Most homes have a tank-type water heater. If you find a large, cylindrical tank standing on end with pipes attached to it, you have a tank-type system. If you don't find a tank, but you have a hot-water heating system that heats your home, it's likely that the hot water reaching your taps is heated by the boiler. In other cases, hot water may be supplied by a stand-alone instantaneous water heater that's mounted near the fixture it serves.
The standard water heater comes in two flavors: electric and fuel-fired. In the latter, the fuel most commonly used is gas, either natural or propane, but oil-fired heaters are popular in many areas. Fuel-fired units have a vent pipe at the top to carry away exhaust gases. Electric models, on the other hand, simply have a power cable that connects the heater to your electric service panel.
The job of the tank-type heater is not only to heat the water, but to store it until it's ready to use. Therefore, in addition to the tank's heating system, every tank is equipped with insulation to help keep the water warm between heating cycles.
On top of every tank you'll find the water supply and delivery pipes. The supply pipe routes cold water to the bottom of the tank through the dip tube. The hot-water delivery pipe takes water from the top. For safety, all water heaters are equipped with a T&P valve (temperature-and-pressure relief valve). This valve opens if either the temperature or pressure of the water exceeds a safe limit. The valve is connected to a pipe that runs down the outside of the tank, ending about 6 in. from the floor. It's a good idea to keep a bucket under the end of the pipe to catch water if the valve opens. The T&P valve should not be connected to a drain. If the valve did open, a sign that a problem exists, you might never know that it had opened.
Most tanks are made of steel, which is glass-lined on the inside to help prevent corrosion. In fact, corrosion is the primary reason that tanks fail. Once rust produces a hole, there are temporary fixes, but the tank should be replaced. All tanks also have an anode rod to control corrosion. The magnesium anode rod protects the tank by corroding in place of the steel. Because the rod is designed to corrode, it will eventually wear away. After this happens, corrosion of the steel accelerates. It's a good idea to check the anode rod once a year, and replace it if necessary. At the bottom of every tank is a drain cock to empty the heater, and a valve on the supply pipe allows you to shut down the hot-water plumbing without affecting the cold-water supply to the house.
The typical electric unit is wired to a 220-volt circuit. To heat the water, the current passes through electrical-resistance heating elements—usually two, one at the middle of the tank and one at the bottom. Power is delivered to each element through a thermostat—a switch that senses the water temperature. When the temperature drops, the switch closes to allow current flow, and it opens when the temperature reaches its preset limit. Thermostats have a dial for setting the maximum water temperature—generally between 130 degrees and 140 degreesF, or as low as about 120 degreesF for increased energy savings and scald protection.
When a hot-water tap is opened, cold water enters the tank through the dip tube and the drop in temperature triggers the thermostat and element at the bottom. As the water at the top of the tank is replaced by cool water, the temperature at the top thermostat drops and its element kicks in. When the tap is turned off, the heating elements continue to carry current until the thermostats are satisfied.
Fuel-Fired Water Heaters
Instead of electrical-resistance elements, gas-fired heaters have a burner that's fed gas through a control valve and a thermostat switch. In an oil-fired heater, the burner is similar to that found on an oil-fired furnace. In either case, the burner is usually situated to throw a flame under the tank. The exhaust gases are vented either through a hollow core at the center of the tank or around the tank sides. Because fuel-fired heaters heat the tank, which in turn heats the water, there will be more wear and tear on the tank than with electric heat. A fuel-fired heater, therefore, may have a shorter life expectancy than an electric heater.
Because water heaters both heat and store water, the rate at which the water is heated and the capacity of the tank affect the supply of hot water at your fixtures.
The speed at which a unit heats water is called its recovery rate. This figure indicates the amount of water in gallons that can be heated to 100 degreesF in 1 hour. Once you draw water faster than it's heated, the temperature drops.
However, because the tank stores hot water, its capacity also affects the ongoing availability at the tap. Choosing a water heater that has an appropriate capacity and recovery rate depends on how much water your home demands and how your unit heats the water. Typically, heaters with low recovery rates have a high tank capacity. Although it takes longer to heat the water, there's more of it for intermittent use. Electric heaters fall into this category. On the other hand, a fuel-fired heater with a high recovery rate needn't have a large tank, because it can heat the water faster. In general, electric models have the lowest recovery rate, and oil-fired units have the highest.
If your home doesn't have a hot-water tank, you probably have a hot-water or steam home-heating system that also heats water for your taps. To accomplish the job, your boiler has a tankless water heater. In this system, a coil of pipe is connected at one end to the cold water supply, and at the other to your hot-water delivery piping. As the boiler heats the water that warms your home, that water heats the coil, creating hot water at your taps.
Because tankless heaters only heat water as it's used, there's no cost for maintaining heat in a large volume of water during periods of low usage. However, they do have a few drawbacks. First, the hot water generated is far hotter than necessary, so a cold-water mixing valve should be installed to reduce the chance of scalding. Second, the boiler must fire to generate hot water—which is efficient during the winter months, but decidedly more wasteful when the weather is warm. Like tank-type heaters, tankless heaters are designed to achieve a specific heating rate. Once the rate is exceeded by demand, the temperature of the water drops. In some cases, storage tanks are connected to the heating coil to increase hot-water availability.
In addition to boiler-mounted tankless heaters, stand-alone units are available. Gas-fired instantaneous water heaters utilize a coil and heat exchanger to heat water as it's required. Like boiler-mounted units, instantaneous water heaters don't use energy to maintain the heat in a volume of water, but only fire as hot water is required. A downside is that stand-alone units typically have a lower flow rate than boiler-mounted systems and may fall short during periods of high demand.