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Sizing Water Heaters

August 11, 2000

Mohammad A. Malek, P.E.

Choos ing t he r i ght s i zed w ater hea te r depends on k now ing t he t e rm inology and de te rm in i ng ho t

water us age

A water heater is required for supplying potable water for showers, kitchens, bathrooms, washing machines, dishwashers

and other purposes in a home or commercial building.

A water heater is defined as a closed vessel used to supply potable water, which is heated by combustion of fuels,

electricity or any other source and withdrawn for use externally to the system at pressures not exceeding 160 psig, and

shall include all controls and devices necessary to prevent water temperatures from exceeding 210F. The water heater is

intended to supply potable water with 100% make-up from a potable water supply system like city water.

Based on use, a water heater may be classified as a commercial water heater or a residential water heater. Automatic

storage type water heaters for residential use range from 5 to 120 gal. nominal tank capacity. Water heaters for

commercial applications are available with storage capacities from 40 to 500 gal. or more.

If you select a water heater more than the capacity required, you will be paying extra needlessly and fuel cost will be high.

If you buy a water heater of less capacity, there may be a shortage of potable water.

Def in it ion o f Term s

There are some terms that are important to know when sizing a water heater:

Storage t ank c apac i t y : The capacity of the tank for storing hot water. The tank permits a large volume of water to be

drawn from the system at flow rates exceeding the recovery capacity of the heatr.

Rec ov ery c apac i t y : The amount of water in gallons per hour, raised at a given efficiency and Btu input.One-hour

draw c apac i t y : The maximum hot water use for one hour at the peak period of the day when the heaviest draw of hot

water will occur.

Input rat ing: The amount of fuel measured in British Thermal Units (Btus) consumed by a gas or oil heater in an hour.

Input for the electric heater is expressed in kilowatts (kW). One kW of electricity is equal to 3, 413 Btus.

Bas is o f Se lec t ion

The best method for selecting a water heater is on the basis of hot water usage.

The selection is a combination factor of heat input, tank size and recovery

capacity.

Heat input provides hot water at the hourly recovery rate hour after hour. Thetank represents instant hot water at greater-than-heater recovery. The supply of

hot water in the tank cannot be replenished until the peak usage period has

ended, and heater recovery is available for this purpose. Theoretically, a water

heater should have a combination of storage and heat capacity equal to usage:

Available Hot Water = Storage + Heat Input (Recovery Rate)

Residential Peak usage, based on accepted practice, is the one hour period during the day when the heaviest draw of hot

water will occur. Table 1 gives the typical residential usage of hot water:

Table 2 shows the minimum permissible water heater sizes allowed by HUD-FHA in its Minimum Property Standards for

One and Two Family Living.

Size Select ion

To determine the size of the water heater

that best suits your needs, follow these

steps:

Determine maximum hot water use from

Table 1.

From Table 2, select a water heater with

a hot water supply rating that is within 3 gal.

of the maximum expected total use of one

hour.

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Example

Here is an example of how to select a water heater for a three-bedroom house, with three residents, two full baths, an

automatic dishwasher and an automatic clothes washer.

Step 1 Maximum hot water required (From Table 1 ):

3 Residents............................15 gal. Automatic Dishwasher.............10 gal. Clothes Washer......................20 gal. 2

Baths..................................20 gal. Total Requirement...................65 gal.Step 2 A water heater should be selected for a

68-gal. of hot water supply rating (65 + 3 = 68). FromTable 2 , for a one hour draw capacity of 70 gal. (next highest

number above 68), one of the following water heaters should supply the minimum potable water requirement for the

example house:

A gas-fired water heater 40 gal. storage capacity, 36,000 Btu/hr

An electric water heater 50 gal. storage capacity, 5.5 kW

An oil-fired water heater 30 gal. storage capacity, 70,000 Btu/hr.

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