Hard Water Physics: Why Tankless Units Scale

To understand maintenance, it helps to understand the thermodynamics of scaling. Lake Michigan water is generally classified as moderately hard, with hardness levels reported by the City of Chicago typically falling into the 7 to 9 grains-per-gallon range. When this water is heated rapidly to 120°F or higher, calcium and magnesium ions precipitate out of solution and form a solid crust known as calcium carbonate.

The Silent Drag on Efficiency

In a traditional tank, these minerals settle harmlessly at the bottom. In a tankless unit, they bake directly onto the fins of the heat exchanger. Scale is a strong insulator — significantly more resistant to heat transfer than the copper or stainless steel it covers. That means the burner has to work longer and hotter to push heat through the scale into the water, which leads to metal fatigue and, eventually, a cracked exchanger.

This is why regular plumbing checkups matter for Chicago and Northwest Indiana residents. Without them, you aren’t just losing efficiency — you’re actively shortening the service life of the appliance.

The Real Cost of Skipping Maintenance

It’s tempting to put off maintenance on a unit that seems to be working. But scale doesn’t announce itself — it builds quietly until the symptoms become impossible to ignore. For more on how maintenance affects long-term lifespan in this region, see our companion guide on how long water heaters last. Here’s what’s actually happening behind the cover panel during years of skipped service.

Year 1 to 2: The Setup

A thin film of mineral begins coating the heat exchanger. Efficiency drops slightly, but you won’t notice it on your gas bill or in your shower. The unit compensates by running its burner a few seconds longer per draw.

Year 3 to 5: The Slow Climb

Scale thickens to a measurable layer. Recovery times lengthen. The unit cycles more frequently. Some homeowners begin reporting “cold water sandwiches” — brief bursts of cold water mid-shower — and minor fluctuations in temperature consistency. Annual gas usage tied to hot water creeps up, although the change is gradual enough that most people attribute it to colder winters.

Year 5 and Beyond: The Failure Cascade

This is where the math gets ugly. Scale buildup begins triggering safety shutdowns. Sensors that depend on accurate temperature readings get confused by the insulating layer. The flame rod, which detects whether combustion is happening correctly, can foul. At a certain point, the heat exchanger develops hot spots, and a hot spot in copper or stainless eventually means a hairline crack. Once the exchanger cracks, the unit is generally not worth repairing.

The fix for all of the above is the same as the prevention: an annual flush. A single hour of maintenance per year prevents the entire failure cascade. Skip it for five years and you’ve effectively traded a routine service call for a full unit replacement — plus the inconvenience of an emergency repair when the unit fails, usually on the coldest weekend of the year.

The Homeowner Maintenance Checklist

You don’t always need a technician for the basics. Keeping the unit clean on the outside and ensuring the right airflow on the inside is something most Chicago homeowners can manage themselves between professional service visits.

Before You Flush: Safety & Setup Check

Before you drag a bucket and a pump down to the basement, run through this short checklist. Every issue here is easier to deal with before you start than during.

• Confirm you have isolation valves. These are color-coded (red for hot, blue for cold) ball valves with service ports, installed between the water lines and the unit itself. If your installer skipped them, a DIY flush is not practical — you’ll need a plumber to install isolation valves first or to flush the unit using their own bypass equipment.
• Inspect for leaks. Look at every fitting, valve, and connection on the unit. Any moisture, mineral staining, or corrosion at a joint means stop and call a plumber. Flushing pressurized lines with damaged fittings is how small problems become flooded basements.
• Ventilate the work area. Even food-grade vinegar produces vapor that’s unpleasant in an enclosed basement. Crack a window or run a small fan if you can.
• Have a plan for the used solution. The vinegar coming out at the end of the cycle is acidic and contains dissolved minerals. Most municipal codes allow it to be flushed down a utility sink, but check your local rules — some Northwest Indiana communities have stricter discharge requirements.
• Verify your manufacturer’s instructions. Manuals from Rinnai, Navien, Noritz, Rheem, and Bosch all have slightly different procedures and approved descalers. A flush done with the wrong solution can affect warranty coverage in many cases.

Mastering the Flush: A Step-by-Step Guide

Flushing is the most important technical skill for a tankless owner. The process circulates a mild acid through the system to chemically dissolve the calcium baked onto the internal surfaces. While you can schedule professional water heater service, many homeowners with the right setup choose to perform this themselves. For an even shorter walkthrough geared toward the basic flush, see our post on how to maintain your tankless water heater.

Required Equipment:

• 1/6 HP submersible pump rated for warm water and mild acid
• Two standard 5-foot utility hoses with garden-hose threaded ends
• 5-gallon bucket (preferably labeled and dedicated to this job)
• 4 gallons of food-grade white vinegar, or a manufacturer-approved descaler

The Procedure:

1. De-energize: Unplug the unit or switch off the dedicated circuit breaker. Turn the gas valve to the off position.
2. Isolate: Close the main cold water inlet and the hot water outlet valves at the isolation valves directly above or below the unit.
3. Relieve pressure: Open the caps on the service ports slowly. Be prepared for a small amount of water to spray.
4. Set up the pump loop: Connect Hose 1 to the cold service port and to the pump. Connect Hose 2 to the hot service port and let its other end hang in the bucket.
5. Circulate: Submerge the pump in the vinegar and plug it in. The vinegar will enter the cold port, travel through the heat exchanger, and exit into the bucket. Run for 45 minutes (longer if your local water is harder).
6. Purge: Dispose of the used solution per local code. Close the cold service port, then open the main cold inlet valve to flush the unit with fresh water for at least 5 minutes, letting it drain out the hot service port into the bucket.
7. Reactivate: Close all service ports, remove hoses, open both main isolation valves, and restore power and gas.

Troubleshooting: Error Codes & Red Flags

A tankless unit’s control board constantly monitors flow, temperature, and combustion. When something goes wrong, it communicates through codes. The exact code numbers vary by manufacturer (Rinnai, Navien, Noritz, Rheem, and Bosch all use different numbering systems), but the categories of problems below show up across nearly every brand. For most Chicago homeowners, these codes are the first sign that maintenance was deferred too long.

Chicago Winters: Protecting Your Exterior Vents

The Windy City presents a unique challenge for tankless venting. Because high-efficiency tankless units produce relatively cool exhaust, that exhaust is typically routed through PVC. The cool exhaust also carries significant moisture.

The Ice Dam Challenge

In sub-zero temperatures, moisture in the exhaust can freeze the moment it hits the cold air, forming icicles on the vent termination. If the ice grows large enough to block the intake or exhaust, the unit will fail-safe and shut down. After every major Chicago snowstorm — and during the deep-freeze stretches in January and February — check your vents (typically located on a side wall of the house) and clear any snow or ice accumulation around them.

The King Tankless Inspection Standard

While a DIY flush handles the scale, it doesn’t address the mechanical health of the unit. King’s NATE-certified technicians perform a deeper inspection of the system’s vital signs during a professional service call:

• Gas pressure manifold test to confirm the unit is receiving the correct BTUs
• Flame rod inspection and cleaning to remove carbon buildup that interferes with reliable ignition
• Combustion analysis testing CO and CO2 levels for safety and efficiency
• Condensate trap cleaning to remove debris that can back up and cause shutdowns
• Expansion tank verification to confirm the system’s pressure management is intact
• Venting inspection for proper slope, secure fittings, and exterior termination clearance

Choosing the Right Plumbing Company

A tankless water heater is one of the few appliances in your home where a service mistake can cause significant water damage, gas safety issues, or both. The company you hire to maintain or replace it matters more than most homeowners realize. A few things to look for, regardless of who you ultimately go with:

Longevity in the local market. A company that’s been around for decades has seen every brand, every failure mode, and every quirk of local housing stock. King has served Chicago and Northwest Indiana since 1968. That means real familiarity with how Lake Michigan water behaves in a heat exchanger, and with the venting realities of older Chicago two-flats and bungalows.

Licensed, bonded, insured plumbers. This protects you if something goes wrong on the job. A handyman or unlicensed installer doesn’t carry the coverage that licensed plumbing work requires — and unpermitted work can complicate insurance claims and home sales down the line.

Transparent pricing in writing. A reputable company quotes the work before they start it, not after. Ask about service fees, hourly rates, and whether the diagnostic call applies toward the repair if you proceed.

24/7 availability. Water heaters fail at inconvenient hours. A company that picks up the phone late on a Sunday night is the one you want when a vent ices over at 2 a.m.

Third-party recognition. Awards and certifications mean an outside party has vetted the company’s work. King has won the BBB Torch Award for Ethics, is a Carrier Factory-Authorized Dealer on the HVAC side, and maintains NATE-certified technicians. None of those credentials on its own proves quality, but together they’re a useful filter.

Reviews that mention specifics. Look for reviews that name technicians, describe the actual work performed, and explain how the company handled any issues that came up. Generic five-star ratings are less informative than detailed three or four-star ones. For more on vetting a service contractor, our post on choosing the right HVAC company applies the same principles to plumbing.

Frequently Asked Questions