Why Does My Tankless Water Heater Go Cold?

You have probably heard of the countless advantages of tankless water heaters; that’s why you bought one in the first place.

Things like no standby energy loss, never running out of warm water, more efficient than tank-style devices, and saving money on the energy bill.

But what about when your unit declines to operate according to your expectations? When instead of perfectly hot water, all you acquire is chilled water? Or why does my tankless water heater go cold?

Let’s be honest; there is nothing better than a long hot shower on a snowy day— except when it’s not warm. When your water heater produces lukewarm water, it puts a curb on your day.

It does not just affect your shower, but it affects your dishwasher, washing machine, and your kitchen sink as well.

If your device is creating cold water, don’t ignore it anymore. You must find the reason for the temperature fluctuation so that you can troubleshoot it.

In this guide, we will educate you about the reasons why your electric tankless water heater goes cold after a few minutes. We will also give some practical tips to help you avert this issue from occurring again.

How Does A Tankless Water Heater Work?

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To understand why my tankless water heater is not getting hot, it is necessary to understand how a tankless unit works.

You are most likely conversant with traditional tank-style water heaters. They heat water and store it in the tank for later usage.

An average device stores 30 to 50 gallons of water. This is the reason they are so big.

Nevertheless, they are undesirable because they waste a lot of energy by warming up water unnecessarily. The standby energy losses can skyrocket your monthly energy bills.

On the flip side, tankless appliances preserve energy and save money. Since they are without storage tanks and are intended to heat water promptly, they are much smaller than tank-style heaters.

Moreover, they are called on-demand or instant units because they produce warm water only when you need it. When you turn on the hot water fixture, your appliance’s flame kindles the heat exchanger.

Then, groundwater enters the inlet pipe at the system’s bottom. As the water moves through the heat exchanger, the unit heats the water to the preferred temperature. Ultimately, the warm water leaves the heater to be supplied through pipes.

Why Does My Tankless Water Heater Go Cold? Top Reasons And Their Solutions

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Plumbing Crossover

Plumbing problems are the biggest cause of temperature changes, and plumbing crossover is one of them. A faulty mixing valve, single-level faucet, or damaged plumbing are the reasons behind this problem.

It causes fraternization of cold and hot water, which decreases the water temperature. To prevent it from occurring, examine the plumbing crossover.

Turn off the cold water supply to your device and turn on all the taps. After 10 minutes, if the water is still flowing out, it means you have a plumbing crossover.

It is very easy to repair the plumbing crossover. Inspect the valve or mixing valve or change the cartridge.

However, remember that transitioning the cartridge may not fix the issue. You may have to purchase a stellar quality faucet or cartridge to get rid of this problem.

Incorrect Gas Line Size

Incorrect gas line size is another reason why my tankless water heater goes cold. It is critical to extend the gas line to support your heater in most cases.

To accommodate your unit, you may have to upgrade your gas lines from ½ to ¾ to accommodate your unit, but “1” is more appropriate for a tankless machine.

Many appliances take cold water to 120 degrees Fahrenheit in a flash. An undersized gas line can cause partial ignition that can lead to equipment failure.

Coldwater Sandwich

It is a state when on activating the faucet, you do obtain warm water, but in between, you acquire cold water. The situation can be very unpleasant for many.

It might be the warm water residue from the last time someone from your family used the faucet. The appliance could not heat water swiftly, so you acquire some cold water before the water is heated again.

If your heater does not have some special features that can reduce this fluctuation, this problem will occur in all tankless devices, especially in electric-powered units.

The only solution to this problem is to wait for the unwanted stream of cold water to pass before taking a shower.

Clogged Water Filter

If you are getting lukewarm or cold water, you need to examine the cold water inlet. It is a tiny plastic connection that allows cold water to enter the water heater.

The cold water inlet filter prevents any debris from entering the unit. However, over time, it gets obstructed with the debris. When it happens, water cannot enter the device; therefore, you get cold water.

To fix this problem, you need to remove and clean the cold water inlet filter. If required, you can also replace it.

Low Water Pressure

One of the most significant reasons why the tankless water heater shuts off during the shower is low water pressure.

The flow within your device is lower than the activation flow rate, resulting in the closing down of the gas burner.

If your system features a standing pilot, the pilot light stays lit; only the main gas burner shuts down.

Check the user manual to know about the minimum flow rate—it is typically around 0.4 or 0.5 gallons per minute.

If your heater is poorly designed and its capacity is smaller than demand, and if extra taps are opened or closed in your residence, there will be a fluctuation in the flow, and you will observe a change or temperature drop. Consequently, you will have lukewarm or cold water to take a shower with.

To resolve this problem, you can upgrade the unit’s capacity, install a second heater, or limit the simultaneous usage of fixtures or taps.

If this issue happens rarely, restrict the concurrent use of showers or outlets in your house. However, if your current device doesn’t fulfill your hot water needs, you can get a second heating system.

Blocked Heat Exchanger

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If there is hard water in your locality or you use well water, you can blame mineral accumulation for water getting cold.

Hard water deposits these minerals like calcium and magnesium onto the components of your system. This collection rusts the pipes and damages the heat exchanger, so it cannot heat your water.

Besides, you should avoid using a clogged heat exchanger because it can be dangerous. To avoid the limescale and mineral buildup, you should frequently flush your water heating machine.

If accumulation has already occurred, contact a licensed plumber to help clean the lines.


All the problems mentioned above could be the cause of why my tankless water heater goes cold. By following the guidelines discussed, you should be able to fix the issues.

However, if the issues look intricate to troubleshoot on your own, we recommend you hire the services of an experienced technician or plumber to help you with the job.

Reference Article Links

How to Protect Your Tankless Water Heater From Hard Water

Hard Water and tankless water heaters



Common Hot Water Heater Problems and How to Fix It

Cold Water Sandwich Effect And The Solution

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