Here’s Why Your Tankless Water Heater Goes Cold After a Few Minutes & Its Solution

Does your tankless water heater go cold after a few minutes?
I know how frustrating it can be when you suddenly get bombarded with cold water in the shower. Whether it is Rinnai, Rheem, Ecosmart Nortiz, or any other brand. Wanna know how to fix it? Let’s learn why this is happening and discover how to fix it.

But first, let me make it clear. Are you facing, what is also called, a cold water sandwich? It’s a fancy pant term for when on opening the tap or shower, you get hot water, then cold water for a minute, and then continues hot water. If that’s the case in your home then you need to install a small tank water heater in line with the tankless water heater to solve this problem. Call your plumber and get it installed.

If you aren’t getting any hot water at all from your water heater then click here.

I’m assuming a cold water sandwich is not what your case is and you are continuously getting cold water after a few minutes of hot water bursting.

6 Possible reasons for your tankless water heater to go cold after a few minutes

1. Calcium and mineral build-up 

scale build up

The mineral buildup in the heat exchanger of the tankless water heater can make the unit act weirdly. One such act can be a tankless water heater sending cold water after a few minutes. Trust me, it’s a pretty common culprit. 

Especially if you have hard water coming into your home. The mineral starts to stack up inside the heat exchanger over time, like in a few months to over a year, these minerals get so large that they prevent the water heater from efficiently heating the water. 

So what you should do first, if you haven’t already, is to get your tankless flushed with vinegar. Here’s the complete guide to help you with that

2. Poor maintenance

Do you have a gas model? Tankless gas water heaters are slightly different and sensitive to maintenance and servicing compared to electric.

Is your tankless gas water heater old? Do you remember the last time you had it serviced? 

Over time, burned gas particles accumulate on the vent, exhaust fan blade, and even gas valve filter. It can cause inefficiency to vent out the flue gases and suffocate the burner and reduces proper gas supply in case of filter. So the burner somehow fires initially but can’t keep it lit resulting in the water turning cold.

Fixing this issue and servicing requires dissembling the unit which can get complicated. I advise you to call a professional plumber with expertise in tankless water heaters for that. 

3. Not calling enough hot water


What I mean by this is that every tankless water heater has an activation flow rate as a safety mechanism. You have to call at least a certain amount of water for the heater to get ignited and heat water. If the incoming water is lower than that then the heater won’t ignite and you get cold water. 

Try running two hot water fixtures at the same time. Are you using a low-flow showerhead and getting cold water during the shower? Keep the bathroom faucet running and then start the shower. If this fixes the problem then replace the showerhead with a standard flow model.

4. Dirty flame sensor

A flame sensor or thermocouple is found in gas-fired water heaters. It’s a safety device that has the role to shut off the gas supply when it detects that there’s no flame. 

It’s a thin rod-type device that sits near the burner. When this device gets dirty it no longer can effectively detect the flame and shut off the supply and you get cold water right after a hot water burst.

You can open the front panel, sand, and clean the flame sensor rod with a lint cloth to see if it works. 

5. Defective flow sensor

sensor device
typical sensor

It can be electric or gas. The flow sensor is found in both types of tankless models. The function it has is to detect the flow of water when water flows through the unit and ignite the water heater. 

If this flow sensor is rendered defective, which it can be if the model is old, then it will fail to detect water flow and switch off the power of the heater and you only get lucky to enjoy a few minutes of hot water. 

Chances are you might need to replace the flow sensor. I advise calling the plumber here.

6. Low gas pressure

I have seen this problem with a few of my clients. The natural gas supply coming to their home didn’t have enough pressure due to either leakage or improper installation of the gas line.

What you can do here is call your gas company and get your gas line inspected for leakage and pressure. If there isn’t enough pressure, then chances are your will have to retrofit the gas line.

If your appliance is too old then maybe it’s time to replace it. Start here with the top selected electric tankless water heaters here.

Your call!

So that’s it for now. I hope I could help you out with this irritating problem. Discussed above were the most cases I have seen in my work experience for a tankless water heater to go cold after a few minutes. 

Go ahead and try them. If nothing works out then call big guns and that would be professional.

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