Few home surprises rank with stepping into the shower and catching a blast of cold water. Maybe you just moved in, came home from a trip, or wrapped up a repair, and now the tank sits cold.
Gas, electric, and tankless units each start a little differently, but one rule holds for all three: the tank has to be full of water before any heat reaches it, or you can wreck the unit in seconds.
Below are the steps for each type, the safety checks worth doing first, and the temperature to set once the heater fires up.
Key Takeaways
- Fill the tank first: Open the cold water valve and bleed air from a hot faucet until water runs steady before sending gas or power to the unit. An empty tank can burn out an electric element in seconds.
- Gas heaters: Open the gas valve, set the knob to “Pilot,” light it with the igniter or a match, hold the knob down about 60 seconds, then turn the dial to “On” and set 120°F.
- Electric heaters: Keep the breaker off, fill the tank until a hot tap runs steady, then flip the breaker on. Power always goes last.
- Tankless heaters: Open the water and the gas or electric supply, press the power button, and set the display to 120°F.
How Do You Turn On a Gas Water Heater?
To turn on a gas water heater, fill the tank and bleed the air out first, then open the gas valve, set the control knob to “Pilot,” light the pilot and hold the knob down while the thermocouple warms, and finally turn the knob to “On” and set the dial to 120°F. Water goes in before gas goes on, every time.
About half of US homes rely on natural gas water heaters. While some older models or units in remote areas might use propane, the lighting process is almost identical.
Modern gas heaters usually have a built-in spark generator (piezo igniter), while older models require you to manually light the pilot with a match. Here is how to get the flame going.
1. Inspect for Gas Smells
Safety comes first. Before you do anything, smell the area around the heater. Natural gas and propane have an additive that smells like rotten eggs. If you smell gas, do not light the heater. Leave the house immediately and call your utility company.
2. Fill the Tank
Never light a water heater that is empty. Locate the cold water shut-off valve above the unit and turn it parallel to the pipe (open). You will hear water moving through the pipe as the tank starts to fill.
3. Bleed the Air Lines
Do not skip this step. Go to a bathroom or kitchen sink and turn on the hot water faucet. You will hear sputtering and hissing as air escapes. Leave the faucet open until a steady stream of water flows out with no bursts of air. That steady stream is your proof the tank is full, and you can then close the faucet.
4. Open the Gas Valve
Locate the gas line feeding into the thermostat control box. Turn the handle so it is parallel with the pipe. This allows gas to flow to the unit. If the handle sits crosswise to the pipe, the gas is still shut.
5. Light the Pilot Light
Before you light anything, wait several minutes for any gas in the compartment to clear. Then locate the control knob on the front of the thermostat.
- For Piezo Igniters (Red/Black Button): Turn the control knob to “Pilot.” Press and hold the knob down. While holding it, press the ignition button repeatedly until you see the small blue flame ignite through the sight glass. Keep holding the control knob down for about 60 seconds to warm the thermocouple.
- For Manual Lighting: If you do not have a clicker button, you will need a long lighter. Turn the knob to “Pilot” and hold it down. Apply the flame to the pilot tube nozzle inside the access panel. Once lit, keep holding the knob for a minute.
That hold is the part people rush. According to water heater manufacturer A.O. Smith, keeping the knob pressed for about 60 seconds, and up to 90 seconds on some models, lets the thermocouple get hot enough to hold the gas valve open on its own (1).
6. Set the Temperature
Release the control knob. The pilot light should stay lit. Turn the knob from “Pilot” to “On” and set your temperature. The U.S. Department of Energy recommends 120°F for most households, which is hot enough for daily use while slowing the mineral buildup and corrosion that shorten a tank’s life. Leaving the dial at 140°F instead wastes roughly $36 to $61 a year in standby heat losses, according to the Department of Energy (2). You should hear a “whoosh” sound as the main burner ignites.
How Do You Turn On an Electric Water Heater?
To turn on an electric water heater, leave the breaker off, open the cold water inlet valve, run a hot faucet until the water flows steady with no air in it, then flip the breaker on. Power goes on last, after the tank is proven full.
Electric models are the simplest type to start and the easiest to destroy if you skip the bleeding step. A heating element that switches on in an empty tank burns out in seconds, a failure known as dry firing, and a replacement element plus the service call costs far more than the two minutes the check takes.
Follow these steps in order.
1. Ensure the Breaker is Off
Go to your electrical panel and verify the circuit breaker for the water heater is in the OFF position. Never fill the tank while the power is on.
2. Open the Cold Water Valve
Turn the cold water inlet valve to the open position to let water rush into the tank. Leave it fully open, not cracked partway.
3. Run a Hot Water Faucet (Essential)
Go to a sink or tub in your house and turn on the hot water handle. Air will push out first, often loudly. Wait until the water runs clear and steady without any air bursts. That is your confirmation the tank is completely full of water and safe to power up.
4. Turn On the Circuit Breaker
Head back to your electrical panel. Flip the breaker switch to the ON position. Unlike gas heaters, electric models are silent, so there is no burner noise to confirm anything happened. The elements are working; you just have to wait and check a hot tap later.
How Do You Turn On a Tankless Water Heater?
To turn on a tankless water heater, confirm the gas or electric supply is on, open both the cold inlet and hot outlet valves, press the power button on the control panel, and set the display to 120°F. There is no tank to fill, so nothing has to be bled first.
Tankless heaters are the easiest type to start for that reason. The procedure differs slightly between gas and electric models.
1. Check Utilities
- Electric Tankless: Ensure the circuit breakers (you may have more than one) are on.
- Gas Tankless: Ensure the gas supply valve is open and the unit is plugged into an electrical outlet (for the digital display and igniter).
2. Open Water Valves
Ensure both the incoming cold water valve and the outgoing hot water valve on the unit are fully open. Many units also have a small inline filter screen on the cold side, so check that its shut-off is open too.
3. Power On and Set Temperature
Press the power button on the control panel. Use the arrow keys to set your desired temperature (120°F is standard). The unit will sit idle until you open a faucet; at that point, it will fire up and heat the water on demand.
What Should You Check Before Turning On a Water Heater?
Before you turn on a water heater, check the temperature and pressure relief valve for drips, clear anything combustible away from the base, and inspect the connections on top of the tank for leaks. Water heaters combine high pressure, electricity, and combustible gas, so the walk-around is worth the few minutes.
Inspect the T&P Relief Valve
Look for the Temperature and Pressure (T&P) relief valve, usually located on the side or top of the tank with a discharge pipe running down. If this pipe is dripping before you even start, you may have a faulty valve or high pressure. Keep an eye on it as the water heats up, since pressure climbs with temperature.
Clear the Area
Water heaters need air to breathe, especially gas models. Ensure the area around the base of the heater is free of combustible items like boxes, paint cans, or rags. Blocking the air intake can create a fire hazard or carbon monoxide buildup.
Check for Water Leaks
Before leaving the unit to do its job, inspect the plumbing connections on top of the tank. If you see any water dripping, tighten the fittings gently or call a plumber. A small leak now can turn into a flooded basement later.
Wait for Hot Water
Recovery time depends on your energy source. Gas heaters typically heat a full tank in 30 to 40 minutes. Electric models are slower and can take 60 to 80 minutes to fully recover. Check the water temperature at a faucet after an hour rather than opening the panel again.
