Hard Water and Tankless Heaters: The Maintenance Nobody Mentions in Florida

Why Florida water is the problem

Much of South Florida’s water supply is drawn from limestone aquifers, which makes it hard — high in dissolved calcium and magnesium. Heat that water and the minerals come out of solution and deposit as scale on the hottest surfaces they touch.

In a tankless water heater, the hottest surface is the heat exchanger, and the passages through it are narrow by design. Scale that would be a nuisance in a storage tank is a genuine threat here: it insulates the exchanger, restricts flow, forces the burner to work harder for the same output, and eventually causes overheating faults.

What scale actually costs you

• Efficiency. A scaled exchanger transfers heat poorly, so you burn more energy for the same hot water.
• Flow. Restricted passages mean less hot water and unstable temperatures.
• Reliability. Overheat and flow-sensor faults become routine.
• Lifespan. The heat exchanger is the expensive part. Neglect turns a 20-year appliance into a 10-year one.
• Warranty. Many manufacturers require documented descaling. Skip it and a claim can be refused.

That last point catches people out. The maintenance is not optional in the manufacturer’s eyes even if nobody mentioned it at installation.

How often to flush in South Florida

General guidance is annually. In hard-water areas — which covers most of the region — every 6 to 12 months is the realistic interval, and heavy-use households should lean toward the shorter end. If you have a whole-home water softener, you can usually stretch it, but do not skip it.

A descale is not a big job: the unit is isolated at its service valves, a descaling solution is circulated through the exchanger for a while, then flushed and the inlet screen cleaned. If your unit was installed without isolation valves, they are worth adding — without them every future flush is more work than it should be.

The warning signs

• Hot water that fluctuates or goes lukewarm mid-shower
• Reduced hot water flow at fixtures
• Error codes about flow or overheating
• Rumbling or crackling from the unit
• Longer waits for hot water than the unit used to need
• A visible rise in gas or electric use with no change in habits

Beyond flushing

Two things are worth doing alongside it. Clean the inlet water filter screen — it clogs with sediment and mimics scale symptoms. And if the water is genuinely hard, consider treatment upstream: a softener or a scale-reduction system protects the heater, the plumbing, the fixtures and the dishwasher at the same time. It is a bigger conversation than the water heater alone, but for many South Florida homes it is the more economical path.

Is tankless right for your home?

Honestly, sometimes not. Tankless makes sense where you want endless hot water, a smaller footprint and a long service life, and you accept a higher up-front cost plus a maintenance routine. A conventional tank remains a perfectly reasonable choice for many households — lower cost to install, simpler, and generally more forgiving of neglect, though with a shorter life and standby losses.

What we do not recommend is buying tankless for the efficiency claims and then never flushing it. That combination gets you the higher price and the shorter life.