why am i not getting hot water from my tankless water heater

Fixed: Why Am I Not Getting Hot Water From My Tankless Water Heater?

Does your tankless water heater shut off during the shower? Or are you not getting hot water from it? Don’t worry, I will help you out with that in this article.

One of the best domestic water heating systems is the tankless water heater. These are small, compact, provide on-demand hot water which makes it super efficient, and are reliable. 

But these very reliable tankless water heaters too are prone to rare issues and glitches. One such common issue is not getting hot water or getting cold water from your tankless water heater. And that’s the issue we are going to investigate and try to solve today. But before we do that let’s learn in brief about how a tankless water heater works. And check this guide for tank water heater heating problems.

How does a tankless water heater work?

When you open a hot water fixture, cold water enters the heat exchanger of the heater where a sensor detects it and starts the appliance. The water then flows through the heat exchanger where it gets heated and reaches the fixture.

The tankless water heater starts only when hot water is called.

8 Reasons you are not getting hot water from your tankless water heater & how to fix it

1. Pushing your tankless water heater to its limit

multiple taps openining

Sure, a tankless water heater can deliver hot water for an infinite time. But there’s a limit to how much hot water it can deliver per minute and if you push this limit then you are going to get lukewarm or worst yet chilling cold water. 

How can you push this limit? If you are running multiple fixtures like showering and washing hands and doing laundry all at the same time then you are possibly pushing your water heater to its limit.

To find out if that’s the issue, close all taps, reset the heater and open only one tap at a time. If you get hot water then you have found the culprit. 

If you face this cold water issue frequently then you might need an upgrade or a second tankless water heater installed parallel to the first one. Check out the best whole house electric tankless water heater here.

2. Excessive scale buildup

This can very likely be a cause if you have hard water coming to your home. Hard water is when your water has a lot of calcium and magnesium contents.

Over time, these minerals start to stack up inside the heat exchanger which reduces its efficiency to heat water. If your tankless water heater is not regularly maintained then at some point, not just these scales prevent water from heating but also can damage the unit altogether. 

If you have hard water coming to your home and haven’t flushed your water heater for over a year then it’s to do it now. Also, try to install a water softener. Here’s the water heater flushing and descaling guide.

3. Clogged ventilation or air intake

obstructed vent line

This is applicable to tankless gas water heaters. And usually, most of them will show errors if there’s any blockage on the vent or air intakes. 

When the vent or air intake is obstructed, which could also happen due to things like birds nests, and rodents living inside, the burner doesn’t get the oxygen required for Ignition. And so then obviously water can’t heat up. 

Check for all vent lines and air intakes for any blockages and clean them if necessary. 

4. Problem with the power source

If you have an electric water heater then make sure the electric power to the unit is working. You will want to check the breaker panel for any tripping. It can trip for various reasons. For now, you will want to reset the breaker so power can be resumed to the unit. 

If you have a gas-fired tankless water heater then you will want to first make sure your gas company is supplying gas to your home. Have you paid the bill? 

Make sure the gas valve is completely ON and also water inlet valve is completely ON. 

5. Dirt build-up on the burner

dirty burner

If a water heater is not maintained regularly, the dirt starts to build up on the different parts of the unit. One such part with a tankless gas water heater is a burner that collects burnt gas particles over time. Now, this can block and reduce gas flow which results in lowering flame size. This low flame is hard to be detected by a sensor called a thermostat which then shuts off the gas supply. And this way water doesn’t get heated. It’s a cycle of cause and effect. 

Make sure the burner is clean and if it’s not then it need to be.

6. Bad ignition rod

ignition rod

The purpose of the Ignition rod is to produce the spark that gas can then catch fire from. If the Ignition rod goes bad it can’t produce the spark necessary for flame activation. 

Make sure it’s clean and neat. Try sanding it a bit or you may need to replace it for which you might need to call the plumber. 

7. Frozen heat exchanger

Are you living in freezing cold climates? Frozen pipes and heat exchangers can prevent hot water from reaching your appliances and fixtures. 

You can use a hair dryer to blow some hot air on the heat exchanger so the ice inside melts.

8. Not calling enough hot water

not calling enough hot water

Are you partially opening the tap? A tankless water heater activates when a hot water tap is opened but there’s a minimum flow rate, called activation flow rate for the heater to start. If this minimum flow is not called then the heater won’t start and send you cold water. This is a part of the safety mechanism. 

The 0.5 GPM is the usual activation flow rate and yours could be different. Try to fully open the tap to see if you get hot water. Learn about the pros and cons of tankless water heaters here.


So you just exhausted the full list of the most common reasons for tankless water heaters to send cold water. But this list is not all-inclusive and there can be deeper issues with your tankless water heater. If the above troubleshooting guide doesn’t help then you may need to call a certified plumber.

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