8 Reasons a Tankless Water Heater Goes Cold After A Few Minutes

By Danika | July 28, 2022

8 Reasons a Tankless Water Heater Goes Cold After A Few Minutes

new Rheem tankless water heater

Tankless water heaters are both convenient and efficient, but they can begin to function erratically from time to time. They are designed to run constantly, supplying a constant flow of hot water. However, like any other electric appliance, it is susceptible to damage. There are a few different reasons why your tankless water heater goes cold after a few minutes. 

Having a faulty or damaged tankless water heater can be frustrating, as it not only interrupts your hot water supply but can also increase your energy bill. Let us explore some possible reasons why your tankless water heater might be running hot and cold and provide some tips on how to fix the problem.

When a Tankless Water Heater Goes Cold After A Few Minutes it gives you a cold water sandwich
Click here to find out about tankless water heater repair

1. The Infamous Cold Water Sandwich

A cold water sandwich occurs when the appliance is used immediately after being turned on or if it has been turned off for a while and then turned back on. When the tankless water heater is first turned on, the water inside the unit is cold. The cold water will mix with the hot water being generated, causing the temperature of the water coming out of the taps to be lower than usual.

This means that the unit first delivers hot water for the first few seconds, then cold water for a few seconds before the heat exchanger can warm up the water again. This will result in an inconsistent flow of hot water, which can be irritating when you are trying to take a shower or do the dishes.


Installing a recirculation pump: Involves installing a small pump that will circulate the water inside the tankless water heater and make it evenly hot. It will ensure that you always have a consistent hot water supply, regardless of whether the unit has been turned on recently. A recirculation pump can be easily installed by a professional and is not very expensive.

Install a small water tank: This is a more expensive solution but will eliminate the cold water sandwich problem. A small hot water tank (30 to 40 gallons) is installed next to the tankless water heater. When the unit is turned on, the hot water in the tank will be circulated through the pipes, providing a consistent flow of hot water until the tankless water heater can heat the water to the desired temperature.

Flush out: Let the unit run for a minute or two before using it to allow the cold water inside the unit to be flushed out. This will ensure you only get hot water when you turn on the taps. However, this method can be time-consuming and is not always practical.

Dirty water filter could be why you get cold water from your tankless heater
Image source: Fresh Water Systems

2. Dirty Water Filter

The water filter is responsible for keeping sediments and dirt out of the unit; if it becomes clogged, it can cause a drop in water pressure. This, in turn, can cause the tankless water heater to overheat, as it will work harder to heat the water. A drop in water pressure can also cause the unit to cycle on and off more frequently, resulting in an inconsistent hot water supply.


Check the water filter: The first step is to check the filter and see if it needs to be replaced. Depending on the model of your tankless water heater, the filter might be located inside the unit or outside. It will need to be replaced every few months or sooner if it becomes clogged.

limescale build up causes your tankless water heater to run inefficiently
Image source: Noritz

3. Limescale Build-Up

Limescale is a chalky substance created when water Hardness minerals (calcium and magnesium) interact. These minerals are found in almost all water sources, but the hardness level varies from place to place.

Limescale can build up inside the tankless water heater and the heating elements. This can cause the unit to overheat the water, leading to the water being too hot when it comes out of the taps. In extreme cases, the limescale build-up can cause the unit to malfunction and stop working altogether.


Descaling solution: You can purchase a descaling solution from a hardware store and use it to remove the limescale build-up inside the tankless water heater. This is a relatively easy process, but it will need to be repeated every few months to prevent the build-up from happening again.

Water softener: This is a more long-term solution and will require the installation of a water softener. A water softener works by removing the Hardness minerals from the water before it enters the tankless water heater. This will prevent the limescale from building up in the first place and extend the appliance’s life.

rheem thermostat for water heater

4. Incorrect Water Temperature

If the water temperature is too high, the tankless water heater will overheat the water. This can cause the water to be too hot when it comes out of the taps and damage the unit. On the other hand, if the water temperature is set too low, the unit will not be able to heat the water to the desired temperature, resulting in lukewarm water. Click here to find out the best temperature for a tankless water heater.


Check the thermostat: The first thing you should do is check the thermostat and ensure it is set to the correct temperature. If it is not, then adjust it accordingly.

Bleed the unit: If the thermostat is set correctly, but the water is still not hot enough, you may need to bleed the unit. This involves opening the bleed valve and allowing some hot water to escape. This will allow cold water to enter the unit, which will then be heated to the correct temperature.

5. Faulty flow sensor

The flow sensor is responsible for detecting the water flow rate and activating the heating mechanism. If the flow sensor is not working properly, it might not be able to detect the water flow, and as a result, It will deliver false readings to the control panel. This can cause the unit to either overheat the water or not heat it enough, both of which can be detrimental to your tankless water heater.

The unit control panel or board will have an error message or code that will indicate that the flow sensor is not working as it should.


Inspect the source of the damage: If the flow sensor is blocked, it will not be able to detect the water flow rate accurately. Use a toothbrush or a small brush to clean the sensor, and remove any dirt or debris that might be blocking it.

Check the wiring: The flow sensor is connected to the control panel by a wire. If the wire is loose or damaged, it can cause the flow sensor to malfunction. Check the wiring and ensure that it is properly connected and that there are no loose or damaged wires.

Replace the flow sensor: Have your unit inspected and serviced by a professional if the problem persists. The flow sensor might need to be replaced if it is damaged beyond repair.

6. A plumbing system crossover

A tankless water heater requires a certain amount of water pressure to function properly. If the incoming water pressure is too low, the unit will not be able to generate enough hot water. This is often caused by a plumbing system crossover, where the cold and hot water lines are connected.

The cold water line is usually under more pressure than the hot water line, and as a result, the cold water will flow into the hot water line, reducing the amount of pressure available to the tankless water heater.


Install a pressure regulator: A pressure regulator (or a pressure balancing valve) will help to maintain the correct water pressure and prevent the cold water from flowing into the hot water line. It regulates the water pressure by allowing a certain amount of water to flow into the hot water line and redirecting the rest back into the cold water line.

Have your plumbing system done by a professional: This is the best and most permanent solution to the problem. A professional plumber can properly assess your plumbing system and make the necessary changes to prevent the cold water from flowing into the hot water line.

7. An improperly gas line size

If the gas line that supplies the tankless water heater is not the correct size, it will not be able to deliver enough water to the unit. This will cause the unit to not function properly, especially during high-demand periods when a lot of hot water is needed.

The manufacturer usually determines the gas line size specified in the product manual. Having the wrong size gas line is one of the most common reasons a tankless water heater is not working and is an easy problem to fix.


Install the correct size gas line: This is the most permanent and effective solution to the problem. The gas line size will be specified in the product manual, and you can purchase the correct size gas line from a hardware store.

Install a gas line booster: A gas line booster will help increase gas flow to the tankless water heater and is a good temporary solution.

8. Frozen pipes

When the pipes that lead to the tankless water heater are frozen, then the water will not be able to flow through them. This can cause the unit to overheat, as it will not be able to get rid of the heat that it is generating. In extreme cases, the frozen pipes can burst and cause extensive damage.


Thaw the pipes: The first thing you need to do is thaw the frozen pipes. You can do this by applying heat to the pipes, using a hair dryer, or a heat lamp. Once the pipes are thawed, the water should start flowing through them again.

Insulate the pipes: Another solution is to insulate the pipes to prevent them from freezing in the first place. You can do this by wrapping them in foam insulation or electric heat tape.


A tankless water heater can greatly improve the quality of your life, as it provides hot water on demand. However, if the unit is not properly maintained or installed, it can cause several problems that will affect its performance and cause your tankless water heater to go cold after a few minutes or right from the get-go.

The best way to prevent this is to have your tankless water heater checked and serviced by a professional. Danika Plumbing has a team of experienced professionals that can provide expert advice and test and repair your unit. Call us today at +1 425-374-1557 or schedule an over-the-phone consultation to find out more.

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