Gas Hot Water Heater
Troubleshooting and Repair Guide

A gas hot water heater troubleshooting guide is a simple guide that helps homeowners deal with the most common problems related to pilot light, thermocouple, gas valve, and gas-related components. Find out what the most frequent causes, symptoms, and solutions are.

Before you try to repair your gas water heater, keep in mind that leaking propane and natural gas can be dangerous. It is good to know that when it is leaking, natural gas tends to rise while propane, as it is heavier than the surrounding air, collects at the floor level.

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In this article:

  1. Safety first
  2. How does the gas water heater work
  3. Problem #1: No hot water
  4. Problem #2: Lack of oxygen
  5. Problem #3: Sooting
  6. Problem #4: Condensation
  7. Problem #5: A burner won’t stay lit
  8. Problem #6: A pilot won’t light or stay on

Gas hot water heater troubleshooting - Safety first

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If you smell a leaking gas near the gas water heater, shut off the pilot light, close the main gas valve, and DO NOT activate any electrical power switch.

Call a plumber or your gas supplier immediately from the remote location (outside, for example). This is a big problem where any spark could be hazardous.

"I smell gas" - you would say... but that doesn't mean there is a problem. Natural gas is odorless, so what you smell is not a gas, but the chemical added as a safety feature so the potential leaking can be easily recognized without calling a professional to identify it.

Before we proceed with the gas hot water heater troubleshooting tips and explain the most common problems and symptoms, we want you to become familiar with how a gas water heater works and its main parts.

Troubleshooting, then, gets much easier and repairs are faster and less costly.

How does the gas water heater work

Gas water heaterGas water heater
  • The cold water from the house plumbing enters the heater tank through the dip tube and fills the tank from the bottom up.
  • The control gas valve keeps the small pilot light on and ignites the gas burner when there is a call from the thermostat - when the temperature in the tank drops below the set temperature. Heating starts immediately.
  • Hot flue gases run through the flue pipe located in the middle of the tank and transfer heat to the surrounding water, making it warmer and finally hot.
  • Hot flue gases tend to rise and, with the help of the venting system, are finally vented out, either naturally or forced by the power vent.
  • Through the openings at the bottom of the unit and draft hood, sufficient air is provided for proper and stable gas combustion.
  • Once the hot water tap is open for the shower or dishes, hot water is drawn from the top of the unit through the hot water outlet, then the plumbing system, and to the opened hot water faucet. Outgoing hot water is then replaced with the incoming cold water.
  • Every time the temperature inside the tank drops below the set temperature on the thermostat, the gas burner kicks in and starts heating water again.

To provide safe, reliable, and long-term heating, gas water heaters are equipped with several safety elements:

  • The temperature and pressure relief valve or TPR prevents extreme pressure and temperature in the tank, helping the excessive air or water get out.
  • The thermocouple is a temperature sensor, designed as the metal tube where one end is enclosed in the pilot flame. When exposed to higher temperatures, it generates and delivers electrical voltage to the gas valve to open and provide gas to the main burner for gas burning. No flame - no gas supply to the burner.
  • The gas control valve is the device from where the gas is delivered to the gas burner. It opens up if the conditions are met. It is also used to control and set the temperature.
  • For the tank and other part protection from corrosion, glass lining on the inside of the tank and sacrificial anode rod are used.

For the proper troubleshooting, it will be assumed that the gas water heater was installed correctly and was functioning correctly before the problem occurred.

The problems that are described below are related exclusively to the tank-type water heater. If you would like to know how to troubleshoot an electric water heater - check out this article, for tankless problems and solution - see this article, and for issues and repairs that are common for both gas and electric tank-type, use this guide.

Note: Basic knowledge of gas and electrical systems is necessary for proper troubleshooting of gas water heaters.

Common problems and solutions

Problem #1: No hot water

Before troubleshooting a gas water heater, check if the gas is supplied to your house and if it is delivered to your water heater; there might be obstructions, shortage, or the main gas valve is simply closed.

If hot water suddenly becomes cold, the water usage might have been exceeding the tank capacity. If that is the case, wait for the water heater to recover.

Another problem related to no the hot water issue is that the pilot light has gone out. Relight your pilot flame according to the instructions printed on the unit, or find the details here. If the pilot doesn't relight, you might need a new thermocouple - use this article to learn how to install it and what can go wrong with the thermocouple.

If the water heater doesn't perform as it used to do, hot water is actually lukewarm; there might be a lot of sediment build-up, which interrupts the heat transfer.

Did you check if the gas valve is working?

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Problem #2: Lack of oxygen

For the proper operation of gas water heaters, fresh air needs to be delivered for complete gas combustion. The air should enter the gas chamber from the base of the unit and enter the flue vent from below the draft hood.

If there is insufficient air flow, problem symptoms are seen as follows:

  • Pilot outage
  • Yellow burner flame
  • Sooting burner flame
  • The smell of burnt gas in the room
  • The possible presence of carbon monoxide.

The solution is prevention; check and provide enough airflow; check the flue vent and base of the water heater for possible obstructions, and do not cover it with the insulation blanket. Clean the flame arrestor regularly.

Problem #3: Sooting

Check for the potential lack of air if your gas water heater is running and the gas chamber and flue vents are sooted while the burner is clean.

If all of these elements are sooted, then: a wrong orifice might be used, there is excessive gas pressure, a gas burner and gas connection in the burner assembly are loose.

If the pilot flame or the main burner flames are too large or small, there might be some dirt and debris in the orifice or gas supply tubing, or an incorrect orifice size has been used. Follow the manufacturer's manual for the right specs.

Problem #4: Condensation

While the gas is burning during the heating operation, the flue gases can cool to the "dew point," resulting in condensation, which can now go back to the gas chamber and affect the flame.

What can cause the flue gases to cool are cold air supply and cool flue pipe surface. It usually happens when a new heater is filled with cold water for the first time.

Condensation also happens if a large amount of hot water is used in a short time and when the incoming water is very cold.

Another reason for condensation is the increased combustion efficiency when above 87.5% of the flue heat is transferred to the surrounding water.

The symptoms of the condensation are water around the heater right after the gas water heater operation; water dripping, small black and red spots on the gas burner, water at the top of the heater, and vent piping (that is the reason for using PVC or more expensive stainless steel vent pipes).

The problem is when the condensate falls onto the burner - you can hear a sizzling or frying noise. A large amount of condensate can even put out the fire.

The solution is to raise the supply air temperature, stored water temperature, or even tank size (make sense if the existing one is too old and low-performing).

Problem #5: Burner won't stay lit

The gas burner won't stay lit due to the following reasons:

  • High ambient room temperature
  • Dirty or clogged air intake
  • Obstructions in the vent
  • Flame arrestor openings are blocked

See why a gas burner is not working.

Problem #6: Pilot light won't light or stay lit

Check if there is a gas at all or if the gas pressure is low, and if there is a problem with the gas supply, call the utility company.

Bleed the gas line if there is some air trapped (it should be done by the certified technician).

When attempting to light the pilot, be sure that the pilot light button is fully depressed. Check if the pilot electrode is sparking to the pilot and if the piezo igniter is working properly.

There might also be a defective magnet combined with the thermostat-gas valve, so replace it if necessary.

If the pilot tube or orifice is clogged, clean it. If the thermocouple connection is loose with the gas valve or in the pilot light assembly bracket, tighten it with your finger and 1/4 turn with the wrench; it might need just a simple adjustment.

The thermocouple has to be appropriately secured and in contact with the pilot flame. If it is broken, replace it. If you have melted insulation on the igniter wire or shorting of igniter wire, replace it.

If the pilot light goes out periodically, the following might be the reasons: condensation may be extinguishing the pilot light, insufficient combustion air supply, clogged or incorrect vent system, or inconsistent gas supply and pressure.

Avoid problems by checking the vent system

The venting system is significant for safe water heater operation and its efficiency and performance. A vent with a 90-degree elbow installed on the top of the tank draft hood can reduce the vent efficiency by up to 50%.

The recommendation is to install the venting system that rises straight up. For horizontal vent pipes, different installation rules are available (use the manufacturer's manual for info).

Decompression is another reason why the pilot flame will fail. If air cannot enter as fast as it is being used, the air pressure drops below the outside pressure, creating an effect called "capping."

The solution for decompression is to make two air vents on the outside wall, one close to the ceiling and the other closed to the ground. If the room is on the balcony, for example, provide a venting opening on the door. Check out how properly to vent a gas water heater.

And, remember, the gas can explode.

Before troubleshooting a gas hot water heater and performing any work on the unit, make sure that the gas supply is off and always refer to the user manual.

Some problems with gas water heaters are easy to troubleshoot and cheap to fix with the correct tools and knowledge, but calling a professional is recommended.

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