6 Reasons Why a Tankless Gas Water Heater May Stop Working


In Brief: Why a Tankless Gas Water Heater Stops Working

A tankless gas water heater can stop working if the narrow apertures of the heat exchanger get plugged by scale. There could also be a problem with the burner or the ignition. Sometimes, the load on the tankless heater exceeds its capacity. Another problem could be blockage of the air intake or venting pipes.

Tankless water heaters are rapidly replacing storage-tank water heaters in homes and commercial spaces. This is because these newer heaters are built to give limitless hot water faster and also save resources.

Their unique technology saves homeowners money, energy, and water. But like all home appliances, they too can face problems if not taken care of.

1. If the heat exchanger needs descaling

If the cold water being supplied to the tankless heater has mineral content, it can cause scale build-up. This scale can end up blocking the heat exchanger.

The heat exchanger of a tankless water heater passes water through very small apertures. These apertures can get plugged due to scale.

With not enough room for the water to pass, the heater is unable to increase the water temperature effectively. This could be a reason for your heater to stop working.

To fix this, you will need to descale the water heater and flush it with white vinegar. To prevent this from happening again, install a water softener, and flush your tankless heater periodically.

2. The burner may be clogged

For gas-powered tankless water heaters, another possible cause could be a problem with the burner. Sometimes, the burner tube gets blocked, causing the combustion quality to suffer.

This could be because of debris, dust, or even an insect that crawled inside. To know if this is the issue, check if the flame is affected. A blocked burner can have a lazy yellow flame instead of a clear blue one.

To solve this problem, you will need to disassemble the burner and look inside. To clean the burner tube, you may have to use a brush, pipe cleaner, or an air compressor.

Other issues with combustion

If you detect a problem with the flame but the burner tube isn’t blocked, check the ignition. A bad ignitor may be defective in starting the combustion.

If the combustion isn’t done properly, there won’t be enough heat energy to transfer to the water. Sometimes, due to poor quality, a previously working ignitor may stop being effective. Get a professional to look at your heating unit to see if the ignitor is working okay.

If the problem persists despite a functional ignitor, there may be an issue with the gas supply. Make sure that your gas bills are paid on time and consult the local gas authorities for assistance.

3. Too much load

One of the most common causes of a tankless water heater to stop working is too much load. If the number of fixtures running simultaneously or their GPM requirements exceeds the capacity of the heater, it’ll stop working.

The product manual will specify the maximum flow rate capacity of the heater. Compare this with the sum of required flow rates for all fixtures and appliances using hot water together.

The easiest way to solve this problem is to stay within the limits of the tankless heater’s capacity. You can manage that by prioritizing fixture use. Otherwise, contact your plumber or retailer for an upgrade to suit your needs.

4. Blocked vent/ air intake

Continued combustion requires steady airflow and its byproducts need to be effectively removed. If the air intake of the gas tankless water heater is blocked, there won’t be enough air for combustion.

Similarly, if the venting pipes are blocked, the fumes from the combustion can’t escape. Both these issues affect the functioning of a tankless heater.

These openings may be blocked because of debris, dust bunnies, or insects stuck inside. These are all common problems faced by gas tankless heater owners.

To fix it, you will need to thoroughly clean the passages. You can prevent this to some extent from happening in the future by using insect screens.

5. Minimum flow rate is too high

If your tankless water heater is a relatively old model, it may have a high activation flow rate. This is the required GPM of the feed water for the flow sensors to detect inflow.

For flow rates lower than this, the sensors will not activate the heating unit. To avoid overheating or malfunction, the low flow rate makes the heater turn itself off. As a result, the water coming out will not be hot.

If you want to solve this problem, you will have to increase the feedwater flow rate. Another option is to upgrade your tankless water heater. Newer models have much lower activation flow rates that go as low as 1/4 GPM.

6. Maybe it’s a cold water sandwich

One more problem that you could face while using a gas tankless water heater is a cold water sandwich. This is when the faucet first gives out the hot water, then cold water suddenly, and then gradually hot water.

This can be quite uncomfortable to experience when in the shower. It happens because the heat exchanger takes some time to heat up.

The initial hot water could be what’s left inside the pipe from the previous use. When the heat exchanger is hot enough, it replaces the cold water with that of the desired temperature.

You can install a mini heater to heat water till the tankless heater’s heat exchanger gets hot. A simpler solution is to wait to jump in the shower till it’s consistently hot.


A gas tankless water heater can stop working because of many issues. These could involve the heat exchanger, the burner, the ignition, load, or the air passages.

Sometimes, the problem could be because the heat exchanger needs to take some time to get hot enough. If that happens, you may experience a cold water sandwich.

These problems can usually be fixed by close examination and servicing. For continued smooth operation, conduct regular maintenance and care practices for your gas tankless water heater.

About Jennifer Moore

I'm Jennifer and I am a hardcore car racing lover. I have been writing on automotive topics for more than 8 years now, specializing in performance enhancement. My passion is to help people find the best of what they are looking for, be it an engine or a set of tires. If you want to know something about cars and how they work, just ask me!