There are a handful of moments in life that remind you just how much you depend on hot water. Moving into a new home and discovering the shower runs ice cold is one of them. Coming back from a long vacation to find the pilot light went out is another.
If you have found yourself in either of those situations recently, you are probably asking the same basic question a lot of homeowners ask at some point: how do you turn a water heater on? The answer depends on what type of unit you have, but the good news is that for most households, it is something you can handle yourself with a little patience and the right information. This guide walks through the process for gas, electric, and tankless models, along with common mistakes to avoid and a few situations where calling a professional is genuinely the smarter move.
Turning on a water heater comes down to three things: making sure the unit has fuel or power, checking that the thermostat is set correctly, and following the startup sequence specific to your type of unit. Gas water heaters require you to light the pilot or ignite the burner. Electric units just need the circuit breaker switched on and the thermostat set. Tankless water heaters, whether gas or electric, typically need both the power and the water supply turned on before they will fire up.
Before you start: what to check first
It sounds obvious, but a lot of people skip the basics and go straight to diagnosing something complicated when the actual issue is much simpler. Before doing anything else, make sure the cold water supply valve to the unit is fully open. This is the valve on the pipe feeding into the top of the tank. If it is closed, no water enters the tank, and a dry tank on a gas unit can cause serious damage to the heating elements or the tank itself. On an electric unit, running the element dry will burn it out almost immediately.
Also check that the pressure relief valve, the small lever or valve usually on the side of the tank, has not been tripped. If it is in the open position, water will leak out and pressure will not build properly. Most of the time these are fine, but it is worth a quick look before startup.
How Do You Turn a Water Heater On
Gas water heaters are the most common type found in North American homes, and starting one up is not difficult once you understand the sequence.
Step 1: Check the gas supply. Locate the gas shut-off valve on the supply line leading to the unit. It should be in the open position, meaning the handle is parallel to the pipe. If it was turned off, open it slowly and give it a moment. If you smell gas at this point, stop immediately and ventilate the area before proceeding. A faint smell of gas that clears within a minute or two is normal as the line pressurizes, but a strong persistent smell means something is wrong and you should leave the area and call your gas company.
Step 2: Set the thermostat. The temperature dial is typically located on the gas valve assembly near the bottom of the tank. Set it to the lowest position before lighting the pilot. This is a step a lot of people skip, and it can cause problems if the thermostat is set too high during startup.
Step 3: Light the pilot. Most modern gas water heaters have an electronic igniter, which means you do not need a match or lighter. Turn the gas valve knob to the “pilot” position, press and hold it down, and then press the igniter button repeatedly until you see the pilot flame through the small viewing window, if one is present. Keep holding the knob down for about 30 to 60 seconds after the flame lights. This allows the thermocouple, the small sensor that detects the flame, to heat up enough to keep the gas flowing. If you release too soon, the pilot will go out and you will need to start again.
Step 4: Turn to the desired temperature. Once the pilot is confirmed lit, turn the gas valve knob from “pilot” to “on,” then set the thermostat to your desired temperature. For most homes, 120 degrees Fahrenheit is the recommended setting.
Step 5: Wait for the tank to heat. A full tank of cold water can take 30 to 60 minutes to reach temperature, sometimes longer for larger tanks. Give it time before assuming something is wrong.
How to turn on an electric water heater
Electric units are arguably simpler to start because there is no pilot or gas line involved. The process is mostly about making sure the power is on and the settings are right.
Step 1: Fill the tank first. This is the most important step and the one most often skipped. Before turning on the power to an electric water heater, make sure the tank is completely full of water. Turn on the cold water supply valve, then open a hot water faucet somewhere in the house. When water flows steadily from that faucet, the tank is full and you can close the tap.
Step 2: Restore power at the breaker. Electric water heaters typically run on a dedicated 240-volt circuit. Go to your electrical panel and flip the breaker for the water heater to the on position. If it was already in the on position after a move or power outage, try switching it off and back on to reset it.
Step 3: Set the thermostats. Most electric tank water heaters have two thermostats hidden behind access panels on the side of the unit, one for the upper heating element and one for the lower. Using a flathead screwdriver, remove the panels and set both thermostats to the same temperature, again 120°F is a good starting point. Mismatched thermostat settings are a surprisingly common cause of uneven hot water, and most people never think to check this.
Step 4: Replace the panels and wait. Electric units take a bit longer to heat a full tank than gas units, typically 60 to 90 minutes. The upper element heats first, which is why you will get some hot water before the whole tank is ready.
How to turn on a tankless water heater
Tankless water heaters work differently from tank-style units because they heat water on demand rather than storing it. That means the startup process is a bit different too.
For a gas tankless unit: Make sure the gas supply is open, the cold water inlet valve is open, and the unit is plugged in or connected to power. Most tankless water heaters have an electronic ignition and do not require manual pilot lighting. Turn on a hot water tap and the unit should fire up automatically. If it does not, check the display panel for any error codes, as most modern tankless units will tell you exactly what the problem is.
For an electric tankless unit: Make sure the circuit breaker is on, the water supply valve is open, and the unit’s settings are configured to your desired temperature. Like the gas version, it should activate automatically when a hot water tap is opened.
One thing worth knowing about tankless units: they require a minimum flow rate to activate, usually around 0.5 gallons per minute. If you open a tap to just a trickle, the unit may not turn on. That is not a malfunction, it is just how the technology works.
Common mistakes people make
The biggest mistake with gas water heaters is not holding the pilot button down long enough. People press it, see the flame, let go too quickly, and then cannot figure out why the pilot keeps going out. You need to hold it for a full 30 to 60 seconds, even after the flame appears. That is just how the thermocouple safety system works.
With electric units, the most common error is turning on the power before the tank is full of water. Running a dry element will burn it out almost instantly, and replacing an element costs anywhere from $20 to $50 for the part alone, plus labor if you bring in a plumber, typically $100 to $200 for the job. That is an avoidable repair.
Another mistake that comes up more than it should is ignoring error codes on newer units. Modern water heaters, especially tankless models, have diagnostic systems built in. A lot of homeowners see a blinking light or a code on the display and just reset the unit without looking up what the code means. Sometimes that code is telling you something important, like a blocked venting issue or a failing sensor, and ignoring it leads to a bigger repair bill down the road.
When to call a professional
There are a few situations where it is worth picking up the phone instead of troubleshooting yourself.
If you smell gas strongly and it does not clear quickly, do not try to start the unit. Leave the area, avoid using any switches or open flames, and call your gas supplier or a licensed plumber. Gas leaks are genuinely dangerous and this is not the place to experiment.
If your water heater has been sitting unused for a long time, years rather than months, a professional inspection before startup is a reasonable precaution. Anode rods corrode over time, seals can dry out, and venting can develop blockages. A plumber can check all of this in under an hour.
If you restore power or light the pilot and the unit still does not produce hot water after a full hour or two, there is likely a component failure involved. On an electric unit, this is often a burned-out heating element or a failed thermostat, with repair costs typically running $150 to $300 depending on the issue. On a gas unit, a faulty thermocouple is the most common culprit and usually costs $20 to $50 for the part plus labor.
What it might cost if something goes wrong
It is worth knowing what you are potentially dealing with financially before you start digging into things.
A thermocouple replacement on a gas water heater is one of the cheaper fixes, usually $75 to $150 total with labor. A gas valve replacement is more significant, running $200 to $400 depending on the unit and your location. On an electric unit, replacing a heating element typically costs $150 to $250 installed. If the control board on a tankless unit fails, parts alone can run $200 to $500, making it worth comparing against the cost of a new unit, especially if the heater is already over 8 to 10 years old.
New tank water heaters run $400 to $900 for the unit, plus $200 to $500 for professional installation. Tankless units cost more upfront, usually $800 to $1,500 for the unit and $500 to $1,000 installed, but tend to last longer and use less energy over time. In some parts of the world, including the UK and Australia, prices vary depending on local labor rates and whether the unit is gas or electric, but the ranges above are a reasonable reference point.
A few practical tips most people overlook
If you are turning on a water heater that has been off for several weeks or more, it is a good idea to flush a little water from the drain valve at the bottom of the tank before heating. This clears out any sediment that may have settled. It takes about five minutes and can extend the life of your unit noticeably.
Also, if you are not sure what type of water heater you have, look for a label on the side of the unit. Most manufacturers include a diagram of the startup procedure right on the label. It is not always easy to read, and the font is usually tiny, but it is there.
Read more: What temp should hot water heater be
Getting a water heater started for the first time, or restarted after a shutdown, is one of those tasks that seems intimidating until you actually do it. Gas, electric, and tankless units each have their own process, but none of them are especially complicated when you take it step by step. The most important things are filling the tank before turning on power, lighting the pilot correctly on gas units, and not skipping the basic checks before you begin.
If you follow the steps above on how do you turn a water heater on and something still is not working after an hour or two, that is a good sign something mechanical has failed and a licensed plumber is your next call. Most water heater repairs are not catastrophically expensive, and catching a problem early almost always saves money compared to waiting until the unit fails completely.
