Hot Water Heaters
The domestic hot water (DHW) is designed for an output temperature of 140 degrees. The tank itself is usually designed for a rise of 100 degrees above the incoming water temperature. A gas-fired tank has an hourly recovery rate about the same as the tank volume. Electric is usually slower. A good recommendation for tank capacity is 10 gallons per person for gas-fired and 20 gallons per person for electric.
The tanks are usually steel and their corrosion resistant liners are made of glass, porcelain or cement (stone). Hot water rises to the top of the tank and is discharged.
Electric heaters usually have two elements, one near the top and the other lower down. Usually only one element is used at a time. As hot water is drawn from the top, the lower heating element is activated. If usage is large, the upper element is triggered keeping the water hot on an instantaneous basis.
Most tanks have magnesium anode rods that protect the steel around the tappings inside that keep them from corroding. Five-year warranty tanks usually have one anode rod; ten-year warranted tanks have two.
Every tank should be drained every six months to remove sludge and silt by the drain valve at the base of the tank. This is seldom done and significantly reduces the heaters’ efficiency.
Standby losses waste about half of the fuel used to heat domestic water. 10-25% is lost in the tank and the rest in the piping. Insulation on the tank and pipes will cut that loss in half!
Gas-fired heaters have safety control devices such as a pressure control for surges at the main gas inlet and a thermocouple that the pilot light keeps warm. If the pilot light goes out, the gas valve will close and the heater shuts down. It must manually be re-lit. There are also automatic valves that are shut down by sensors located in the upper part of the tank if the water temp reaches 210 degrees.
Gas burning equipment must be vented to the outside. In some areas regulations require a separate flue when a gas burner is vented with a coal or oil burner. If a single flue is allowed, the gas pipe should be connected to the chimney above the oil or coal pipe. If there is only a single chimney opening, the gas pipe should join in a Wye, not a Tee, and should have individual draft hoods.
Gas burning equipment requires a vent or draft hood at the flue outlet for an increase in chimney drafts or for back drafts. Additional air would be drawn through the draft hood, not the burner, in the case of increased chimney drafts and at down drafts, baffles in the hood force the air to exit without affecting the burner.
Oil-fired heaters use the same burners as furnaces and boilers. They may be installed in the same chimney flue, or connected with a Wye to the same smoke pipe. Each smoke pipe should have its own regulator.