Causes and Solutions for Tankless Water Heater Fluctuating Temperature

Do you feel like your tankless water heater is not providing the same consistent hot water temperature it used to? Dealing with tankless water heater fluctuating temperature is a common issue many homeowners face. Fluctuating temperatures can be caused by a variety of problems, from sediment buildup to thermostat malfunctions.

In this blog post, we’ll explore some of the common causes and solutions for tankless water heaters fluctuating temperature. We’ll provide helpful tips to help diagnose and resolve the issue, so you can start enjoying consistently hot showers again.

With our tips and advice in hand, you’ll have the knowledge to fix the problem yourself or decide when it’s time to call in a professional. So let’s get right into it!

Tankless Water Heater Fluctuating Temperature 101

Tankless water heaters are an increasingly popular choice for modern homes. Unlike traditional tank-style water heaters, tankless versions don’t require large amounts of energy to heat stored hot water. Instead, they rely on a flow sensor and thermostat to activate the heating element which heats the flowing water.

However, even the most reliable tankless water heaters can experience problems with the temperature. Knowing how your water heater should function will help you identify and address any problems that might arise. 

Tankless Water Heaters and How They Work

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Tankless water heaters are an energy-efficient way to heat your home’s water compared to traditional storage tank water heaters. They work by heating water on demand rather than constantly storing it at a set temperature like traditional water heaters. This allows you to save energy and money in the long run. 

Tankless water heaters consist of three main parts – a gas burner or electric element, a heat exchanger, and control valve. The burner or electric element heats the incoming cold water as it passes through the unit.

The heat exchanger then transfers heat from the burner or electric element to the cold incoming water, raising its temperature. The control valve efficiently manages the release of hot water, accurately providing just what is needed without wasting any energy. This regulation ensures that the user always has exactly enough hot water to fulfil their needs.

The biggest advantage of a tankless system is its efficiency. The periodic use of energy can reduce annual utility costs by up to 25%, yielding a considerable financial benefit.

Furthermore, there’s no need for a large storage tank of heated water (which takes up valuable space). Tankless systems can free up significant space in homes or apartments where space is at a premium.

Additionally, they last longer than traditional storage tanks due to their lack of standing hot water and constant heating elements.

Common Causes of Tankless Water Heater Fluctuating Temperature and Effective Solutions

Now that you have a better understanding of how tankless water heaters work, let’s take a look at some of the common causes of tankless water heater fluctuating temperature and what you can do to fix them.

Insufficient Water Pressure or Flow Rate

Low water pressure affects temperature stability in tankless water heaters because it prevents a consistent and reliable flow of hot water. This can cause the heater to shut off before reaching the desired temperature or not heat up properly. To resolve this tankless water heater fluctuating temperature issue, you should take the following steps:

  • Check the inlet supply valve for any debris that could be blocking the flow of water. Clean debris from the valve if necessary. 
  • Check for any leaks in your plumbing system that could be reducing your home’s water pressure and causing fluctuations in temperature. Inspect pipes, fittings, and fixtures for any signs of leakage or damage. 
  • Install a booster pump to increase the pressure from your main line into your tankless heater unit, if necessary. 
  • Make sure to use at least 50 psi of pressure when flushing out the system after installation to prevent scaling buildup on heating elements over time and ensure proper operation of temperature control features within the system itself. 
  • If all else fails, consider replacing your existing tankless unit with one designed to work well with low-pressure systems, or contact a professional plumber to troubleshoot further.

Malfunctioning Thermostat or Temperature Sensor

A malfunctioning thermostat or temperature sensor is often the cause of a tankless water heater fluctuating temperature, which can be due to a faulty component, build-up of dirt and debris on its contacts, improper wiring to the unit, or a faulty sensor.

Here’s how to fix malfunctioning thermostats or temperature sensors in tankless water heaters:

  • Check the thermostat/temperature sensor wiring and connections for any signs of damage or loose connections.
  • Use a multimeter to test the thermostat/temperature sensor and check whether its readings are correct. 
  • If necessary, use a brush to clean the contacts of the thermostat/temperature sensor to ensure proper contact. 
  • If necessary, replace the thermostat/temperature sensor with a new one that is compatible with your tankless water heater model. 
  • Reset your tankless water heater unit and make sure it is functioning properly by testing its temperature output from multiple outlets in your home.

Mineral Deposits Buildup in the Heat Exchanger

The mineral deposit is another common cause of tankless water heater fluctuating temperature, as the heated water can contain high levels of calcium and magnesium. These minerals will settle and accumulate in the heat exchanger, which reduces water flow and decreases efficiency.

To clean the buildup, flushing or a chemical descaler may be required. Additionally, there are preventive measures that can help reduce future buildup. Some methods include:

  • Installing a quality sediment filter.
  • Using softening agents such as sodium phosphate.
  • Monitoring water hardness and installing an inline scale inhibitor.
  • Flush the heater at least twice per year to maintain system performance and extend the life cycle of the unit.
  • Regularly inspect your plumbing system and replace filters as needed to minimize mineral deposits from entering your tankless water heater.

Gas Supply Issues

Gas supply issues can cause fluctuations in temperature in tankless water heaters. This is due to the fact that a gas-fueled tankless water heater relies on a steady flow of gas to heat up the water. If the gas supply is inconsistent or interrupted, then the heater will not be able to maintain an even temperature. The result will be water that is either too hot or too cold for use.

To resolve common gas supply problems:

  • Ensure that your gas lines are properly installed and regularly maintained.
  • Use an appropriate regulator to regulate the pressure of your fuel supply.
  • Install a surge protector to protect from power outages and voltage drops.
  • Avoid overloading your system by adding too many appliances at once.
  • Regular maintenance and servicing of tankless water heaters can also help ensure consistent temperatures.

Cold Water Sandwich Phenomenon

Cold Water Sandwich Phenomenon is an issue that can often occur in tankless water heaters, creating an undesired effect of intermittent warm and cold water. It occurs when hot water has been depleted and the water heater sends cold water to the tap while the heating system slowly refills with hot water.

This phenomenon causes lukewarm showering experiences at a minimum and can be uncomfortable for many people. To fix this problem:

  • Check your incoming cold water pressure to ensure it’s adequate for the heater.
  • Ensure that your tankless unit is sized correctly for your home.
  • Install recirculation systems to reduce the wait time for hot water.
  • Optimize temperature settings on the heater and adjust them as needed.
  • Regularly inspect and maintain your system to help ensure consistent temperatures.

Improper Installation

Improper installation of tankless water heaters can also lead to temperature fluctuations. Common mistakes during installation include incorrect sizing, using a weak pressure relief valve, improper venting or exhaust configuration, inadequate gas supply, and dry firing of your new water heater. To ensure proper installation:

  • Select a tankless water heater based on the size of your home and hot water needs.
  • Use a high-quality pressure relief valve to prevent damage from sudden changes in temperature or pressure.
  • Follow local regulations for venting and exhaust requirements, and make sure that the configuration meets manufacturer specifications.
  • Make sure that you have an adequate gas supply for your water heater.
  • Before turning on the power or gas supply, ensure that the unit is filled with water and do not dry fire it.

Clogged Water Filter

Tankless water heater fluctuating temperature can as well be caused by clogged water filters as the filter traps contaminants and sediment, thus reducing the amount of incoming water flow. This causes an imbalance between supply and demand, resulting in the heater working harder to meet the set temperature.

This can lead to fluctuating temperatures or even lukewarm water. It is important to keep your tankless water heater’s water filter clean for optimal performance as it ensures that enough hot water is provided at a consistent temperature.

Signs of a dirty or clogged filter include discolored or smelly hot water, lukewarm water, reduced pressure when showering, and low hot-water flow rate. Steps to clean or replace the filter include: 

  • Shut off all power to the tankless water heater.
  • Disconnect the cold-water line from the unit.
  • Remove any old filter materials from inside the unit.
  • Clean or replace any dirty or clogged parts.
  • Reattach all parts, making sure they are secure.
  • Turn on power and test operation.

Overuse or Overloading the System

Overuse or overloading the system in tankless water heaters can also cause temperature fluctuations and instability. When the flow rate is greater than what the heater is designed to handle, it will struggle to keep up and may fluctuate between hot and cold water.

The power source of a tankless water heater determines its capacity, so if you try to draw more water than it can handle, it won’t be able to function appropriately.

To determine if your heater is overloaded, you should check for a sudden drop in temperature when multiple faucets are running at once.

Solutions to prevent overuse and optimize system performance include:

  • Limiting the number of fixtures active simultaneously.
  • Upgrading capacity with larger unit sizes and proper insulation of piping.
  • Adding an accumulator tank may help as it will store incoming cold water until a demand is made for hot water. This helps avoid pressure drops by allowing more time for the water heater to respond appropriately and reduces sudden requests for high volumes of hot water.
  • Installing flow control valves on each outlet can also help balance the load and ensure that pressure remains consistent across all outlets.


Taking the proper steps to maintain and care for your tankless water heater will ensure you have many years of reliable hot water. It is important to pay attention to the temperatures and adjust them if there are any fluctuations.

Keep an eye out for warning signs such as a loud noise, leaking, or excess corrosion. If any of these or other issues should arise, contact a professional to handle the repair safely and efficiently.

Regular maintenance will keep your tankless water heaters running smoothly and safely so that you can continue to enjoy the savings and convenience that come with having one in your home. No matter what type of tankless system you own, following basic procedures can help you keep it running great!

I'm a dedicated home improvement enthusiast with a passion for energy-efficient living and a wealth of experience in the tankless water heater industry. I'm committed to providing comprehensive and practical guidance to homeowners seeking the perfect tankless heater solutions for their homes.