Top 5 Reasons Why Your Gas Water Heater Won’t Heat

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There’s no good time to discover only cold water filling your kitchen sink when the dinner dishes are piling up or, even worse, nothing but a chilling spray is coming out of the showerhead. But having your water heater fail to heat your home’s water during the coldest season of the year is especially agonizing.

Before you suffer through another icy cold shower, let’s examine how a properly-operating gas water heater does its job and the five most likely reasons why the unit is failing to do that job.

How a gas water heater heats the water

How a gas water heater operates, and the efficiency of that operation is determined by three factors: the water heater’s tank, the combustion process, and air flow.


Both the incoming cold water pipe and the outgoing hot water pipe connect to galvanize steel pipes on the water heater tank. Since many homes have copper plumbing, a dielectric union is needed to connect the two types of piping. Without this union, the pipes can deteriorate due to electrolytic corrosion, a process that occurs when water flows across two dissimilar metals. The incoming cold water pipe attaches to the water heater’s dip tube which distributes the incoming water to the bottom of the tank to ensure that all of the water is heated thoroughly. If the dip tube deteriorates and breaks, the incoming water will stay near the top of the tank where it won’t heat properly.

Although the interior of the tank is metal, it’s coated with enamel which prevents salt, chemicals, and other minerals in the water from attacking the metal. To further protect the tank and dip tube, an anode rod is used to attract corrosive minerals, keeping them away from the interior wall of the tank. Over time, mineral deposits collect on the bottom of the tank which can produce a loud popping or knocking sound when the water is being heated. To avoid deposit build-up, every few years you should open the drain valve located near the bottom of the tank and flush out the water.

If the water temperature or pressure increases above proper operating levels, the temperature and pressure relief valve (usually located near the top of the tank) will open to stabilize the pressure. A discharge tube must be installed on the pressure relief valve and positioned six inches from the floor to prevent scalding water from spraying out of the tank and causing injury.


Gas water heaters will ignite a burner positioned directly under the bottom of the tank using either a pilot or a hot surface ignition system. On water heaters using a pilot, the pilot and burner are regulated by a thermostatically-controlled gas valve. When the valve detects a decrease in water temperature, it opens, allowing the pilot to light the burner and the water to be heated. Most pilot systems will use either a thermocouple or thermopile to monitor the pilot. If the pilot goes out, the thermocouple or thermopile will prevent the gas valve from opening. Water heater models with a thermopile will also incorporate a limit thermostat to prevent the gas valve from opening if the burner or the surrounding area gets too hot.

A hot surface ignition system also uses a thermostatically-controlled gas valve along with a control board. In addition, a power vent blower fan will exhaust the fumes generated by this system to the outside of the home. When the gas valve detects a decrease in water temperature, the control board begins a timed ignition sequence by sending 120 volts of alternating current to a draft inducer fan motor which turns on to draw air into the burner. Once the draft inducer fan motor reaches its maximum speed, a pressure switch closes, and the control board sends 120 volts to the hot surface igniter. Next, the control board energizes a solenoid on the gas valve which allows gas to flow into the burner and be lit by the igniter. A flame sensor is used to monitor the burner flame to ensure it is lit while gas is flowing through the valve.

Air Flow

Water heaters with gas burners need to exhaust fumes containing carbon monoxide to the outside of the home. The exhaust goes through a backdraft insert and flue. The backdraft insert prevents air from entering the top of the water heater and improves heat distribution in the tank. The flue allows the fumes to be exhausted to the outside of the home. If the flue deteriorates or is damaged, carbon monoxide can contaminate the air. This is why carbon monoxide detectors should always be installed near the home’s bedrooms and the detectors’ batteries checked regularly. If an exhaust leak is detected, you should shut off the gas supply immediately.

In addition to keeping you safe from harmful fumes, good air flow improves the efficiency of the water heater. A unit located in a utility closet, or a cramped storage area, may not have enough available air for the burner to function properly. Closet doors should be louvered, or space left at the bottom of the door, to increase air flow. Newer models will have a ring filter at the bottom of the heater to prevent dust and debris from entering the unit. If the filter becomes clogged, air flow to the burner will again be reduced. One more safety warning: since gas water heaters operate using an open flame, combustible materials such as paint should never be stored close to the unit.

5 causes of a gas water heater not heating the water

So, now that we’ve gone over how a gas water heater is supposed to heat the water, why would a unit fail to do so? Here are the five most likely causes:

  1. Restricted pilot – Carbon deposits can create a restriction in the pilot, preventing it from lighting the gas burner. You can try cleaning the pilot with a stiff brush or compressed air, but that old pilot may need to be replaced with a new one.
  2. Defective igniter or spark electrode – The igniter or spark electrode is the component the lights the pilot. A defective igniter or electrode will stall the ignition process and prevent the water from heating. You can use a multimeter to test an igniter for electrical continuity to determine if a continuous electrical path is present in the component. If the igniter tests negative for continuity, the part is defective and will need to be replaced.
  3. Faulty gas valve & thermostat assembly – The gas valve opens to allow gas to enter the burner and be lit by the igniter. A faulty gas valve may prevent gas from entering the burner. You should first confirm the ignition source is working properly and the gas valve has pressure before replacing the valve assembly.
  4. Defective thermocouple or thermopile – If the pilot goes out, the thermocouple or thermopile will prevent the gas valve from opening. A defective thermocouple or thermopile may prevent the pilot from staying lit. If you’ve determined the other ignition components are functioning normally, you will likely need to replace the thermocouple or thermopile to solve your water heating problem.
  5. Malfunctioning pressure switch – As noted earlier, on water heaters using a draft inducer fan motor, a pressure switch will close once the fan motor reaches its maximum speed, signaling the control board to continue the ignition process. A malfunctioning pressure switch may fail to close even with adequate air flow. Again, you can use a multimeter to test the switch for electrical continuity to determine if the part has failed, provided the switch is connected to the water heater and the draft inducer fan motor is running.

Find the right part to keep the hot water flowing with Repair Clinic

From pilots, igniters, and gas valves to thermocouples, thermopiles, and pressure switches, Repair carries genuine manufacturer replacement parts that match your water heater model, including those manufactured by Rheem, A.O. Smith, Bradford White, Bryant, Carrier, Coleman, Honeywell, and Lennox. Enter the full model number of your water heater into the Repair Clinic website search bar for a complete list of parts compatible with your unit. You can also explore Repair Clinic’s DIY content library to watch videos and read articles concerning common problems affecting water heaters, how to test certain components, and how to install new parts.

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