Tankless Water Heater Goes Cold After a Few Minutes

On a cold day, you’re taking a hot shower, shampoo in your hair, and soap in your eyes and suddenly the water becomes lukewarm. You realize you’d need to pick up the pace and rinse off before the water becomes cold. But alas, you didn’t make it! A torrent of ice-cold water ruins what would have been a relaxing shower.

You think you’ve fixed the problem by buying a tankless water heater. But what happens when your tankless water heater goes cold after a few minutes? This problem cuts across the different brands of tankless water heaters, including Rinnai, Rheem, Ecosmart Nortiz, and others.

Do you want to know the reason this happens and how to fix it? Read on to find out!

What Are Tankless Water Heaters?


Tankless water heaters are also known as demand-type, and they provide hot water only when it is needed. As a result, they don’t produce the standby energy losses that are associated with storage water heaters and, as such, save energy and power.

How Does a Tankless Water Heater Work?

A tankless water heater heats your water instantaneously when the hot water faucet is turned on. When you turn on your hot water faucet, a gas burner or an electric element will heat the cold water as it flows through the heat exchanger unit. Normally, a tankless water heater can provide 2-5 gallons(7.6-15.2 liters) of water per minute. Gas-fired tankless heaters have a higher flow rate than electric ones.[1] [2]

Pros of a Tankless Water Heater

  • Tankless water heaters can be 24%-34% more energy efficient than storage tank heaters for homes that use at most 41 gallons of water. In addition, they are 8%-14% more efficient in homes that use around 86 gallons of water per day.
  • Most tankless heaters have a life expectancy of 20 years and above.
  • Tankless heaters avoid the standby heat losses associated with a storage water heater.
  • Tankless water heaters are small and will be a great option, especially if you have limited space.

Cons of a Tankless Water Heater

  • Tankless water heaters cannot cater to multiple outlets at once.
  • Since tankless water heaters do not have the option of water storage, if the power should go out, so will your hot water.
  • A tankless water heater is way more expensive upfront. For example, the cost of a tankless water heater’s natural/propane gas model is $1000-$1500, and that of the electric models is $500-$1500. And it also costs a sizeable amount for your plumber to install the unit.

Why Does a Tankless Water Heater go Cold?

First, if you turn on and get alternating cold and hot water, your problem is a cold water sandwich. If that’s the case, then all you need to do is install a small tank heater in line with the tankless water heater. So if you’re reading on, you are getting cold water continuously after a few minutes of hot water.

I’m going to give you some possible reasons why your gas or electric tankless water heater goes cold after a few minutes.

1.    Calcium and Mineral Build-up

Mineral build-up in the heat exchanger of your tankless heater can cause several problems. One such problem is the continuous stream of cold water after a few minutes.

This is a common cause of such a problem, especially if hard water comes into your home. Over time, minerals begin to build up inside the heat exchanger. Between a few months and a year, the build-up can accumulate so much that they prevent the water heater from operating effectively.

So, what now? The solution is quite simple, flush your tank with vinegar. To also prevent the occurrence of mineral build-up, get a water softener. It will help clear the pipe and make the water taste much better.

2.    Broken Dip Tube

This is the most common cause of a heater running cold water after an initial stream of hot water. Normally, this issue occurs in old water heaters because, over time, the dip tube becomes damaged. A dip tube is a pipe made of plastic that connects the cold water inlet to the water heater.

This tube carries cold water straight to the bottom of the tank, where the burner is located. Once the water is heated, it rises to the surface and runs through the hot water outlet into your shower, sink, or other appliances.

Hot water is at the top of the tank, and a dip tube prevents the cold water from cooling the already-heated water. Dip tubes are made of plastic; the more they are exposed to varying water temperatures and acidity, the more they will break down. If the tube is defective, the cold water mixes with the hot water.

The only solution to this is replacing the dip tube. Dip tubes can be found online or in local hardware stores. If you need to know the size of your dip tube, go to the manufacturer’s website and get the specifications of your water heater.

3.    Poor Maintenance

This is the main reason why your gas tankless water heater goes cold after a few minutes. This is because gas tankless water heaters are more sensitive to maintenance than electric tankless water heaters.

Over time, burned gas particles can accumulate on the vent, exhaust fan, and even on the gas valve filter. This will then cause inefficiency in venting the gases and suffocate the burner. In the case of the filter, it will reduce the proper gas supply.

The implication is that the gas fires initially but can’t remain lit. Hence, the water that runs initially is hot and becomes cold after a few minutes. Fixing this issue or even servicing the heater will involve taking apart the entire unit. Therefore, you will need the services of a professional plumber.

4.    Low Activation Flow Rate

Every tankless heater has an activation flow rate as part of its safety mechanism. This is to say that a tankless water heater must be able to sense water flow to initiate the operation.

To avoid overheating, tankless heaters require a minimum flow of water, and failure to meet these requirements will make the heater run cold water after a few minutes. A tankless heater must have a 0.5 gallons per minute minimum flow rate for the heat exchanger to be activated

To fix this, ensure that the flow rates at outlets connected to the heater exceed its minimum requirements. Also, try running two hot water fixtures simultaneously. For example, if you’re using a low-flow shower head, keep the bathroom faucet running and then start the shower. If this works, replace your shower head with a standard flow model.

5.    Dirty Flame Sensor

There is a flame sensor or thermocouple found in gas tankless heaters that can shut off the gas supply when it detects no flame. This is a safety device that is thin, rod-like, and sits near the burner. When this sensor gets dirty, it can no longer detect presence\absence of flame accurately and can shut off the gas supply.

Unfortunately, this can also cause a burst of cold water after some time. If you can, open the front panel of your tankless heater and clean the flame sensor with a lint cloth.

6.    Faulty Flow Sensor

This can occur in the tankless water heater’s gas and electric models. The function of a flow sensor is to detect the flow of water through the unit and ignite the water heater.

If the flow sensor is faulty, which can happen if the model is old, it can fail to detect flowing water. The heater’s power will be switched off if flowing water is not detected.

You would have to call a professional plumber to handle this issue because you might need to replace the flow sensor.

7.    Low Gas Pressure

This can also be a problem. When the natural gas supply coming into your home doesn’t have sufficient pressure, either due to gas leakage or improper gas line installation.

You will have to call your gas company for them to inspect the gas line for leakage or low pressure. You may have to retrofit the gas line if there isn’t any pressure.

Does a Tankless Water Heater Take a Long Time to Heat Up?

Normally, an electric tankless water heater gives hot water within seconds. It may take longer with a gas tankless water heater, but I’m talking about seconds, not minutes. Generally, tankless heaters need 15 seconds to heat water to the pre-set temperature, but it may take longer than that for hot water to come out of the shower head.

Does a Gas Hot Water Heater Run All The Time?

Pilot-controlled gas water heaters use a continuously running pilot light and can work even during a power outage. The pilot light is what needs to be lit to heat up your water.

An electric-controlled gas water heater requires electricity to ignite the heat source. So if the power goes out, the water may remain hot for just a few hours, and you’ll need to wait for the electricity to return to reheat it once it gets cold.

Final Thoughts

Even with the higher upfront cost, a tankless water heater quickly becomes a sought-after option. Apart from reducing your electricity bill, this heater is also more eco-friendly and lasts longer than conventional tank heaters.

So, even if you have the abovementioned issues, it’s nothing to worry about. Try out the solutions mentioned and thank me later.

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