Proudly Serving the Knoxville Area

Why Is My Tankless Water Heater Running Cold More Quickly? Do I Need Water Heater Repair To Fix It? | Oak Ridge, TN

Have you experienced hot and cold water running from your tankless water heater? Though not uncommon, it is a problem that requires a tankless water heater repair. Your tankless water heating unit should run smoothly without delays to provide heated water on demand. If left unchecked, the issue can make the water heater less efficient, use more energy, and leave your Oak Ridge, TN home without enough supply of heated water. So, what makes the tankless water heater run cold and hot occasionally? What is the cause of the said fluctuations? How can a water heater repair help resolve the issues?

Cold Water Sandwich

One of the common issues that arise in a tankless water heater is a phenomenon called a cold-water sandwich. This is when the unit produces hot water for several seconds, followed by cooler water, and then applies warm water. You might think your tankless water heater shuts down for a few seconds. This usually arises when a hot water outlet such as a faucet or shower has just been shut off. For instance, if a family member just left a shower and you are next in line when you open the shower, the hot water in the pipes falls first.

This is followed by cold water as the unit heats the water that then starts flowing. Though this isn’t an emergency or a huge issue, if it is occurring persistently, you should have it addressed by a tankless water heater repair professional. The professional might install a secondary water heater or a hot water recirculation system to curb the issue.

Since the heat exchanger delay causes the issue, installing a return loop-to or a recirculating system to your tankless water heater will resolve the issue. However, you’ll incur higher energy bills. Hence, if you have a higher hot water need at your Oak Ridge, TN home, the only realistic and feasible solution is to replace the water heater with a higher capacity one. Remember, a correctly sized water heater can endlessly supply your home with heated water.

Incorrectly Sized Gas Line

Is your water heater gas-powered? Then the cold and hot water instances you experience might also arise if the gas line is the wrong size. In most tankless water heaters, you are required to extend the gas line to ensure an ample supply of gas. Have you already tried this, but the water heater still runs the hot and cold water? Then that could point to an incorrectly sized gas line. Hence, you need a tankless water heater repair service to inspect the unit and upgrade the gas meter to ensure that your unit gets enough hot water supply. Otherwise, you will need a gas line replacement.

Wrong Plumbing System Crossover

A tankless water heater must be directly connected to the return and supply lines. If the plumbing was incorrectly done or the pressure balancing valve requires some repairs, the cold water might flow into the heated water lines. This makes the heated water lukewarm. As a result, you will get some cold and heated water bursts until the line heats up properly after being used continuously. Without the assistance of a tankless water heater repair professional, recognizing this can be a problem.

After inspecting the water heater and noticing it has a wrong plumbing crossover, the professional has two options. They either might raise the thermostat on the tankless water heater or install the pressure balancing valve. Raising the thermostat means you’ll need to use the heated water at all outlets constantly. This allows the temperatures of your heated water to rise, heating the return line steadily. A pressure balancing valve monitors the temperature of your home’s incoming gas line. It also balances the excessive pressure whenever the cold water enters the system, eliminating the bursts of cold and hot water.

A Broken Flow Sensor

The other reason the tankless water heater might run cold and hot is a broken flow sensor. It measures the volume of water entering the unit and sends that information to the water heater control board. The controller will adjust the flame whenever it senses a burst of cold water entering the unit. Hence, you’ll get enough heated water immediately throughout your home. To fix this issue, you need a tankless water heater repair technician to inspect and pinpoint the unit. The professional inspects the thermostat, gas pressure, or water heater assembly. If the issue is a damaged or malfunctioning backflow sensor, have a tankless water heater repair technician replace the sensor.

Mineral Buildup

 The other reason you might experience bursts of cold and hot water at your Oak Ridge, TN, home is if the water heater has accumulated some sediments. This usually arises if your home receives hard water. The minerals dissolved in the water accumulate in the walls of the tankless water heater reducing its heating efficiency. This is a leading cause of water temperature issues in any water heater.

The only solution is having a water softener system installed. To resolve the issue, the plumbing professional will describe the system based on its severity. Remember, these minerals also result in corrosion of the pipes. The professional might also test the water for hardness and make various recommendations, such as changing the water utility provider or installing an ion exchange water softener to safeguard your plumbing.

Let the Reliable Plumbing Service Provider Help

Whenever they notice they are getting these bursts of heated and cold water, some homeowners resort to inspecting the tankless water heater themselves. However, at My Professional Plumber, we highly advise against this. Instead, let our tankless water heater repair professionals help. 

With their many years of experience, up-to-date tools, and training, they are in a better position to resolve the issue. The tankless water heater repair professionals will inspect the unit to diagnose any existing issues and fix them promptly. Do not allow frustrations to get better for you. Call My Professional Plumber today to schedule an appointment.

Photo By ANTHONY PAZ at Shutterstock


118 North Peters Road
Knoxville, TN 37923


8:00 am–5:00pm