduce descaling frequency, but they do not remove hardness from the water. If the goal is to stop descaling entirely, they will not do it. The distinction is in salt-based versus salt-free.

Softener Sizing for a Tankless House

Tankless heaters often appear in high-demand households, which means the softener must handle peak flow as well as capacity. A four-person home at 12 GPG has a daily load of 4 × 75 × 12 = 3,600 grains. Weekly regeneration needs about 25,200 grains, so a 32,000 grain unit covers it.

Peak flow is the second constraint. Two showers plus a dishwasher is about 6.5 gpm, which a 32,000 grain unit in a 10 inch tank with a 3/4 inch valve handles. A larger house with three simultaneous showers needs a 1 inch valve. The split between capacity and flow is covered in capacity versus flow rate.

Five Steps to Protect the Unit

1. Test hardness and record the GPG number before choosing equipment.
2. Install a softener if hardness is 7 GPG or higher, sized on both capacity and peak flow.
3. Descale on the manufacturer’s interval even with a softener, because some scale still forms.
4. Keep the service valves accessible and label them, so descaling does not require moving the unit.
5. Log each descale with the date and the descaler used, which helps at warranty time.

What Not to Do

Do not run a descaling solution through the domestic hot water system; it is for the heater only, connected at the service ports. Do not use a stronger acid than the manual allows, because it can attack the exchanger. Do not skip the clean-water flush, because residual acid will sit in the unit.

Do not assume a softener makes descaling unnecessary forever. It reduces scale dramatically, and the manufacturer’s interval still applies, especially in the very hard band. A softener plus a biennial descale is a reasonable regime; a softener alone with no descale for a decade is not.

If the unit is already erroring on flow, descale first and see whether the error clears before authorising a repair. Many flow errors are scale, not a failed sensor. The same logic applies to softened water and water heater life, where the tank version of this problem is covered.