Tankless water heaters are famous for delivering hot water on demand, saving space, and (often) lowering energy use compared to traditional tanks. But here’s the tradeoff many homeowners don’t hear enough about: a tankless unit is a high-performance appliance with narrow waterways, sensors, and safety systems that need regular attention. Skip maintenance long enough and you can end up with the one thing you bought a tankless heater to avoid—an unexpected no-hot-water day.

This checklist is designed to be practical. You’ll learn what to do monthly, seasonally, and annually, what “normal” looks like, and which warning signs mean it’s time to call a pro. Whether your unit is natural gas, propane, or electric, the goal is the same: keep heat exchange efficient, prevent scale buildup, and catch small issues before they become big outages.

Why tankless units still need regular upkeep

A tankless heater doesn’t store hot water, but it does store problems if you let them accumulate. Minerals in water (especially calcium and magnesium) can deposit inside the heat exchanger. Over time, scale narrows the passages where water flows, forcing the unit to work harder, run hotter, and potentially shut down for safety.

Unlike a tank heater that can “limp along” while slowly losing performance, tankless systems are more likely to protect themselves by throwing error codes or refusing to fire when conditions aren’t right. That’s good for safety, but it’s frustrating when you’re mid-shower. Regular maintenance is basically your insurance policy against those surprise lockouts.

It’s also worth remembering that many manufacturers tie warranty coverage to proper maintenance. Keeping records of flushes, filter cleanings, and professional inspections can matter if you ever need a warranty claim.

Know your setup before you touch anything

Before you start checking boxes, take five minutes to identify what you have. Look for the brand/model label, fuel type (gas or electric), and whether your unit is indoor or outdoor rated. Indoor gas units often rely on specific venting and combustion air requirements; outdoor units have different exposure risks like freezing and debris.

Next, locate the service valves. Many tankless installations include isolation valves and a flush port kit (hot and cold). If you don’t have those valves, descaling becomes much harder and often requires a plumber to retrofit them. Also locate the inlet screen/filter, the condensate line (for condensing models), and the electrical disconnect or breaker.

If you’re unsure what’s safe for DIY and what isn’t, that’s normal. Gas, venting, and combustion adjustments are not casual weekend tasks. You can still do a lot as a homeowner—like cleaning inlet filters and monitoring performance—while leaving combustion and gas-related checks to licensed professionals.

The maintenance rhythm that prevents most no-hot-water days

Think of tankless maintenance like car care: a few quick checks frequently, plus one deeper service on a schedule. The quick checks catch obvious issues (restricted airflow, clogged filters, condensate problems). The deeper service (descaling) protects the heat exchanger, which is the heart of the system.

Below is a simple rhythm that works for most households:

• Monthly: quick visual checks, listen for changes, confirm stable temperature
• Every 3–6 months: clean the inlet filter and inspect vent/air intake
• Annually (or more often with hard water): flush/descale, deeper inspection, test safety components

If your home has hard water, a large family, or frequent hot-water use (laundry, big tubs, multiple showers), you’ll want to lean toward the more frequent end of that range.

Monthly quick checks that take less than 10 minutes

Watch for temperature swings and “cold sandwich” patterns

Tankless heaters can sometimes produce a “cold water sandwich”—a brief burst of cooler water between hot draws. A little of this can be normal depending on plumbing layout, but noticeable changes are your clue that something’s shifting: scale buildup, a dirty filter, or a sensor starting to misread.

Pick one fixture (like the main shower) and mentally note how long it takes to reach your usual temperature and how stable it stays. If it’s taking longer than it used to, or if it surges hot/cold, you’ve got a reason to investigate before it turns into a full shutdown.

If your unit has a digital controller, check the set temperature and confirm it hasn’t been changed accidentally. Many homes do best around 120°F (49°C), but your needs may differ—especially if you have mixing valves or specific health considerations.

Look for error codes and keep a simple log

Even if hot water is still working, error codes that appear and disappear are early warnings. Write down the code and the date. A repeating pattern (even once a month) can point to a venting issue, ignition problem, flame sensing, overheating, or flow sensor trouble.

Most manufacturers publish code charts in the manual. If you don’t have the manual, you can usually find it online using the model number. The key is not to ignore intermittent codes—those are often the easiest to fix before they become constant.

A basic log can be as simple as a note in your phone: “Jan 12: Code 11 once, reset, fine.” When a technician shows up, that history is gold.

Check around the unit for moisture, corrosion, or odd smells

Tankless heaters shouldn’t be surrounded by puddles, dripping fittings, or rust trails. A small leak can drip onto sensors or circuit boards and cause strange, intermittent failures. If you see moisture, don’t just wipe it up—trace it to the source.

Pay attention to smells. A musty smell could indicate a condensate issue (condensing unit) or water sitting where it shouldn’t. A gas smell is an immediate stop-and-call situation—leave the area and follow your local gas safety guidance.

Also make sure the area around the unit stays clear. Storing paint, chemicals, or clutter near a gas appliance can cause corrosion or airflow problems over time.

Every 3–6 months: airflow, filters, and flow rate basics

Clean the cold-water inlet filter (screen)

Most tankless units have a small screen filter on the cold-water inlet. Its job is to catch debris before it reaches the heat exchanger. Over time, that screen can clog and reduce flow, which can lead to ignition issues or temperature instability.

Turn off power to the unit, shut the water supply valve, relieve pressure by opening a hot tap, then remove the filter carefully. Rinse it under running water and use a soft brush if needed—no harsh chemicals. Reinstall it snugly (don’t overtighten), restore water, and check for leaks.

If you find a lot of grit or sediment, that’s a sign your home may benefit from a whole-house sediment filter. Less debris entering the unit means fewer nuisance problems and a longer service life.

Inspect the air intake and exhaust venting (especially for gas units)

Gas tankless units need clean airflow and proper venting to burn efficiently and safely. Leaves, lint, dust, or even small nests can restrict air intake or exhaust. Outdoors, wind-driven debris can accumulate around the termination. Indoors, laundry-room lint is a common culprit.

Do a visual inspection: make sure vent pipes are properly connected, supported, and free of obvious damage. Check the vent termination outside—clear any debris and confirm there’s no snow buildup in winter climates.

If you notice soot, discoloration, or a strong exhaust smell, don’t keep troubleshooting on your own. Those can be signs of combustion problems that require professional testing.

Confirm hot-water flow feels “normal” at multiple fixtures

Tankless heaters rely on minimum flow to activate. If your flow drops below the activation threshold—because of clogged aerators, partially closed valves, or plumbing restrictions—the unit may cycle on and off or not fire at all.

Check a couple of faucets and showers. If one fixture is weak, clean the aerator or showerhead. If the whole house seems weaker, the inlet filter, a valve, or a bigger plumbing issue may be involved.

Flow issues are one of the sneakiest causes of “sometimes hot, sometimes not” complaints. Catching them early can save you from chasing the wrong problem.

Annual (or semi-annual) deep maintenance: the descaling flush

Why flushing matters more than almost anything else

If you only do one maintenance task, make it descaling. Scale acts like insulation inside the heat exchanger. That means the burner (or heating elements) must work harder to transfer the same heat into the water. Efficiency drops, temperatures fluctuate, and the unit can overheat and shut down to protect itself.

Hard water accelerates this dramatically. Some homes need flushing every 6 months; others can go annually. If you see white mineral crust on faucets, showerheads, or inside kettles, assume your tankless heater is seeing the same minerals.

Flushing also helps maintain consistent performance—hot water that arrives faster, stays steadier, and doesn’t force the unit to run at the edge of its capacity.

What you’ll need for a standard flush

Most DIY flushes use a small pump, two short hoses, a bucket, and a descaling solution (often food-grade vinegar or a manufacturer-approved cleaner). You’ll also need isolation valves/flush ports already installed. If you don’t have them, talk to a plumber about adding a service valve kit.

Plan for about 60–90 minutes the first time you do it, including setup and cleanup. After that, it’s usually faster. Always follow your manufacturer’s instructions—some brands specify certain cleaners and flush times.

And if your unit is under warranty, check what the manufacturer recommends so you don’t accidentally void coverage with an unapproved chemical.

Flush steps at a high level (and where people slip up)

The general flow is: power off, close water valves, connect hoses to the service ports, circulate descaler through the unit for the recommended time, then rinse thoroughly and restore normal plumbing. The most common slip-ups are forgetting to fully rinse, leaving a valve partially closed afterward, or failing to check for leaks when you return the unit to service.

After the flush, run hot water at a faucet for a few minutes to clear air and confirm stable temperature. If you have a controller, check for any codes. A properly flushed unit often sounds “smoother,” with less strain during heating.

If the flush water comes out extremely cloudy or gritty, that’s a sign you’re dealing with heavy buildup. In those cases, a professional may recommend additional cleaning or a water treatment upgrade to keep the problem from returning quickly.

Condensing tankless units: don’t ignore the condensate system

Understand what condensate is and why it can shut you down

Condensing tankless heaters squeeze extra efficiency by extracting more heat from exhaust gases. The byproduct is condensate—slightly acidic water that drains through a condensate line (often through a neutralizer kit) to a drain.

If that line clogs, freezes, or backs up, the unit may shut down to prevent damage or unsafe operation. Homeowners sometimes mistake this for a “heater problem” when it’s really a drainage problem.

Keeping the condensate path clear is one of the easiest ways to prevent random error codes and shutdowns, especially in colder months.

Check the condensate drain line for kinks, clogs, and freezing risk

Visually inspect the line from the unit to the drain. Make sure it slopes correctly and isn’t kinked or pinched behind storage items. If you see algae or buildup in clear tubing, it may be time to clean or replace that section.

In climates where freezing is possible, confirm the drain routing is protected. Frozen condensate lines can crack or block flow, and that can mean no hot water until thawed and repaired.

If you have a condensate pump, listen for it cycling during hot-water use. A pump that’s silent when it should be running (or running constantly) needs attention.

Inspect or replace the condensate neutralizer media if installed

Some systems include a neutralizer cartridge filled with media that reduces acidity before draining. Over time, that media dissolves and becomes less effective. If it’s depleted, you may see corrosion risks downstream or fail local code requirements.

Check your manual or ask your installer how often it should be replaced. Many homeowners forget this component exists until a drain line or fitting corrodes.

Keeping the condensate system healthy is a small task that protects both the heater and the plumbing around it.

Gas tankless safety checks that are worth scheduling with a pro

Combustion quality and venting verification

Gas appliances need the right fuel-to-air mix. If it’s off, you can get poor efficiency, nuisance shutdowns, or unsafe combustion byproducts. While homeowners can do visual checks, proper combustion analysis requires specialized tools.

A professional can verify gas pressure, combustion readings, vent integrity, and that the unit is operating within manufacturer specs. This is particularly important if you’ve remodeled, enclosed a mechanical room, or changed ventilation in the home.

If you ever notice headaches, unusual odors, or soot near the unit, treat it as urgent and get it checked.

Ignition components, flame sensor, and burner cleaning

Over time, burners and ignition components can accumulate dust or residue, especially in laundry rooms or garages. That can cause delayed ignition, rumbling sounds, or intermittent failure to light—classic “it works sometimes” complaints.

A technician can clean burners properly, check flame rods/sensors, and confirm safe ignition. This kind of service often resolves recurring error codes that don’t show up consistently.

It’s also a good time to inspect gaskets and seals, which can harden with age and heat cycles.

Gas line sizing and supply stability (often overlooked)

Tankless heaters can demand high BTUs when firing at full capacity. If the gas line is undersized, you may get hot water at low demand but failures when multiple fixtures run at once. This can feel like “random” cold water events.

A pro can confirm the line is properly sized for your total gas load (furnace, stove, fireplace, etc.). If you’ve added appliances since the heater was installed, this check matters even more.

Fixing supply stability can transform performance—especially in winter when overall gas demand is higher.

Electric tankless maintenance: different focus, same goal

Scale prevention still matters for electric models

Electric tankless heaters don’t have burners or venting, but they still have heat exchangers and heating elements that can scale up. Mineral buildup can reduce heat transfer and cause the unit to work harder for the same output.

Descaling flushes are still relevant, particularly in hard-water areas. The result is steadier temperature and less strain on components.

If you notice the unit struggling to reach set temperature, scale is one of the first things to rule out.

Electrical connections and breaker behavior

Electric tankless units often require multiple high-amperage breakers. If a breaker trips, you may get lukewarm water (partial heating) rather than a total shutdown, depending on how the unit is staged.

Pay attention to any breaker that trips more than once. Repeated trips can indicate a failing element, loose connection, or an electrical supply issue. Those are not DIY guess-and-check situations—get an electrician or qualified technician involved.

Also, make sure the unit’s area stays dry. Moisture and high-amperage electrical components don’t mix.

Flow sensor and inlet filter checks

Just like gas units, electric tankless heaters rely on flow sensing to activate. A dirty inlet filter or debris can reduce flow and cause cycling or inconsistent heating.

Cleaning the inlet screen and checking fixture aerators can prevent a lot of “it suddenly went cold” complaints, especially in older homes with sediment in the plumbing.

When performance changes, start with the simple flow-related checks before assuming the unit is failing.

Water quality upgrades that make maintenance easier

When a water softener makes sense

If your home has hard water, a softener can dramatically reduce scale formation. That doesn’t mean you’ll never flush again, but it often extends the time between flushes and keeps efficiency higher over the long run.

Softened water also helps faucets, showerheads, dishwashers, and washing machines. If you’re already battling mineral crust around the house, the tankless heater is just one victim of the same water chemistry.

If you’re unsure, you can start with a simple water hardness test strip and decide from there.

Consider a sediment filter if you see grit or frequent clogs

Not all water problems are about hardness. Some neighborhoods deal with sediment—especially after municipal work or in homes with older pipes. Sediment can clog inlet screens, reduce flow, and wear components.

A whole-house sediment filter is relatively straightforward and can reduce the frequency of inlet filter cleanings. It also protects other appliances and valves throughout the home.

If your inlet screen is dirty every time you check it, a sediment filter is often the “fix the cause” move rather than “keep cleaning the symptom.”

Flush frequency based on real-life signals

If you want a simple rule: flush at least annually, and flush more often if you notice longer time-to-hot, temperature instability, or recurring error codes tied to overheating/flow. Water quality isn’t static—seasonal changes and municipal adjustments can shift mineral behavior.

Also consider lifestyle changes. A new baby, guests, a basement suite, or a renovated bathroom can increase hot-water demand and push the unit harder.

Matching maintenance frequency to actual usage is one of the best ways to avoid surprise outages.

How to spot “call a pro” moments before you lose hot water

Repeated resets, lockouts, or error codes that return

If you’re resetting the unit more than once in a blue moon, something is going on. A reset can clear a temporary glitch, but it shouldn’t become part of your routine.

Recurring codes often point to a root cause: scale, venting restriction, gas pressure fluctuation, ignition issues, or sensor problems. The sooner you address it, the less likely you are to face a total shutdown on a busy morning.

Bring your error-code log to the appointment—it speeds up diagnosis and reduces trial-and-error.

Water leaks, corrosion, or signs of overheating

Any leak around a tankless unit deserves attention. Even small drips can damage electronics or create corrosion that spreads. Corrosion on fittings may also indicate galvanic issues or condensate problems (for condensing units).

Signs of overheating can include unusually hot casing surfaces, a burning smell, or frequent high-temperature shutdown codes. Overheating is often linked to scale, restricted flow, or airflow/venting problems.

These are not “wait and see” situations. Addressing them early can prevent more expensive repairs later.

Major changes in performance during high demand

If hot water is fine when one faucet runs but fails when two showers run, you could be hitting capacity limits—or you could have a supply issue (gas line sizing, electrical staging, or flow restrictions). A professional can test the unit under load and determine whether it’s a maintenance issue, a sizing issue, or both.

Sometimes the fix is simple (descale, clean filter, clear vent). Other times, it’s about setting realistic expectations for what the unit can deliver simultaneously.

Either way, diagnosing under real conditions is the fastest path to a reliable solution.

Building a checklist you’ll actually follow

Create a recurring calendar reminder

Maintenance plans fail because life gets busy. Put reminders on your calendar: a monthly quick check, a 3–6 month filter clean, and a yearly flush. If you prefer seasons, tie tasks to something you already do—like changing HVAC filters or switching smoke alarm batteries.

Consistency beats intensity. A 10-minute monthly habit often prevents the “two hours of panic troubleshooting” scenario later.

If you rent out part of your home, consider doing checks a bit more often—tenant usage patterns can be unpredictable.

Keep a small maintenance kit near the unit

If you plan to flush yourself, keep the pump, hoses, and bucket together. Label them “water heater only” so they don’t walk away to other projects. Having everything in one place makes you far more likely to do the flush on schedule.

Also keep a flashlight and a small soft brush for the inlet filter. These simple tools remove friction from the process.

And store your model number, manual link, and error code chart in a note on your phone for quick reference.

Decide what you DIY and what you delegate

Many homeowners comfortably handle inlet filter cleaning and visual checks. Descaling can be DIY if you have isolation valves and follow instructions carefully. But combustion analysis, gas pressure checks, and venting verification should be handled by a qualified technician.

If you’d rather have a single provider handle the full routine, look for teams that offer complete water heater services so you’re not coordinating multiple appointments when something goes wrong.

And if you’re in Arizona and want a local expert who handles diagnostics, maintenance, and repairs, working with a plumbing company in Phoenix that’s familiar with tankless systems can make the whole process smoother—especially when you need help quickly.

Maintenance checklist you can print (and stick near the unit)

Monthly

1) Temperature stability check: Run hot water at a regular fixture and note time-to-hot and steadiness. If it’s changing, investigate.

2) Error code scan: Check controller/display for codes. Log any code, even if it clears.

3) Visual inspection: Look for leaks, corrosion, moisture, or unusual smells. Keep the area clear of clutter.

Every 3–6 months

4) Clean inlet filter: Power off, shut water, relieve pressure, remove and rinse screen, reinstall, check for leaks.

5) Air intake/vent check (gas units): Clear debris at termination points. Visually confirm vent connections look secure.

6) Flow sanity check: Confirm multiple fixtures have normal flow. Clean aerators/showerheads if needed.

Annually (or every 6 months with hard water)

7) Descale flush: Circulate approved descaler through the unit, then rinse thoroughly. Confirm valves are returned to normal positions.

8) Condensate system inspection (condensing units): Verify drain line slope, clear clogs, check pump function, replace neutralizer media if applicable.

9) Professional inspection (recommended for gas units): Combustion analysis, gas pressure check, burner/ignition cleaning, vent verification.

Planning ahead: maintenance that pairs well with upgrades

When it’s time to think about a new unit

If your tankless heater is older, frequently locking out, or struggling to meet demand even after descaling and filter cleaning, it may be nearing the point where repairs become less cost-effective. Technology also improves over time—newer models can offer better modulation, efficiency, and smarter diagnostics.

Another sign is repeated component replacement: flow sensors, control boards, ignition assemblies, or persistent heat exchanger issues. At some point, you’re paying for downtime and inconvenience as much as parts.

When replacement becomes the smart move, getting the sizing right matters as much as the brand. A properly sized unit matched to your fixtures and usage patterns is the difference between “endless hot water” and “why is the shower cold when the dishwasher runs?”

Installation quality affects maintenance frequency

A well-installed tankless system is easier to maintain. Isolation valves make flushing simple. Correct venting prevents combustion issues. Proper gas/electrical supply prevents nuisance shutdowns. Even the placement of the unit—protected from debris, freezing, and tight airflow—reduces headaches.

If you’re upgrading or moving a unit, it’s worth working with a team that specializes in tankless water heater installation so the system is set up for long-term reliability, not just “it works today.”

Good installation is also what makes your maintenance checklist realistic. When the service ports are accessible and the drain routing is correct, you’re far more likely to keep up with care.

Small habits that protect hot water all year

Beyond scheduled maintenance, a few habits help: avoid setting the temperature unnecessarily high, fix dripping hot taps (they can cause short-cycling in some setups), and clean fixture aerators when you notice flow changes.

In colder regions, protect exposed piping and pay attention to freeze warnings for outdoor units. In dusty environments, keep the area around the unit clean so intake screens and internal components don’t load up with debris.

These are small moves, but they add up to fewer error codes, fewer cold surprises, and a longer-lived heater.

No-hot-water days are usually preventable

Most tankless “failures” start as minor issues: a slightly clogged inlet filter, scale slowly building up, a vent termination getting blocked, or a condensate line starting to back up. The good news is that these problems rarely appear out of nowhere—you typically get warning signs first.

Use the checklist above to keep tabs on performance, stay ahead of scale, and treat error codes as early alerts rather than annoyances. With a little consistency, your tankless heater can do what it was meant to do: deliver dependable hot water, day after day, without the surprise cold-shower moments.