Why Does My Tankless Water Heater Go Cold?

running water in faucet

Isn’t it annoying when the water suddenly goes cold when showering? It beats the purpose of having a water heater in the first place.

So, what’s going on? Why does my tankless water heater go cold?

The most common reasons for your tankless water heater going cold include sediment build-up, system overload, cold water sandwich effect, blocked water supply, and ignition failure. Cold water sand sandwich will resolve itself within a few seconds, while others require troubleshooting and resolving the underlying cause.

Read on to find out more and avoid those cold showers.

Why Am I Not Getting Hot Water from My Tankless Water Heater?

Here are a few reasons why you might not be getting hot water from a tankless water heater and what you can do about it.

Sediment Build-Up

Sediment build-up is one of the most common issues with all types of water heaters. It mostly happens if the incoming water is hard water. This type of water has a high mineral content, particularly calcium and magnesium. The two deposits on the heat exchanger impairing its effectiveness.

If sediment build-up on the heating element is mild, the hot water flow will be as usual. However, if there is excessive build-up, the outlet water will either be lukewarm or completely cold.

Solution. Regularly flush your water heater as part of routine care and maintenance. If your water supply is hard water, channel it through awater softener before connecting it to the water heater to eliminate calcium and magnesium ions. If the heat exchanger is completely damaged due to sediment build-up, replace it with a new one per the manufacturer’s recommendations. It is best to have a professional undertake the heat exchanger replacement.

System Overload

Each tankless water heater has an indicated water heating capacity in Gallons Per Minute (GPM). Smaller tankless hot water heaters have a capacity of 3-5GPM, while larger ones have a capacity of 6-12GPM.

The higher the GPM, the higher the demand for hot water the water heater can support at a time. For instance, if your water has a GPM ranging between 9-12, you can use a showerhead, have a dishwasher running, and brush simultaneously without the water running cold. However, if it is lower than 5 GMP, you might only be able to use one hot water outlet at a time.

Solution. The most obvious solution to this issue is ensuring that you do not overload the tankless water heater’s capacity by limiting the number of hot water fixtures open simultaneously. If the cold-water issue persists despite closing some hot water fixtures, consider resetting the water heater.

Also, you might want to invest in a larger unit or an additional unit if your demand for heated water is consistently high. Consult with a water heater specialist to help you determine the right size of a tankless system for your home.

Cold Water Sandwich

Although tankless water heaters are also known as instant hot water heaters, they take a few seconds to heat water. Therefore, when you first open the faucet or showerhead, cold water will flow out for few seconds before hot water comes in.

You may also experience a cold-water sandwich if you jump into the shower immediately after another person. Since there was hot water already flowing out of the faucet, it will continue to flow until the water lines are drained. A few seconds later, you will be blasted with cold water as the tankless heater begins to heat a new batch of hot water. After a few seconds, hot water will resume.

Solution. There isn’t anything much you can do to resolve the temperature fluctuation other than patiently waiting for the cold water to flow out and for hot water to come in.

Blocked Air Supply

Gas and propane tankless water heaters require an adequate supply of air into the gas burner to produce enough heat to heat your water. If the air supply or vent pipe is blocked, the combustion process will be inefficient, and there’ll be no heat to heat your water.

Most tankless water heaters present an error code if the air supply is blocked. For instance, the Rinnai tankless water heaters produce error code 10 on the digital display. The air supply may be blocked as a result of improper installation or broken or punctured vent pipes. The vent pipe may also be blocked by bird nests, rodents, or the accumulation of dirt.

Solution. A blocked air supply or vent pipe should be addressed immediately as it could lead to a fire hazard or carbon monoxide leakage. It is best to hire the services of a professional plumber to undertake proper installation or unblock the air vent.

Ignition Failure

Often an ignition failure in a gas or propane tankless water heater results from an issue with the gas supply. If there is no gas flowing into the hot water heater, the combustion chamber cannot produce any heat. If you’ve not paid your gas bill, the suppliers may have cut out your supply. Check your propane tank to ensure it is not low on LP.

Other possible causes of ignition failure in gas tankless water heaters include a faulty gas valve or water valve. Otherwise, your ignition pack may have failed and needs to be replaced.

In the case of an electric water heater, ignition failure may result from a power cut. The hi-limit switch may have tripped, or the electric connection to the water heater may be faulty. Also, the ignition rod connection or electrode may be loose. Check if the electronic board has bad connections or a malfunction.

Solution. Ensure that you have paid your natural gas or electricity bill and ascertain a gas supply into your home. If the propane tank is low, refill it. Also, confirm that the water and gas valves are fully opened. If the ignition pack is faulty, contact your water heater supplier for replacement parts.

To resolve ignition issues in an electric tankless water heater, turn off the unit at the main socket and then put it back on to reset it. If the electrode is dirty, clean it gently with 100 grit sandpaper. If the electrode or flame rod is loose, install it properly. Ensure that the electronic board is connected to the right power voltage. If it is faulty, have an electrician replace it.


If hot water suddenly goes cold, give it a few seconds to see if it will resolve itself. If it doesn’t reset the water heater as per the manufacturer’s instructions. If the problem persists, consult with the supplier or a plumber.

Tankless Water Heater FAQs

Still have questions when it comes to your tankless water heater? Read on for answers to some of the more commonly asked questions.

What Temperature Should a Tankless Water Heater Be Set At?

On average, 120F is sufficient. 110F-130F degree is a good temperature range. Note that the higher the temperatures, the more likely the tankless water heater is to cause scalding.

How Do You Reset a Tankless Water Heater?

Most tankless water heaters can be reset by shutting down the unit and then starting it up again. Some manufacturers provide specific instructions for resetting. Check your manual for resetting instructions specific to your tankless water heater model.

Can You Run Out of Hot Water with a Tankless Water Heater?

Yes. If you overload it by exceeding its hot water capacity.

How Many Years Does a Tankless Water Heater Last?

Most tankless hot water heaters have a life expectancy of about 20 years with proper care and maintenance.

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