Common Tankless Water Heater Problems and How to Prevent Them

Tankless water heaters are a great fit for many Crestwood and Chicagoland homes. They heat water only when you need it, take up less space than a traditional tank water heater, and can provide a steady supply of hot water when the system is properly sized, installed, and maintained.

But like any high-efficiency appliance, a tankless water heater is not a “set it and forget it” system. When homeowners call about tankless water heater problems, the issue is often related to mineral buildup, improper sizing, venting problems, gas supply issues, lack of maintenance, or installation that was not done correctly.

The good news is that many of the most common tankless water heater problems can be prevented with the right setup and routine service.

At MR Plumbing Heating Air Electrical, we help homeowners throughout Crestwood and the Greater Chicagoland area repair, maintain, and replace tankless water heaters. If your hot water is inconsistent, your unit keeps shutting off, or you are wondering whether a tankless system is still the right choice for your home, here is what you should know.

How Tankless Water Heaters Work

A tankless water heater, also called an on-demand water heater, heats water as it passes through the unit. Instead of storing 40, 50, or more gallons of hot water in a tank, the system activates when you open a hot water faucet, start a shower, or run an appliance that needs hot water.

Inside the unit, either a gas burner or electric heating element heats the water as it flows through a heat exchanger. When you turn the hot water off, the unit shuts down.

This design can reduce standby energy loss because the system is not constantly reheating a storage tank. It can also help a properly sized system deliver hot water for longer periods of time. However, tankless systems still have limits. They must be sized for your home’s flow rate, incoming water temperature, fuel supply, venting requirements, and household usage.

1. Mineral Buildup Inside the Tankless Water Heater

Mineral buildup is one of the most common causes of tankless water heater problems. If your home has hard water, minerals such as calcium and magnesium can collect inside the heat exchanger. Over time, that scale can restrict water flow, reduce efficiency, trigger error codes, and make the unit work harder than it should.

Signs of mineral buildup may include:

• Water that is not getting as hot as it used to
• Fluctuating hot water temperatures
• Lower hot water pressure
• The unit shutting down during use
• More frequent error codes
• Higher energy bills

In the Chicagoland area, hard water can be tough on plumbing fixtures, appliances, and water heaters. If your faucets have white scale, your showerhead clogs often, or your glassware comes out spotty, your tankless water heater may also be dealing with mineral accumulation.

How to prevent mineral buildup

Most tankless water heaters should be flushed on a regular schedule using the manufacturer’s recommended process. This usually involves circulating a cleaning solution through the unit to dissolve scale inside the heat exchanger.

For many homes, annual flushing is a good starting point. Homes with harder water, heavier hot water usage, or older equipment may need more frequent maintenance. If mineral buildup is a recurring problem, MR Plumbing may recommend a water softener or water treatment option to help protect the system.

Ignoring scale buildup can shorten the life of the water heater and turn a simple maintenance visit into a more expensive repair.

2. The Tankless Water Heater Was Not Sized Correctly

A tankless water heater does not run out of stored hot water like a tank unit, but it can still be overwhelmed by too much demand at one time.

Every tankless system has a maximum flow rate, usually measured in gallons per minute. If multiple showers, a dishwasher, a washing machine, and several sinks are all pulling hot water at once, the unit may not be able to keep up.

This can lead to:

• Warm water instead of hot water
• Temperature drops during a shower
• The water heater shutting down
• Hot water that works fine at one fixture but not when multiple fixtures are running

Proper sizing is especially important in Northern Illinois because winter groundwater temperatures can be cold. The colder the incoming water is, the harder the tankless unit must work to raise it to your desired hot water temperature.

How to prevent sizing problems

Before installing a tankless water heater, a qualified plumber should look at more than the number of bathrooms in the home. The right system depends on how your household actually uses hot water.

A professional sizing evaluation should consider:

• How many showers may run at the same time
• Whether the washing machine and dishwasher are often used during showers
• The number of people in the home
• The home’s incoming cold water temperature
• The required temperature rise
• The unit’s flow rate at that temperature rise
• Gas line capacity or electrical capacity
• Whether one unit or multiple units are needed

If your tankless water heater has never performed well, the problem may not be the brand. It may be undersized for your household.

3. Inconsistent Hot Water Temperature

Few tankless water heater problems are more frustrating than a shower that keeps switching from hot to lukewarm or cold. Inconsistent temperature can have several causes, so it is important to diagnose the system carefully instead of guessing.

Common causes include:

• Mineral scale inside the heat exchanger
• A clogged inlet filter
• Too much hot water demand at once
• A dirty flame sensor or burner issue on a gas unit
• Low gas pressure
• Improper venting
• A failing flow sensor
• Plumbing cross-connections that allow cold water to mix into hot lines

One common complaint is sometimes called the “cold water sandwich.” This happens when hot water is used, turned off briefly, then turned back on. A small pocket of cooler water may pass through before the unit fully reignites and stabilizes.

How to prevent temperature swings

Routine tankless maintenance can help keep water temperature more stable. The technician should clean or inspect key components, check the filter, look for scale buildup, test operation, and confirm that the unit is firing correctly.

If temperature swings happen only when several fixtures are used at once, the issue may be capacity. If the problem happens at one fixture only, the cause may be a faucet cartridge, mixing valve, shower valve, or fixture-specific issue rather than the water heater itself.

4. Low Water Pressure or Reduced Hot Water Flow

If your cold water pressure seems normal but your hot water pressure is weak, the tankless water heater may be part of the problem. Tankless units have filters and narrow internal passages that can be affected by debris, scale, or sediment.

Possible causes include:

• A clogged inlet filter
• Mineral buildup in the heat exchanger
• Debris from plumbing work
• Partially closed valves
• Undersized or restricted piping
• Fixture aerators clogged with sediment

Low flow can also prevent the unit from activating. Tankless systems need to sense enough water movement before they turn on. If the flow is too low, you may get little or no hot water at the fixture.

How to prevent low-flow issues

Filters should be checked and cleaned as part of regular tankless maintenance. If mineral buildup is present, the unit may need to be flushed. If the issue started after plumbing work, a technician can inspect the valves, lines, and fixtures for debris or restrictions.

If the problem is only at one sink or shower, the fix may be as simple as cleaning the aerator or showerhead. If the issue affects the whole house, the water heater and main plumbing system should be inspected.

5. Ignition Failure or Flame Failure

Gas tankless water heaters rely on proper ignition and combustion. If the unit cannot ignite or cannot maintain a stable flame, it may shut down and display an error code.

Common causes include:

• Gas valve issues
• Low gas pressure
• An undersized gas line
• Air in the gas line
• A dirty burner
• A dirty or faulty flame sensor
• Blocked intake or exhaust vents
• Condensate drainage problems on condensing units

This is not a problem to ignore. If a gas tankless water heater is not burning properly, it needs professional attention.

How to prevent ignition problems

A tankless water heater should be installed with the correct gas line size, regulator setup, venting, clearances, and combustion air requirements. During maintenance, the technician can inspect the burner, flame sensor, venting, and error code history.

If you smell gas, leave the area and call your gas utility or emergency services right away. Do not try to relight or repair the unit yourself.

6. Improper Venting

Venting is one of the most important parts of a gas tankless water heater installation. The unit must safely bring in combustion air and exhaust gases out of the home. If the venting is blocked, undersized, improperly pitched, or installed with the wrong materials, the water heater may shut down or operate unsafely.

Improper venting can cause:

• Frequent shutdowns
• Error codes
• Combustion problems
• Condensation issues
• Reduced efficiency
• Unsafe exhaust conditions

In Chicagoland winters, vent terminations also need to be positioned and maintained so they are not blocked by snow, ice, leaves, or debris.

How to prevent venting problems

Tankless water heater venting should always follow manufacturer specifications and local code requirements. A licensed plumber can verify that the system has the correct vent type, slope, length, termination, clearances, and combustion air setup.

If your tankless water heater shuts down during windy weather, cold weather, or heavy snow, the venting and exterior termination should be inspected.

7. Corrosion and Condensation Problems

Tankless water heaters are less likely to fail from tank corrosion because they do not store a large volume of water. However, corrosion can still damage components, especially if condensation is not draining correctly or if water is leaking inside or around the unit.

Corrosion may appear near:

• Water connections
• The heat exchanger
• The burner area
• Condensate lines
• Vent connections
• Service valves

Small leaks should never be ignored. Moisture inside a tankless water heater can damage electrical components, affect gas combustion, and lead to premature failure.

How to prevent corrosion

Routine inspection helps catch leaks, condensation problems, and corrosion early. On condensing tankless units, the condensate drain should be checked to make sure acidic condensate is being removed properly and safely.

If you see rust, water stains, green or white crust around fittings, or moisture below the unit, schedule service before the problem gets worse.

8. Error Codes That Keep Coming Back

Modern tankless water heaters are designed to protect themselves. When something is wrong, the unit may display an error code and shut down. Resetting the system may temporarily restore hot water, but recurring error codes usually point to an underlying issue.

Depending on the brand and model, error codes may relate to:

• Ignition failure
• Flame failure
• Overheating
• Blocked exhaust
• Scale buildup
• Sensor problems
• Water flow problems
• Electrical or control board issues

It is helpful to write down the error code before turning the system off. That information can help your plumber diagnose the problem more efficiently.

How to prevent recurring error codes

Do not keep resetting the unit without finding out why the code is appearing. A professional technician can check the manufacturer’s code chart, inspect the system, test water flow, check gas pressure, inspect the venting, and look for maintenance issues.

Some error codes are caused by simple maintenance problems. Others involve gas, exhaust, electrical components, or safety controls and should be handled by a licensed professional.

9. Lack of Routine Tankless Water Heater Maintenance

Many tankless water heater problems come down to one issue: the unit has not been maintained. Tankless systems are efficient and reliable, but they still need regular service to keep working properly.

A professional tankless water heater maintenance visit may include:

• Flushing the heat exchanger
• Cleaning the inlet filter
• Checking for leaks or corrosion
• Inspecting venting and air intake
• Checking condensate drainage
• Testing operation and temperature output
• Reviewing error codes
• Inspecting valves and connections
• Checking gas operation on gas-fired units

Skipping maintenance can reduce efficiency, increase repair costs, shorten equipment life, and make hot water less reliable.

When Should You Call a Plumber for Tankless Water Heater Problems?

Some simple issues, such as checking whether a breaker has tripped or making sure a valve is fully open, may be safe for a homeowner to review. But many tankless water heater problems involve gas, electrical components, venting, combustion, water pressure, or manufacturer-specific diagnostics.

Call a professional plumber if you notice:

• No hot water
• Hot water that turns cold during use
• Error codes on the water heater
• Water leaking from or around the unit
• Rust, corrosion, or water stains
• Low hot water pressure throughout the home
• The unit shutting off repeatedly
• Unusual noises during operation
• A gas smell near the unit
• Hot water problems after a recent installation

If your tankless water heater is older, repairs are becoming more frequent, or the unit was never sized properly, replacement may be the smarter long-term solution. MR Plumbing can help you compare repair, maintenance, and replacement options so you can make an informed decision.

Tankless Water Heater Maintenance Tips for Homeowners

To help your tankless water heater last longer and perform better, follow these basic maintenance tips:

• Schedule routine tankless water heater maintenance
• Have the unit flushed according to manufacturer recommendations
• Ask about water treatment if your home has hard water
• Keep the area around the unit clear
• Check exterior vents for snow, leaves, and debris
• Do not ignore recurring error codes
• Do not overload the system by running too many hot water fixtures at once
• Have installation issues corrected by a qualified professional

The best time to maintain a tankless water heater is before it fails. A little preventive service can help you avoid cold showers, emergency repairs, and premature replacement.

Tankless Water Heater Repair and Maintenance in Crestwood and Chicagoland

MR Plumbing Heating Air Electrical has served Crestwood and the Greater Chicagoland area since 1972. Our licensed, background-checked technicians can diagnose tankless water heater problems, perform maintenance, explain your options, and provide up-front pricing before work begins.

We service traditional and tankless water heaters, and we can help with related plumbing issues such as hard water, water pressure problems, water quality concerns, leaking pipes, and fixture problems.

Whether your tankless water heater needs a flush, a repair, or a full replacement, our team is here to help you get reliable hot water back in your home.

Frequently Asked Questions About Tankless Water Heater Problems

Why does my tankless water heater keep going cold?

A tankless water heater may go cold because of mineral buildup, low water flow, an overloaded system, ignition problems, a clogged filter, or improper sizing. If the problem keeps happening, the unit should be inspected by a plumber.

How often should a tankless water heater be flushed?

Many tankless water heaters should be flushed about once a year, but the right schedule depends on water hardness, usage, manufacturer recommendations, and the age of the unit. Homes with hard water may need more frequent service.

Can hard water damage a tankless water heater?

Yes. Hard water can leave mineral scale inside the heat exchanger. That buildup can reduce efficiency, restrict flow, trigger error codes, and shorten the life of the water heater.

Why does my tankless water heater show an error code?

Error codes are designed to help identify a problem. They may point to ignition failure, overheating, blocked venting, poor water flow, scale buildup, sensor issues, or other system faults. Write down the code and schedule service if it returns.

Is a tankless water heater worth repairing?

It depends on the age of the unit, the type of repair, maintenance history, and whether the water heater was properly sized and installed. If the system is newer and generally reliable, repair may make sense. If it is older, neglected, undersized, or frequently breaking down, replacement may be more cost-effective.

Call MR Plumbing for Tankless Water Heater Service

Do not wait until a small tankless water heater problem leaves your home without hot water. If your system is showing error codes, running cold, leaking, losing pressure, or overdue for maintenance, call MR Plumbing Heating Air Electrical today.

For expert tankless water heater repair, maintenance, and replacement in Crestwood and the Chicagoland area, call 708-385-8607 or request service online.

Schedule your tankless water heater service with MR Plumbing today.