How Water Heaters Work (and Why They Fail)
A water heater is one of the hardest-working appliances in your home, and one of the least understood. Most people don’t think about theirs until it stops working. In the best-case scenario for a failing water heater, you take a few cold showers; in the worst-case scenario, your home faces massive water damage.
Once you understand how a water heater works, it’s a lot easier to understand why it eventually fails, and what you can look for to avoid a surprise.
This guide covers three things: what’s inside a water heater; the different types on the market and how each works; and the specific ways water heaters break down over time. We’ll close with how we designed Reservoir’s predictive water heater to avoid common failure points.
Anatomy of a Water Heater
Strip away the brand name, and almost every water heater—gas, electric, or heat pump—is built from the same five basic parts.
Anatomy of a typical tank water heater. The main components are the tank (except with tankless), heating element, plumbing intake and outlet, anode rod, and controls.
Water Storage
Most water heaters (except tankless models, which we’ll get to) store a supply of hot water in an insulated steel tank, usually somewhere between 30 and 80 gallons for a home. The tank has a glass or enamel lining on the inside to slow down corrosion from the water it holds. Insulation wraps the outside so the water stays hot between uses instead of losing heat to the surrounding room.
Heating Elements
The part that adds heat to the water is the part that varies the most between types. A gas water heater uses a burner underneath the tank. An electric resistance water heater uses one or two metal heating elements submerged directly in the tank, similar to the coil in a toaster. A heat pump water heater uses a compressor and refrigerant loop to pull heat out of the surrounding air and transfer it into the water (the same basic principle as a refrigerator, running in reverse).
In indirect water heaters, heat is generated by a separate appliance, such as gas, oil, or propane furnace. That furnace heats a separate supply of water, which heats the water in the standalone tank through a heat exchanger.
Plumbing (In and Out)
Every water heater has a cold water inlet, where water enters from your home’s supply line, and a hot water outlet, where heated water exits toward your fixtures. Hot water is taken from the top of the tank, since heat rises. Inside the tank, a dip tube directs incoming cold water to the bottom, so it doesn’t mix with the hot water sitting near the top waiting to be used.
Corrosion Protection (Anode Rod)
The non-lined sections of steel tanks would rust through in a matter of months if left unprotected, so nearly every tank water heater includes a sacrificial anode rod: a rod made of magnesium or aluminum that corrodes instead of the tank. The water attacks the rod first, because the rod’s metal is more chemically reactive than the tank’s steel. Once the anode rod is fully consumed, the tank itself becomes the next thing corrosion goes after. This is one of the single biggest factors in how long a water heater lasts, and it’s why the typical way you find out your water heater failed is when it’s already leaking on your floor.
Controls (Thermostat and Switching Logic)
At its simplest, a water heater’s control system works like an oven: a thermostat measures the water temperature, and when it drops below a set point, the heating element or burner turns on until the tank is back up to temperature. That’s it, on the most basic units. More advanced water heaters add sensors, diagnostics, and smarter logic on top of that same basic loop.
Types of Water Heaters
Overview of the Main Types
There are four common types of water heaters in U.S. homes today: gas tank, electric resistance tank, heat pump, and tankless (which comes in both gas and electric versions). They all deliver hot water to your fixtures, but they differ in how they generate heat, how efficient they are, and what they cost to install and operate.
Types of water heaters | Upfront cost | Operating cost | Approximately what % of homes in the US have each type |
|---|---|---|---|
Gas tank | $ | $$ | 50% |
Electric resistance tank | $ | $$$ | 45% |
Tankless (gas or electric) | $$$ | $ (gas) or $$$ (electric) | 5-10% |
Heat pump or hybrid tank | $$ | $ | 1% |
Relative upfront and operating cost of different types of water heater. Efficient models like heat pump water heaters can be eligible for energy-efficiency rebates, which brings the upfront cost down further. Operating costs can be reduced for electric heaters by using onsite solar, or taking advantage of time-of-use (TOU) rates from your utility.
How Gas Tank Water Heaters Work
A gas tank water heater burns natural gas or propane in a burner assembly underneath the tank. The flame heats the bottom of the tank directly, and combustion gases rise through a flue that runs up through the middle of the tank before venting outside, transferring extra heat to the water along the way. Gas water heaters recover quickly and work even during a power outage (as long as the gas supply and pilot ignition are functioning), but they’re among the least efficient options since a lot of heat escapes up the flue. Plus, they need a proper gas line and venting, which many homes don’t have.
How Electric Resistance Tank Water Heaters Work
An electric resistance water heater uses one or two heating elements submerged directly in the tank; think of it as a very large, well-insulated version of an electric kettle. Electricity runs through the element, and resistance in the metal converts that electricity directly into heat. These units are simple, cheap to install, and don’t need any venting, but resistance heating is a one-to-one conversion: for every unit of electricity you put in, you get one unit of heat out. That makes them the least efficient option to operate over time, even though they’re often the cheapest to buy.
Water heater type | kWh of electricity use per year, according to Energy Star | Annual electricity cost for hot water, assuming $0.30 per kWh |
|---|---|---|
Electric resistance | 3,500 - 4,500 | $1,000 - $1,400 |
Heat pump or hybrid | $420 |
Comparison of annual operating cost of an electric resistance water heater versus a heat pump water heater, assuming $0.30 per kWh (a typical cost in Boston, MA).
How Heat Pump Water Heaters Work
A heat pump water heater doesn’t generate heat directly. Instead, it moves heat that already exists in the surrounding air into your water. This is the same principle a refrigerator or air conditioner uses, but running in the opposite direction. Because it’s moving heat instead of creating it from a fuel source, a heat pump water heater typically uses two to four times less electricity than a standard electric resistance unit to heat the same amount of water. The tradeoff: it draws heat from the room it sits in, so it cools and dehumidifies that space slightly, and some models recover more slowly than a gas or resistance unit during high-demand periods.
Heat pump water heaters use a heat pump, which is essentially a refrigerator or air conditioner running in reverse, to heat water while using very little electricity.
How Tankless Water Heaters Work
A tankless water heater skips storage altogether. Instead of keeping a tank of hot water ready at all times, it heats water on demand: when you turn on a hot tap, a flow sensor detects the water movement, and either a gas burner or an electric heating element rapidly heats the water as it passes through a heat exchanger, on its way to the fixture. Because there’s no standing tank of water losing heat over time, tankless units avoid standby heat loss and take up less space. However, they require higher instantaneous output to keep up with demand, and they can struggle to keep up if multiple fixtures draw hot water simultaneously. Some homes even end up adding a tank to their tankless setup to make up for this.
Gas tankless systems are expensive to install, but fairly efficient to operate. Electric resistance tankless systems require so much power, and such a substantial electrical circuit, that they are impractical in most homes.
How Water Heaters Fail
Understanding the anatomy makes the failure modes make a lot more sense. Nearly every water heater failure traces back to one of a few root causes.
How Water Heater Tanks Fail
The leading cause of tank failure is corrosion. Once the anode rod is used up, the water starts attacking the tank’s steel directly, and that process is slow and invisible until it suddenly isn’t. Over years, corrosion works through the tank wall from the inside out. There’s rarely a warning: because the failure happens on the inside, most homeowners don’t see a pinhole leak coming, they just find water on the floor. Sediment buildup makes this worse. Minerals in the water settle at the bottom of the tank over time, which can trap heat against the tank wall, waste energy, and accelerate corrosion in the areas underneath the sediment layer. Pressure and temperature issues can add additional stress to the system and exacerbate corrosion, leading to even faster failures.
The sacrificial anode rod keeps the water heater tank from corroding. Once it breaks down, the interior of the tank is vulnerable to rust, and the broken-down material and sediment accumulates at the bottom of the tank, worsening efficiency.
How Tankless Water Heaters Fail
Tankless units fail differently, because there’s no tank to rust through. The most common issue is mineral scale building up inside the heat exchanger, especially in hard water areas; scale acts as insulation, forcing the unit to work harder and eventually restricting flow or triggering error codes. Without yearly service to clean the heat exchanger, which can be expensive, a tankless system will fail.
Igniters, flow sensors, and heat exchangers themselves are also common failure points, particularly on gas units, and they tend to fail as individual components rather than as a catastrophic single event. However, tankless units with digital controls often signal a problem through an error code well before it fails outright, giving you a chance to service it rather than replace it.
When Is It Time to Replace Your Water Heater?
A few signals to watch out for:
Age. Tank water heaters typically last 8–12 years; tankless units often last 15–20 years with proper maintenance. Check your tank for a sticker or label with an installation date.
Rusty water or a rusting tank exterior. This usually means the anode rod is long gone and the tank itself is corroding.
Popping or rumbling noises. This is often sediment buildup hardening at the bottom of the tank, reducing efficiency and accelerating wear.
Leaks anywhere around the tank base. Even a small amount of water is a sign the tank has already started to fail.
Inconsistent temperature or reduced hot water supply. This can point to a failing heating element, a bad thermostat, or (for tankless units) scale buildup restricting flow.
If you’re seeing more than one of these at once, replacement is almost always more cost-effective than repair, especially on a tank unit past the 8-year mark.
How We Built Reservoir for Longevity
Most water heaters give little warning before they fail, because the whole category has historically been built with dumb controls and minimal corrosion protection. We built Reservoir’s heat pump water heater to change that.
Reservoir was designed to avoid the most common failure modes for water heaters. We want your Reservoir to be in service for a long time.
Predictive controls that alert you before damage happens. Instead of the basic thermostat-only loop found in most water heaters, Reservoir continuously monitors water temperature, tank conditions, and system performance, not only in the heater itself, but in your broader plumbing system. If any indicator starts to drift out of range, you get a warning before it turns into a flooded basement.
An electric anode rod for corrosion control and less sludge. Traditional sacrificial anode rods wear out, requiring the homeowner to replace them or causing the whole tank to fail. Reservoir uses an electric anode, which runs a small current through the rod, preventing it from degrading over time. It also produces less of the mineral sludge that a standard magnesium or aluminum rod leaves behind, which means less sediment buildup at the bottom of the tank over the unit’s lifetime.
Stable, robust components at the points that see the most stress. We paid particular attention to the parts of the system that take the most wear—fittings, welds, and connection points—using quality materials built to hold up under years of thermal cycling and pressure.
The result is a water heater designed to alert you before anything goes wrong.
Frequently Asked Questions
How long does a water heater last? Tank water heaters typically last 8 to 12 years. Tankless water heaters often last 15 to 20 years with regular maintenance, since there’s no tank to corrode, but this ongoing service is an additional fee on top of the installation cost.
What causes most water heater failures? Corrosion is the leading cause of tank failure, usually once the sacrificial anode rod has fully worn out and the tank’s steel starts corroding directly.
Is a heat pump water heater worth it? A heat pump water heater typically uses two to four times less electricity than an electric resistance tank water heater to heat the same amount of water.
How do I know if my water heater is about to fail? Watch for rusty water, rumbling or popping sounds, any leaking around the base, or inconsistent hot water. Any one of these on a unit older than 8 years is a good reason to start planning for replacement.
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Ready to upgrade to a water heater that will last? Take our 5-minute Virtual Assessment and a member of our team will get back to you soon.
For more detail about water heaters, our team highly recommends The Water Heater Workbook: A Hands-On Guide to Water Heaters by Larry and Suzanne Weingarten.