Hot water reaches your tap because a water heater in your home raises the temperature of incoming cold water using gas burners, electric elements, or heat transferred from surrounding air. The specific process depends on which type of water heater you have, but they all do the same basic job: take cold water from your municipal supply or well, heat it up, and send it to your faucets, showerheads, and appliances on demand.
Storage Tank Water Heaters
The most common setup in homes is a storage tank water heater, a large insulated cylinder that holds 30 to 80 gallons of water and keeps it hot around the clock. Cold water enters the bottom of the tank through a long internal pipe called a dip tube, which pushes it down to where the heating happens. A thermostat monitors the water temperature and triggers the heat source whenever it drops below the set point, typically around 120°F.
In a gas model, a burner underneath the tank ignites and heats the metal bottom, warming the water from below. In an electric model, one or two metal heating elements submerged inside the tank do the work. Either way, hot water naturally rises to the top of the tank, where an outflow pipe draws it off and sends it through your home’s plumbing whenever you open a hot water tap. The tank constantly refills with cold water to replace what you use, so it’s always full.
One downside of this design is standby heat loss. Even when nobody is using hot water, the tank slowly radiates heat into the surrounding space, and the heater cycles on periodically to maintain temperature. That idle energy use adds to your utility bill. Gas tank heaters recover faster than electric ones, reheating about 30 to 40 gallons per hour compared to 20 to 22 gallons per hour for a standard electric unit. That recovery rate matters when multiple people are showering back to back or you’re running the dishwasher and washing machine simultaneously.
Tankless (On-Demand) Water Heaters
Tankless water heaters skip the storage tank entirely. Instead of keeping a reservoir hot, they heat water only when you need it. The moment you turn on a hot water faucet, the unit detects the flow and fires up.
Gas tankless heaters use a flow sensor that detects water movement and immediately ignites a gas burner. Cold water passes through a heat exchanger, a series of coils surrounded by the flame, and comes out hot on the other side. Electric tankless models work slightly differently: a temperature sensor detects the incoming cold water and signals a control board, which activates copper or stainless steel heating coils (similar to the elements in an oven) to warm the water as it flows through.
Because there’s no tank to drain, you theoretically get endless hot water. The trade-off is flow rate. A tankless unit can only heat so many gallons per minute, so running multiple fixtures at once can overwhelm it. High-efficiency gas tankless models carry a Uniform Energy Factor (UEF) of 0.95 or higher, meaning they convert nearly all the fuel they burn into usable heat.
Heat Pump Water Heaters
Heat pump water heaters are the most energy-efficient option available for electric homes. Instead of generating heat directly, they move heat from the surrounding air into the water tank using the same principle as a refrigerator, just in reverse. A refrigerator pulls heat out of the food compartment and dumps it into your kitchen. A heat pump water heater pulls heat from the ambient air around it and transfers that energy into a storage tank of water.
This process uses a compressor and refrigerant loop to concentrate low-grade warmth from the air into high-temperature heat for the water. Because it’s moving existing heat rather than creating it from scratch, a heat pump water heater uses roughly two to three times less electricity than a standard electric tank model. ENERGY STAR-certified heat pump water heaters must achieve a UEF of 3.30 or higher, meaning they produce more than three units of heat energy for every unit of electricity consumed. They work best in spaces that stay between 40°F and 90°F, like a garage or basement, and they slightly cool the surrounding air as a side effect.
Combination Boilers
If your home uses a boiler for heating, you may get your hot water from a combination (combi) boiler. These units handle both space heating and domestic hot water in a single appliance. When you turn on a faucet, a diverter valve inside the boiler redirects the heating water away from your radiators and over to a separate heat exchanger dedicated to your tap water. Your incoming cold water passes through that exchanger, absorbs the heat, and flows to your faucet.
This is called DHW priority mode. The boiler devotes its full capacity to heating your tap water until you close the faucet, at which point it switches back to space heating. That means your radiators briefly pause while someone showers, but the interruption is short enough that most people never notice a temperature change in the house. Combi boilers are compact and popular in smaller homes or apartments where space for a separate water heater tank is limited.
What Temperature Your Water Should Be
Most water heaters ship with a default setting of 120°F, which is hot enough for comfortable showers and effective dishwashing. This temperature also sits below the threshold where water can quickly scald skin, making it a reasonable balance between safety and comfort.
There’s a less obvious concern, though: bacteria. Legionella, the bacteria responsible for Legionnaires’ disease, thrives in water between 77°F and 113°F and can survive at temperatures as low as 68°F. The CDC recommends storing hot water above 140°F to kill Legionella effectively and ensuring circulating hot water never falls below 120°F. The problem is that 140°F water at the tap can cause serious burns in seconds. The solution is a thermostatic mixing valve, a small device installed near your fixtures that blends the super-hot stored water with cold water to deliver a safe temperature at the tap while keeping the tank hot enough to prevent bacterial growth.
Keeping Your Water Heater Working
Every tank-style water heater contains a sacrificial anode rod, a metal rod (usually magnesium or aluminum) that hangs inside the tank and corrodes on purpose. Corrosive minerals and sediments in your water attack the anode rod instead of the tank walls, which protects the tank from rusting through. Once the rod is fully corroded, the tank itself starts to deteriorate. Replacing the anode rod every three to five years is the single most effective thing you can do to extend your water heater’s life.
Sediment also builds up at the bottom of the tank over time, especially in areas with hard water. This layer of mineral deposits insulates the water from the heat source, forcing the unit to work harder and raising your energy bill. Draining a few gallons from the tank’s drain valve once or twice a year flushes out loose sediment before it hardens into a thick crust.
Every water heater also has a temperature and pressure relief valve (T&P valve) on the side or top of the tank. This is a critical safety device. If the temperature or pressure inside the tank climbs dangerously high, the valve opens automatically to release water and prevent a potential rupture. You can test it by lifting the small lever on top. Water should flow briefly through the discharge pipe and stop when you release it. If it drips constantly or doesn’t release at all, the valve needs replacing.
Gas vs. Electric: Practical Differences
Gas water heaters heat water faster and cost less to operate in most regions because natural gas is cheaper per unit of energy than electricity. A standard gas tank model reheats water nearly twice as fast as a comparable electric one, which means you’re less likely to run out of hot water during heavy use. Gas units do require venting to the outside, either through a chimney or a direct vent through a wall, and they need a gas line connection.
Electric water heaters are simpler to install, don’t produce combustion gases, and work anywhere you have a 240-volt outlet. They’re less expensive upfront but typically cost more to run month to month, unless you upgrade to a heat pump model. For homes without a natural gas connection, a heat pump water heater is the most cost-effective electric option over its lifetime, despite a higher purchase price. ENERGY STAR-certified gas storage tanks need a UEF of at least 0.81, while certified heat pump models must hit 3.30 or higher, illustrating just how large the efficiency gap has become between conventional and heat pump technology.