When Hot Water Goes Cold: Troubleshooting Your Tankless Water Heater

Tankless Water Heater

A lack of hot water from a tankless water heater is an annoyance at best. This in-depth manual explores the root causes of such problems and provides helpful advice. We’ve got you covered whether you’re experiencing low water pressure from your tankless water heater or no hot water in the shower altogether.

Understanding the Issue

The efficiency and convenience of a tankless water heater are well-known. However, there are times when people using tankless water heaters won’t get any hot water. Let’s analyze the common causes behind this issue and present specific answers to assist you diagnose and address the problem successfully.

Electrical Issues

Electrical issues are a common reason why tankless water heaters stop producing hot water. When there are electrical problems, the heating process is disrupted and no hot water is produced. The most frequent electrical problems, as well as their causes and fixes, are detailed here.

No hot waterTripped circuit breakerReset any circuit breakers that may have tripped in the electrical panel.

Malfunctioning heating elementIf the circuit breaker hasn’t been tripped, check the heating element for problems and replace it if necessary.

Gas Supply Problems

Problems with the gas supply can render tankless gas water heaters inoperable, leaving users without access to hot water. For the heating process to go smoothly, a steady supply of gas is required. The following are the warning signs, root causes, and potential remedies for gas supply issues:

No hot waterClosed gas valveEnsure that the gas valve is open to allow gas flow to the heater.

Insufficient gas supplyVerify the gas supply to the heater, and fix any problems you find if it is not sufficient.

Maintenance Needs

Tankless water heaters require routine maintenance to avoid problems like running out of hot water. Water flow might be impeded by mineral deposits and debris, which can reduce the efficiency of the heater. Problems with maintenance? Here are the signs, what’s causing them, and how to fix it.

No hot waterBuild-up of mineral deposits and debrisDescaling and cleaning the water heater on a regular basis helps keep the water flowing freely.

System Overload

Tankless water heaters are great, but they can fail if you use too many hot water appliances at once. Be careful with your hot water consumption to avoid this problem. The signs, root, and treatment of system overload are as follows:

No hot waterSimultaneous use of multiple hot water devicesTo avoid taxing the water heater, reduce the number of devices that utilize hot water at once.

Using the aforementioned tables, you may quickly diagnose and repair whatever problem is plaguing your tankless water heater. Following these troubleshooting procedures should get your hot water running smoothly again.

Installation Mishaps

Pipe Sizing

When setting up a tankless water heater, it’s crucial to use the right sized pipes. Problems with water pressure and a shortage of hot water are only two of the concerns that can arise from improper pipe sizing.

  • Consult Manufacturer Guidelines: Check the Manufacturer’s Instructions First, make sure you have read the tankless water heater installation handbook from start to finish. This detailed document offers specific recommendations regarding pipe sizing, which may vary based on the model, capacity, and fuel source (electric or gas);
  • Calculate Flow Rate: Determine the maximum simultaneous consumption of all hot water-using appliances in your home, such as showers, sinks, dishwashers, and washing machines, to get an idea of your household’s peak hot water demand. This flow rate must be calculated precisely;
  • Select Proper Pipe Size: Determine Appropriate Pipe Size: Utilize Manufacturer’s Pipe Sizing Charts to Determine Appropriate Pipe Size Based on Determined Flow Rate. The diameter of the pipes must be large enough to handle the heater’s flow rate without reducing pressure too much;
  • Professional Installation: Hiring a professional plumber is recommended if you are not confident in your plumbing knowledge or if you are unsure of the appropriate pipe size. If you choose a professional plumber, you can rest assured that your pipes will be installed properly and in accordance with all building requirements.

Gas Line Sizing

Gas-powered tankless water heaters require an appropriate gas supply to work successfully. If there isn’t enough gas, the heater won’t work as well, and there won’t be any hot water to use. Learn all about gas line sizing in this comprehensive guide:

  • Consult Manufacturer Specifications: If you want to know what size gas line your tankless water heater needs, you should check the manufacturer’s instructions and specs. The BTU (British Thermal Unit) rating of the heater is often used to gauge the appropriate size of the gas line;
  • Verify Existing Gas Line: It is important to double check that your home’s current gas line can handle the gas flow required by the heater. If it falls short, visit a professional gas technician to upgrade the gas line to fulfill the heater’s requirements;
  • Professional Gas Line Work: Always hire a certified professional to work on your gas lines. Tasks involving gas require expert knowledge and adherence to local construction norms and safety laws.

User Misunderstandings

If you want to prevent a situation where the shower has no hot water but the other faucets do, learning the capacity and restrictions of your tankless water heater is crucial. Allow me to elaborate on this point:

Flow Rate Limitation

The maximum flow rate capacity is one of the major restrictions of tankless water heaters. The flow rate is the amount of water per minute that can be heated by the heater. If you go above this limit, the temperature of your hot water may decrease, or in the worst case scenario, you may not have any hot water at all. Knowing the maximum flow rate for your particular tankless water heater will help you prevent these kinds of problems. Here are samples of the top brands/models.

ModelFlow Rate (GPM)Power Input (BTUs)Fuel TypeKey Features
Rinnai RU180iN10180,000Natural GasEnergy Star certified, efficient for up to five appliances, condensing feature
Rheem RTEX-13Up to 4ElectricIdeal for smaller households, cost-effective
Stiebel Eltron Tempra 36 PlusUp to 7.5Electric99% efficiency, silent operation, Advanced Flow Control
Rinnai RE199iN9.8199,000Natural GasSuitable for larger households, non-condensing, more affordable
Rheem RTEX-36Up to 8.8ElectricCompact design, 99.8% energy efficiency, self-modulation technology
Rinnai RL75iN7.5180,000Natural GasWi-Fi capability, can be paired with external pump for on-demand recirculation

Avoid using multiple hot water outlets at once if the combined flow rate would exceed the heater’s capacity. To find out how much water can run through your heater at once, see the user manual or manufacturer’s website.

Temperature Rise

The capacity of your tankless water heater to produce hotter water is also an important consideration. The heater’s ability to raise the temperature of incoming water is measured in terms of temperature rise. It may take longer for the water heater to heat up the incoming water to the necessary temperature if the water is particularly cold, as it often is in the winter.

The effectiveness and speed with which your water heater provides hot water are both affected by its maximum allowable temperature rise. Depending on the model, a tankless water heater can increase the temperature by anywhere from 20 to 70 degrees Fahrenheit (11 to 39 degrees Celsius).


Maintaining your tankless water heater on a regular basis is essential to keeping it running smoothly and avoiding interruptions to your hot water supply. Inadequate performance and, in the worst case scenario, no hot water, might result from putting off necessary maintenance activities like descaling. Here are some important guidelines to keep in mind when maintaining your tankless water heater:

  • Descaling: The efficiency of a heater might degrade over time as mineral deposits build up in the heat exchanger and require descaling. To prevent these deposits from hindering the unit’s efficiency, it should be descaled on a regular basis, as directed by the manufacturer;
  • Flushing: Water flow and heating efficiency can be enhanced by flushing the system to remove sediment accumulation and restore the heater’s original performance. For information on when and how to flush your particular model, go to the manual;
  • Air Vent Inspection: Inspect the air vents to make sure there is adequate ventilation for combustion and heat production;
  • Filter Cleaning: If your tankless water heater contains a water filter, be sure to clean or replace it as directed to keep the filter working properly and keep the water flowing freely;
  • Gas Line Inspection: Inspect the gas line that feeds the heater to make sure it is in good working order and has no leaks.

By regularly performing these checks, you can keep your tankless water heater running smoothly and for many more years.

Consult User Manual

Last but not least, the user handbook that came with your tankless water heater is an excellent resource for learning about its capabilities, limitations, and troubleshooting advice. Flow rate, temperature increase, maintenance instructions, and troubleshooting procedures are all model-specific details included.

For particular information on how to use your model, it is always best to consult the handbook. Many companies will either post a digital copy of the manual on their websites or send you a replacement if you request one after losing the original.

Advanced Troubleshooting

Some issues with a tankless water heater can be fixed with a little bit of troubleshooting, but if the problem persists, as when there’s no pressure, you may need to call in a professional plumber. Let’s talk about how to approach difficult problems.

Internal Leaks

An interior leak in a tankless water heater is a major cause for alarm. Water damage, decreased efficiency, and other complications can result from internal leaks. Signs of an interior leak may include water gathering around the unit, strange sounds during operation, or a drop in hot water supply.

  • Immediate Action: If you detect a leak inside the water heater, you must immediately turn off the water supply and the gas or electricity to the unit. This will stop any future harm or safety risks from occurring;
  • Professional Plumber: Plumber Consult a licensed plumber who specializes in tankless water heaters for assistance. They may check the whole thing out to find out where the leak is coming from and fix it. Repairs could entail replacing individual parts or the complete device, depending on the nature of the problem.

Mechanical Failures

Tankless water heaters have different mechanical components that can fail over time, such as the heat exchanger, pressure relief valve, or flow sensor. A lack of hot water or other operational issues may result from such failures.

  • Troubleshooting: Before calling a repairman, it’s a good idea to check the manufacturer’s troubleshooting manual for common problems. Error codes and diagnostic indicators on the heater’s screen can sometimes shed light on the nature of the issue;
  • Professional Diagnosis: Call a licensed plumber or HVAC expert if the problem persists or if you can’t figure out what’s causing it. They have the training and equipment necessary to identify mechanical problems.

Importance of Professional Help

When it comes to addressing tankless water heater problems, seeking professional assistance is of paramount importance. Whether you’re dealing with safety concerns, intricate mechanical issues, warranty considerations, or the long-term reliability of your unit, professional plumbers bring a wealth of expertise to ensure your tankless water heater functions optimally. Let’s delve into the significance of professional help in greater detail:


  • Dealing with Hazardous Situations: Tankless water heaters, especially gas-powered ones, can present safety hazards when not handled correctly. Professionals are well-versed in working safely with these appliances and follow stringent safety protocols during repairs;
  • Gas Safety: Gas-powered tankless water heaters involve gas lines, combustion, and ventilation systems. A professional plumber understands the potential risks associated with gas appliances and can safeguard against gas leaks, carbon monoxide emissions, and other safety issues.

Precision Repairs

  • Intricate Systems: Tankless water heaters are intricate systems comprising numerous components, including heat exchangers, flow sensors, and control boards. Pinpointing the exact problem in such a complex system requires expertise;
  • Accurate Diagnosis: Professional plumbers have the knowledge and experience to diagnose issues accurately. Their ability to identify the root cause ensures that repairs are carried out with precision, minimizing the risk of further damage;
  • Avoiding DIY Pitfalls: Attempting DIY repairs on complex tankless water heater problems can worsen the situation or lead to additional damage. Professionals have the skills and tools necessary to perform effective repairs without causing unintended complications.

Warranty Considerations

  • Preserving Warranty Coverage: If your tankless water heater is still under warranty, DIY repairs or hiring an unqualified technician can void the warranty. Many manufacturers require that warranty repairs be conducted by certified professionals;
  • Manufacturer Certification: Professional plumbers are often certified by manufacturers to perform warranty repairs. Choosing a certified technician ensures that your warranty remains intact, providing peace of mind and potential cost savings in case of future issues.

Long-Term Reliability

  • Preventing Future Problems: Professional repairs extend beyond immediate fixes. A qualified plumber can identify potential issues that could arise in the future. By addressing these concerns during repairs, they help prevent further breakdowns and ensure the long-term reliability of your tankless water heater;
  • Efficiency and Performance: Professionally serviced tankless water heaters tend to operate more efficiently and deliver better performance. This translates into consistent hot water supply, energy savings, and enhanced appliance longevity.

Preventive Measures

Regular maintenance is key to preventing situations where there’s no hot water from a tankless water heater. Let’s delve deep into the various aspects of tankless water heater maintenance, from annual inspections to descaling and cleaning procedures.

Schedule Annual Inspections

Annual inspections by a qualified technician are the cornerstone of tankless water heater maintenance. These inspections involve a comprehensive assessment of your unit to identify any issues or potential problems that could lead to a lack of hot water. Let’s break down the components of an annual inspection in more detail:

Annual Inspection Checklist

Inspection AreaDescription
Gas ConnectionsCheck for leaks or corrosion in gas lines.
Water Inlet/OutletInspect for blockages or mineral buildup.
Venting SystemEnsure proper venting and check for obstructions.
Electrical ComponentsExamine wiring, circuits, and connections for issues.
Heat ExchangerInspect for scale buildup and clean if necessary.
Safety FeaturesTest pressure relief valve and other safety mechanisms.

Regularly Descaling and Cleaning Your Unit

Mineral scale buildup is a common issue with tankless water heaters, especially in areas with hard water. Regular descaling and cleaning are essential to maintain the unit’s efficiency and prevent clogs. Let’s explore the descaling and cleaning process in more detail:

Descaling and Cleaning Instructions

1. Turn Off PowerEnsure the unit is not receiving power before starting.
2. Isolate Water SupplyShut off both hot and cold water supply to the unit.
3. Drain the UnitConnect a hose to the drain valve and empty the unit.
4. Remove Inlet/OutletDetach the water inlet and outlet pipes from the unit.
5. Use Descaling SolutionMix a descaling solution as per the manufacturer’s instructions.
6. Circulate SolutionConnect a pump to circulate the solution through the unit.
7. Rinse and ReassembleFlush the unit with clean water and reattach pipes.
8. Restore PowerTurn on the power and water supply. Check for leaks.

Water Quality Considerations

The quality of the water supplied to your tankless water heater can significantly impact its performance and maintenance needs. If you live in an area with hard water (high mineral content), you may need to take extra precautions. Here are some additional measures to consider:

  • Water Softening: Installing a water softener can help reduce mineral buildup in the unit;
  • Sediment Filters: Use sediment filters on the water inlet to prevent debris from entering the heater;
  • Regular Water Testing: Periodically test your water quality to gauge the level of hardness and adjust maintenance accordingly.

Temperature Adjustment and Efficiency

To maintain the energy efficiency of your tankless water heater and avoid sudden changes in hot water availability, consider the following:

  • Set the Desired Temperature: Adjust the temperature setting according to your preferences but avoid excessive heat settings to prevent overheating;
  • Use Low-Flow Fixtures: Installing low-flow fixtures can help reduce hot water demand, increasing the heater’s efficiency;
  • Consider a Thermostatic Mixing Valve: These valves can help ensure a steady and safe hot water temperature at the tap.

Monitor and Address Issues Promptly

Even with regular maintenance, issues may arise over time. Monitoring your tankless water heater’s performance and addressing problems promptly can prevent more significant breakdowns. Common issues to watch for include:

  • Fluctuating Water Temperature: If you notice inconsistent hot water supply, it may indicate scale buildup or flow rate issues;
  • Error Codes: Modern tankless water heaters often display error codes. Consult your unit’s manual to interpret these codes and take appropriate action;
  • Leaking: Any sign of leaks should be addressed immediately to prevent damage to your unit and property.


Addressing issues of no hot water from a tankless water heater often involves understanding the root causes, which can range from electrical and gas supply problems to installation errors and lack of maintenance. Regular upkeep, including annual inspections and routine cleaning, is critical in preventing issues like low pressure or no hot water in the shower. In situations where troubleshooting proves complex, seeking professional assistance ensures not only the restoration of your hot water supply but also the longevity and efficient operation of your tankless water heater system.


Why is my tankless water heater not providing any hot water?

Common reasons include electrical issues, gas supply problems, or maintenance needs.

What should I do if there’s no pressure in my tankless water heater?

Check for blockages and ensure proper installation. If the problem persists, consult a professional.

Why is there no hot water in my shower, but other taps are fine?

This could be due to system overload or user error in understanding the heater’s capacity.

How often should I maintain my tankless water heater?

Annual inspections and regular cleaning and descaling are recommended.

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