Tankless Water Heater Goes Cold After a Few Minutes

Having the water go cold only after a few minutes as you’re enjoying your shower after a long day is undoubtedly one of the most frustrating experiences.

This temperature fluctuation could be caused by a couple of factors that we will discuss in detail in this guide, as well as the most effective methods of solving this issue permanently.

Keep on reading to discover how to properly troubleshoot your water heater and prevent this tankless water heater problem in the future.

Why does the tankless water heater go cold after a few minutes?

While there is no universal answer to this question, you shouldn’t despair. After detailed troubleshooting, you should be able to detect the core issue and solve it quicker than you thought. 

1. Cold water sandwich

One of the main reasons why your hot shower quickly turns into a cold shower is an issue known as the cold water sandwich.  

This happens when a tankless water heater is installed too close to the incoming cold water line. 

As hot water is used and leaves the tankless unit, it immediately gets replaced with cold water, causing the temperature to drop.

2. Clogged tankless heater

Clogged tankless heater filters or a blocked heating component could suddenly cause your water to go cold.

This happens due to a buildup of minerals and sediment in the heating component or filters, which restricts the flow of hot water.

To fix this issue, regularly clean and replace your tankless heater filters. If the problem persists, it may be time to call a professional to check on the heating component. 

The mineral buildup could cause numerous other issues, decreasing the efficiency and lifespan of your tankless water heater. 

3. Faulty flame sensor

Another issue that could be the reason your tankless water heater goes cold after a few minutes is a faulty flame sensor.  

The flame sensor detects the flame from the burner and communicates to the control board whether or not it is lit. 

If the sensor is not working correctly, the control board may shut off the gas supply, causing the water to go cold. 

Your flame sensor may not be working correctly due to the buildup of debris or corrosion. In this case, it can be cleaned and potentially fixed. 

However, if the sensor is damaged beyond repair, it will need to be replaced for your tankless water heater to function correctly again.

4. Faulty flow sensor

A faulty flow sensor could be the culprit for your tankless water heater suddenly losing hot water

A flow sensor is a small device that detects when water flows through the heater and signals the heating process to begin. 

If this sensor is not functioning correctly, it may cause the heating process to shut off prematurely, resulting in a loss of hot water. 

You can try cleaning the flow sensor to see if that resolves the issue. However, if it is damaged or malfunctioning, it requires a replacement. 

5. Activation flow rate issue

The activation flow rate issue is a common problem with tankless water heaters. Not enough water flows through the heater, causing it to cool down quickly.

6. Gas burner or gas line issue

If you have a tankless gas water heater, there could be a problem with the gas burner or the gas line.

One possible issue could be that the gas burner is not functioning correctly. This could cause the water to cool down quickly after only a few minutes. 

Another possibility is that the gas line is not providing enough fuel to sustain a constant hot water supply. 

How to fix a tankless water heater that goes cold after a few minutes

Your tankless heater doesn’t necessarily require professional attention. Once you’ve detected the issue, try one of these fixing methods to ensure the water temperature doesn’t fluctuate.

However, if you detect a severe defect or a more complex issue, such as a blockage in the gas line, it is recommended to at least consult with a professional. 

1. Check for the cold water sandwich 

A cold water sandwich happens when the hot water is used up quickly, and the tankless water heater needs time to heat more water

This can be caused by a low flow rate or a shortage of gas or electricity needed for heating. 

To prevent this, try spreading out hot water usage among multiple fixtures or increasing the flow rate of your system. 

You may also want to check that your gas or electricity supply is sufficient for your water heater’s demands. 

If the problem persists, it could be a sign of a faulty component, and you may need to call a professional for repair or replacement.

2. Unclog your water heater

To unclog and thoroughly clean your tankless water heater, shut off the gas and water supply to the unit

Then, flush out any buildup or sediment within the unit using a flushing kit or vinegar solution. Additionally, check for blockages in the water lines leading to the heater. 

Once everything is cleaned out, turn on the supply and test the heater to see if it maintains hot water consistently.

To ensure that the blockages don’t form in the future, regularly flush the unit and monitor the water lines for any buildup. 

Also, if you have hard water, try to limit the amount of mineral buildup by installing a water filtration system. This will extend the life of your tankless water heater and keep it functioning at its best.

3. Fix the flame sensor

To fix a faulty flame sensor in your tankless heater, start by shutting off the gas and electricity to the unit. 

Then, locate the flame sensor (typically near the burner assembly) and use a small brush or sandpaper to clean it. 

Finally, turn the gas and electricity back on and test the heater’s operation. If the problem continues, consider replacing the flame sensor entirely.

It’s always a good idea to discuss gas-related repairs with a professional. However, if you’re confident enough to replace the flame sensor yourself,

Here are the steps to do it

  1. Shut off the gas and electricity to the unit. 
  2. Locate and remove the old flame sensor (usually near the burner assembly). 
  3. Install the new flame sensor, making sure it’s securely in place. 
  4. Turn on the gas and electricity, and test the heater’s operation.

4. Fix the flow sensor

If a faulty flow sensor seems to be the issue, the first step would be to clean it thoroughly by removing any debris or buildup. 

If that doesn’t solve the issue, you may need to replace the flow sensor altogether.

Here are the steps to replace the flow sensor in your tankless water heater:  

  1. Turn off the water heater’s power.
  2. Remove the access panel to expose the flow sensor.
  3. Disconnect the wires and remove the old flow sensor.
  4. Install the new flow sensor and reconnect the wires.
  5. Place the access panel back and turn on the power to test it for proper function.

5. Adjust the activation flow rate

To adjust the activation flow rate setting, you will need to locate the water heater’s control panel

From there, increase the activation flow rate until the water stays hot for extended periods. 

If this does not resolve the issue, it may signify a more significant problem, such as a faulty heating element or insufficient gas supply.

6. Check the gas burner and the gas line

If the gas burner isn’t working correctly, check to see if the gas supply valve is completely open and if there aren’t any obstructions in the gas line. 

If the burner still isn’t igniting, it could be due to a faulty thermocouple or ignition control module, which will require professional repair.

Another possible cause for a tankless water heater going cold after a few minutes is an issue with the gas line

Ensure the gas line can provide enough pressure to support the water heater’s demands. If not, you may need to upgrade your gas line or adjust the flow rate on the water heater.

Related Article: Rinnai Water Heater Code 12

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