How often should you descale a tankless water heater?

Short answer

Descale the tankless water heater once a year. With hard water, do it every 6 months, and with a water softener feeding the unit, every 2 years is usually enough. Descaling means pumping vinegar through the heater in a loop for a while to dissolve the mineral scale inside its heat exchanger. It takes 60 to 90 minutes, most of it waiting on the pump, and costs $20 to $150, with the high end being a one-time descaling pump kit you reuse every year.

Why it matters

A tankless heater has no tank to hold sediment, so people assume it needs no cleaning. The minerals are still in the water, though, and they come out of it in the one place the water gets hot: the heat exchanger, a coil of narrow passages wrapped around the burner or the heating elements. Scale forms on the inside of those passages like the crust in a kettle.

Scale is an insulator. As it thickens, the burner has to run hotter and longer to push the same heat through it into the water. Efficiency drops, the metal of the exchanger runs hotter than it was designed to, and the passages narrow until the flow drops and the unit starts throwing error codes. Left long enough, the exchanger cracks or clogs outright, and that repair costs a large share of a new unit. Most makers also void the warranty on a unit that wasn't descaled, and they can tell from the state of the exchanger. A yearly flush is what keeps a tankless heater running its full 20 years instead of giving up at the same age as a tank.

When to do it more often

Hard water: every 6 months. If your faucets and showerheads crust over, so does the heat exchanger, only faster, because that's where the water gets hot.
A water softener upstream: every 2 years is usually enough. Softened water leaves far less scale behind, though it still leaves some.
A unit that already shows temperature swings or a flow drop: descale now, then settle into the schedule.

What you need

A submersible pump with hoses, sold as a descaling kit, that circulates vinegar through the unit
A bucket to hold the vinegar and catch the return flow
White vinegar, enough to fill the bucket a few inches deep, or a descaling solution made for tankless heaters

Steps at a glance

Turn the unit off and shut off its gas or electrical supply, then close the cold inlet and hot outlet valves.
Connect the pump's hoses to the service valves on the inlet and outlet lines, with the pump in the bucket.
Fill the bucket with vinegar and open the service valves so the vinegar can flow through the heater.
Run the pump so vinegar circulates through the exchanger and back into the bucket.
Shut off the pump, drain the vinegar, and flush the unit with clean water through the same hoses.
Close the service valves, open the inlet and outlet valves, and restore gas or power.
Clean the inlet filter screen, then run a hot tap until the water flows clear and steady.

You're done when a hot tap runs at full flow with a steady temperature and the unit shows no error.

Signs it's overdue

Water temperature that swings from hot to cool and back while a tap is running.
Lower hot water flow than you had when the unit was new.
Error codes on the unit's display, often one that points at the exchanger or the flow sensor.
A longer wait for hot water to reach the tap.

DIY or pro

This is a moderate job. The only tricky part is the first time, when you're finding the service valves and working out how the hoses connect. Once the kit is set up, each year's flush is a matter of connecting the hoses and turning on the pump. If your unit has no service valves, which is common on older installations, have a plumber add them, because without them the job can't be done.

A plumber charges $150 to $300 to descale a tankless heater, and most will look over the venting, the gas connection and the inlet filter while they're there. If you'd rather not handle the gas valve, or the unit is somewhere awkward, that's good value once a year. Otherwise the pump kit pays for itself on the second flush.

Safety

Gas: shut off the gas valve to the unit before you open any water connection, and don't restore it until the water lines are reconnected and the valves are open. If you smell gas when you turn it back on, shut it off again and call the gas company.
Electricity: an electric tankless unit draws a lot of power. Switch off its breaker before you start, and don't turn it back on until the unit is full of water, since the elements burn out in seconds if they heat dry. A gas unit also has an electrical supply for its controls, so unplug or switch that off too.

FAQ

Can I descale a tankless water heater myself?

Yes, with a descaling kit and service valves on the unit. The pump does the work, and the steps are the same every year.

How long does descaling take?

Plan on 60 to 90 minutes. The vinegar needs time circulating to dissolve the scale, so most of that is waiting.

Do I need to descale if I have a water softener?

Yes, but less often. A softener removes most of the minerals, so every 2 years is usually enough.

What happens if you don't descale a tankless water heater?

Scale insulates the heat exchanger, efficiency falls, the flow drops, and the exchanger eventually clogs or cracks. The warranty is usually void by then.