Water Heaters

A water heater is a sealed reservoir with an energy source that produces hot water for use by the occupants. The reservoir may be a cylindrical tank with a burner at its base, sidearm unit on a furnace, coils in a steam boiler, exhaust jacket on an internal combustion engine, heat exchanger in an exothermic industrial process, even piping loops immersed in a thermal hot spring. The heater's storage capacity is a function of the fixture units it serves and the water temperature it produces, and each is generally classed as a residential or commercial unit depending on the occupancy it serves. Residential unit capacities typically range from 5-125 gal, while commercial capacities range from 40-500 gal or more. The energy source may be electricity, gas, or fuel oil; and each has in addition to its storage capacity a draw capacity (maximum gal/hr of hot water the unit can produce during its peak hour of daily use), recovery capacity (maximum gal/hr at which the unit can replenish its hot water supply), and an input rating (maximum energy the unit's heat source can consume in 1 hr, measured in Btus for gas and oil heaters and kW for electric heaters). The maximum water pressure a unit can deliver is 160 psig, each unit requires controls that limit its water temperature to 210° F at near-sea-level elevations and lower temperatures at higher elevations, each unit must have a pressure relief valve and a tap for flushing and cleaning, the unit's floor should be waterproof and slope at least 1/8 in/ft to a drain, the unit's weight should be added to structural loads (total load = shipping weight x gallon capacity x 8.33 lb/gal), and each unit should be installed in a well-ventilated area. In any water heating system, three temperatures must be known before it can be properly designed:

Entering temperature: the temperature of the cold water entering the tank. This is usually the groundwater temperature (the local AAT), which usually ranges from 35°-90°. The colder the entering water, the more heat that is required to raise it to the exiting temperature.

Ambient temperature: the temperature of the air around the tank. During winter this may be much lower than indoor temperatures if the unit is in a basement. The colder the ambient temperature, the faster the contained water cools and the more the unit must operate to maintain the exiting temperature. This variable does not affect unit capacity.

Exiting temperature: the temperature of the heated water as it leaves the tank; this is also known as the preset hot water temperature or the hot water plumbing's tap temperature. This varies according to use requirements and energy conservation measures. Typical exiting temperatures s 120° F for residential general use, 140° for residential dishwashing, and ^ 170° F for commercial use. The higher the exiting temperature, the smaller the unit, the less it weighs, the less floorspace it takes up, and the less it costs; but its unit operating costs are higherdue to the greater heat loss of standing water. Lower exiting temperatures also greatly reduce tank corrosion and scale accumulation.

In addition to making hot water for the occupants, today's commercial water heaters have a second function: to kill Legionella bacilli. These deadly germs enter humans via inhalation of aerosolized water droplets and particles containing the bacilli (no infection via ingestion of contaminated water or from other persons is known), and are more common than one might think: Random water samples from more than 100 New York City commercial buildings in the mid-1990s found Legionella bacilli in nearly 20 percent of them. Legionella bacilli cannot grow below 68° F (but they can remain dormant at lower temperatures); they propagate between 68-122° F (ideal growth range is 95-115°); and they die within 6 hours at 131°, within 32 minutes at 140°, within 2 minutes at 151°, and instantly at 158° (disinfection range is 158-176°). In addition to high temperatures these bacteria can be killed by desiccation, UV radiation, and a few minutes' exposure to air. Thus the upper limit of AT calculations for commercial hot water systems is usually taken at 170°, as the extra 12° considers typical thermostat cycling to ensure a continuous hot water tank temperature of 158°. For this reason commercial reservoirs are often sized to hold only a day's supply of water. Legionella bacilli also require some kind of sediment to grow in; this can be a layer of mineral particles settling out of the water supply, scale buildup in a tiny recess of a fitting or the bottom of a water heater, or even a slimy biofilm of algae or other biota anywhere in the hot water supply. These stable habitats for Legionella bacilli can be eliminated by bleeding and flushing all water heaters and cooling towers twice a year. The threat of infection is also reduced if the system's piping is not oversized, has a minimum of elbows and other fittings, has straight runs after valves and other constrictions, and contains no air chambers, stems for future expansion, or other unused sections of piping. •

A perennial dilemma with hot water delivery is that when a hot water faucet is turned on after it has been off for awhile, the pipe's standing water between the faucet and the HWH has become cold and must flow out before hot water arrives, which forces the HWH to heat an added volume of water equal to the displaced cold water. In poorly designed systems, this wasted mass heating load can be as much as 5 gallons. This problem is minimized by placing hot-water fixtures near water heating sources, insulating the intervening piping, and installing small electrically operated point-of-use water heating units near the faucet. The latter method is the only practical solution for remote fixtures that use little hot water. The general formula for sizing a domestic water heater is

HW = no. occupants x 5 + dishwashing (5 if by hand, 10 if by machine) + no. clothes washers x 20 + no. full baths x 12

Then, assuming possession of a manufacturer's catalog of unit storage capacities, select a unit whose gallon capacity ^ HW.

A procedure for designing a commercial water heating system is:

1. Find the capacity of each hot-water plumbing fixture, count them up, then apply a diversity factor. This is the format that underlies formulas 4C4J1 and 2 on the next page.

2. After determining the unit's optimal capacity, adjust this amount based on the unit's entering, ambient, and exiting temperatures.

• The primary sources for the data on Legionella were (1) Matthew Freije's "Legion-

3. Consider the unit's energy source. Gas, oil, electric? Which is cheapest and most convenient? Gas and oil units require a flue and outdoor fresh air, but electric units have a slower recovery rate which usually requires installing a unit with 15-20 gal greater storage capacity, which costs more to operate; which may be OK if local electric rates are significantly lower.

4. Make sure the mechanical room door is wide enough to get the unit inside. Don't use the door's catalog width, but know its actual clear opening between the jamb side trim and the hinge side edge that projects furthest when the door is fully open.

Every hot water heater is usually more thermally efficient if it is designed to maintain 80 percent combustion efficiency, is equipped with intermittent ignition, has fan-assisted combustion or a flue damper (commercial units only), and is encased in minimum R-12.5 insulation.

Step 1. Find the theoretical capacity of the water heater.

Step 1. Find the theoretical capacity of the water heater.

Gas heaters: 220 (F + 2 Fn) - Cg (T„ - Tc) 2 Electric heaters: 245 (F + 2 Fn) - Ce (Th - Tc)

F = number of hot water fixture units served, 51 units Fn = amount of nonfixture unit hot water required, gpm. This is sometimes relevant for industrial applications. Here Fn = 0. Cg = capacity of gas hot water heater, ? gal Ce = capacity of electric hot water heater, ? gal. Not applicable. Th = temperature of heated water, 130° F

Tc = temperature of cold water, ° F. Use Tc = 55° or local AAT unless more definitive data is available. Here Tc = 55°.

Step 2. Find the water heater's effective capacity from the fixture unit diversity bar graph below. At a theoretical capacity of 150 gal 4 effective capacity :> 89 gal.

Theoretical capacity

Diverse or effective capacity Fig. 4-20. Diversity bar graph.

ella, Preventive measures for Domestic Water Systems", PM Engineer magazine ||

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