Domestic hot water (DHW) is the heated water used in your home for everyday tasks like showering, washing dishes, and doing laundry. It’s distinct from the hot water that circulates through radiators or underfloor heating to warm your house. DHW is the water that comes out of your taps, and heating it accounts for roughly 18% of a typical home’s energy use, making it the second largest energy expense after space heating and cooling.
How Domestic Hot Water Systems Work
Every DHW system has three basic parts: a cold water supply, a heat source, and pipes that carry the heated water to your fixtures. Cold water enters your home from the municipal supply or a well, flows into a water heater, gets heated, and then travels through pipes to your kitchen sink, shower, washing machine, or anywhere else you need it.
In a typical tank-style heater, cold water enters through a dip tube that channels it to the bottom of the tank. As the water heats up, it naturally rises to the top (hot water is less dense than cold) and exits through an outlet near the top of the tank. This simple physics principle means you always draw the hottest water first.
Types of Water Heaters
Storage Tank Heaters
These are the most common type in American homes. A large insulated tank, usually 50 gallons, keeps water heated and ready to use at all times. The heat source is either a gas burner beneath the tank or electric heating elements inside it. The advantage is straightforward: the hot water is always there when you need it. The tradeoff is that you’re paying to keep that water hot around the clock, even when nobody’s using it.
Tankless (On-Demand) Heaters
Tankless heaters don’t store hot water at all. They fire up only when you open a faucet or start an appliance, heating water as it passes through the unit. Whole-house tankless models typically produce 3 to 4 gallons per minute, which is enough for one or two fixtures running simultaneously. If you try to run the dishwasher, the washing machine, and a shower all at once, you may overwhelm the unit’s capacity.
Electric tankless models can struggle in colder climates. In Consumer Reports testing, some electric units failed to reach 120°F when incoming water was 58°F, though they performed fine with warmer groundwater. Gas tankless models handled cold incoming water without issue. The colder your groundwater, the longer you’ll wait for hot water to arrive at the tap.
Direct vs. Indirect Heating
Most standalone water heaters are “direct” systems, meaning they have their own dedicated burner or heating element. An indirect water heater takes a different approach: it doesn’t have its own heat source at all. Instead, it connects to your home’s boiler. Hot water from the boiler circulates through a coil inside the storage tank, transferring its heat to your domestic water supply. This setup is common in homes that already have a boiler for space heating and can be highly efficient during the heating season, since the boiler is already running.
Solar Water Heating
Solar thermal systems use rooftop collectors to capture heat from the sun and transfer it to your water supply. They come in two broad categories. Active systems use pumps to circulate water or a heat-transfer fluid through the collectors. In climates where freezing is a concern, the active system circulates antifreeze fluid through the collectors and uses a heat exchanger to warm the domestic water indirectly. In mild climates, the system can pump household water directly through the collectors.
Passive systems skip the pumps entirely. A thermosyphon system, for example, heats water in a rooftop collector and relies on natural convection (hot water rising) to move it into a storage tank, typically around 40 gallons. Passive systems cost less and tend to last longer, but they’re less efficient than active ones. Both types usually include a conventional backup heater for cloudy days and periods of high demand.
Temperature: The Safety Balancing Act
Setting the right temperature for your domestic hot water involves balancing two risks that pull in opposite directions: bacterial growth and scalding.
Legionella, the bacteria responsible for Legionnaires’ disease, thrives in water between 77°F and 113°F. Below that range it survives but grows slowly; above 120°F its growth is effectively suppressed. The CDC recommends storing hot water at 140°F or above and ensuring that water in circulation never drops below 120°F. These temperatures reliably prevent Legionella colonization in your plumbing.
The problem is that water at 140°F can cause a serious burn in just a few seconds, especially for young children and elderly adults. That’s why many homes use a mixing valve (sometimes called a tempering valve) at the water heater outlet. The heater stores water at 140°F to kill bacteria, but the valve blends in cold water before it reaches your taps, delivering water at a safer 120°F. If your system doesn’t have a mixing valve, setting the tank to 120°F is the standard compromise between infection risk and scald prevention.
Why You Shouldn’t Drink From the Hot Tap
Your domestic hot water is fine for bathing and cleaning, but the EPA recommends using only cold tap water for drinking, cooking, and preparing baby formula. Hot water dissolves contaminants more readily than cold water, and in homes with older plumbing, it can leach higher levels of lead from pipes, solder, and fixtures. Boiling doesn’t help here, since it actually concentrates lead rather than removing it. If you need hot water for cooking, draw it cold and heat it on the stove or in a kettle.
Maintenance That Extends Tank Life
Storage tank water heaters have a built-in defense against corrosion: a sacrificial anode rod. This is a metal rod, usually magnesium or aluminum, suspended inside the tank. It corrodes in place of the tank’s steel lining, essentially sacrificing itself to protect the more expensive component. Once the anode rod is depleted, the tank itself starts to rust, which is often what kills a water heater.
Checking the anode rod every two to three years and replacing it when it’s heavily corroded can add years to your water heater’s lifespan. A degraded anode rod also contributes to that rotten-egg smell some people notice in their hot water. That odor comes from hydrogen sulfide produced by bacteria reacting with the rod’s material. Newer “powered” anode rods use a small electrical current instead of sacrificial metal, offering permanent corrosion protection and eliminating sulfur odors within about 24 hours of installation.
Beyond the anode rod, flushing a few gallons from the tank’s drain valve once or twice a year removes sediment that settles at the bottom. Sediment buildup insulates the water from the heating element, forcing the system to work harder and driving up energy costs. In areas with hard water, sediment accumulates faster and makes regular flushing more important.
Reducing Your Hot Water Costs
Since water heating is typically a home’s second-largest energy expense, even small changes add up. Low-flow showerheads and faucet aerators reduce the volume of hot water you use without a noticeable drop in water pressure. Insulating exposed hot water pipes, especially the first six feet leaving the heater, cuts heat loss during distribution. Wrapping an older tank in an insulation blanket reduces standby heat loss, the energy wasted keeping stored water hot.
If your water heater is more than 10 to 12 years old, replacing it with a newer, more efficient model often pays for itself through lower utility bills. Heat pump water heaters, which pull warmth from surrounding air rather than generating it directly, use roughly half the electricity of a standard electric tank and have become increasingly affordable in recent years.