This is the third Article for helping designers to choose the
appropriate type and calculate the required power rating for the chosen type of
Electrical Water Heater.
In the first
Article " Electrical Water Heaters Power Rating Calculations – Part One", we gave you a brief about the following points:
 Hot Water System Components,
 Different types of Water Heaters used in domestic and commercial buildings,
 How to choose the best type of water heater for any application?
With this
brief, you will be familiar with the different types and construction of common
Water Heaters.
Also, in the
second article "Electrical Water Heaters Power Rating Calculations –Part Two", we explained the Design Methodology of Electrical Water
Heaters for any Building which is represented in Fig.1, where we are
going to calculate the following:
 The Minimum number of plumbing facilities for a given type of building/occupancy,
 The total demand of Water in GPH or GPM,
 The size (volume) in gallons or liters for the required electrical water heaters,
 The power rating in KW for the required electrical water heaters.
Fig (1): Design Methodology of
Electrical Water Heaters for any Building
And in this
second article, we explained the first two steps which were:
 Step#1: Determination of the Building/Occupancy Type,
 Step#2: Calculation of The Minimum Number of Plumbing Facilities for a Given Type of Building/Occupancy by using the Calculations Spreadsheet for Minimum Number of Plumbing Facilities Required
Today, we will explain in detail the remaining steps.
Step#3: Calculation of The
Total Demand Of Water in GPH or GPM
Step#4: Calculation of The size (volume) in gallons
or liters for the required electrical water heaters

There are several methods for calculating the water load of a
building. One method is not always better than another, although one method
may be better suited for some particular application. The methods that we are
going to explain here are as follows:
First: Methods used for new/existing buildings:
Second: Methods used only for existing buildings:

First: Methods used for
new/existing buildings

1 International Codes Methods
The International codes methods explained in below are from
different International societies like the American Society of Plumbing
Engineers (ASPE), ASHRAE and the National Bureau of Standards. These codes
methods are as follows:

Method# 1: Estimating Hot
Water Demand on Fixture Types
Method#1 utilizes average hourly data (gallons per hour) for
various types of buildings and occupancies provided by ASPE (American Society
of Plumbing Engineers) Data Book: Vol.4.Service Hot Water Systems.
Table1: Hot Water Demand
per Fixture for Various Types of Buildings (Gallons[Liters] of Water per Hour
per Fixture, Calculated at a Final Temperature of 140°F [60°C])
Notes:
Calculation procedures by using Method#1:
Advantages and disadvantages of Method#1:
But this method has the following limitations:
Example#1:
A new apartment building will have 16 threebedroom units. Each
unit will have a clothes washer, kitchen sink, dishwasher, two lavatories,
and two tubs with showers.
The owner has requested a storage tank system. Assume that each
apartment will house a maximum of 5 people. The owner is marketing the
building to middleincome households. Calculate the actual minimum storage
tank size?
Solution:
Using Table 1 (The sizing chart from ASPE Data Book) the total
demand can be calculated as follows:
16 kitchen sinks x 10 gallons/hour = 160 gallons/hour
16 dishwashers x 15 gallons/hour = 240 gallons/hour
32 lavatories x 2 gallons/hour = 64 gallons/hour
32 tubs/showers x 20 gallons/hour = 640 gallons/hour
Subtotal = 160 + 240 + 64 + 640 = 1104 gallons/hour
From Table1, Demand factor = 0.30
Total demand (minimum recovery per hour) = Demand factor x
Subtotal = 0.30 x 1104 gallons/hour = 331gph
From Table1, Storage factor = 1.25
Minimum recommended storage capacity = Total demand (minimum
recovery per hour) X Storage factor =
331gph x 1.25 = 414 gallons
Based on information and experience, the engineer must determine
how much storage is most suitable to the application. Remember that in
selecting storage tank equipment, the calculated storage volumes are “usable”
storage volumes. With the stratification typical inside a storage tank, only
approximately 70% of calculated total volume is usable.
For this example, the actual minimum storage tank size = Minimum
recommended storage capacity / Tank Volume usage factor = 414 gallons / 0.70
= 591 gallons.

Method# 2: Estimating Hot Water Demand on Fixture Units
The fixture unit concept is based on theory of probability by assigning fixture unit's (w.s.f.u: water supply fixture unit) value to each type of fixture based on:
All the above factors together determine the maximum
probable rate of flow.
Table2 lists the demand weights in “fixture units”
as determined by the National Bureau of Standards.
Table 2: Demand weights of plumbing items in ‘water
supply fixture unit, w.s.f.u
From the Table above, the designer can assign fixture
unit weights to the specific
fixtures in his design. When these are added their total provides a basis for determining the maximum probable flow that may be expected in a water pipe.
As a rule, separate hot and cold water demand can be
taken as ¾ the total portable water demand; for example, a lavatory faucet
with a total demand of 2 w.s.f.u would be counted as 1½ fixture unit on the
cold water system, and 1½ fixture unit on the hot water.
Fixture Unit – Flow Relationship Once the total The Minimum Number of fixture units is obtained from step#2, the next step is to assign the probable water demand. There is a complex formula to get it, but we will use in this method a simple chart and table2 to determine the probable water demand.
Figure2 in below shows the probability of flow as a
function of fixture unit count.
Fig (2): Flow  Fixtures Units Relationship
Calculation procedures by using Method#2:
Example#2:
Estimate the hot water flow rate for a small hotel building consisting of 52 flush valve water closets, 30 flush valve urinals, and 40 lavatories. Solution: Step 1: Determine the total fixture unit load for all the fixtures serviced by your water heater application using the Fixture Units in Table2
Since the hot water is required only at lavatories,
the total fixture load is 60 f/u.
Step 2: Using Hunter Curves (Figure2), enter the
graph from the bottom at 60 fixture units and go up to curve C. Then move to
the left horizontally to read approximately 27 gallons per minute of hot
water capacity required.
So, Total Demand of Water = 27
GPM
Disadvantages of Method#2:

Method #3:
Estimating Hot Water Demand on Occupants/Units
The Method#2 in above provides the demands in gallons per hour for various types of fixtures and for various types of buildings. However, it does not provide the time factor usage rate. So, in method#3, two basic determinations must be made For more realistic results:
Maximum load:
The maximum load of a water heater is the maximum
amount of water used daily per person per hour. It is also called hourly peak
demand since the amount of daily water used is spread over several hours. The
amount of water varies with style of living and type of building. To
determine the size of the hot water heater for a building, consider the
maximum hourly use and number of users.
Working Load:
Working load is influenced by the duration of that
peak demand and is
defined as the percentage of maximum load expected under normal conditions in any given hour. Table3 below is an empirically derived approach that relies on the historical actual measured data for specific building categories which is provided by ASHRAE Applications Handbook, Chapter 45, Table 7. Table3: Peak Hot Water Demands and Use for Various Types of Buildings
Example#3:
Determine the monthly hot water consumption for a
2000student high school.
Solution:
Refer to Table3, Average day consumption = 1.8 gal per student per day Total monthly hot water consumption = 2000 students × 1.8 gal per student per day × 22 days = 79,200 gal. 
Method #4:
Estimating Hot Water Demand on Daily Use
In method #4, we
are going to calculate the hot water demand using the estimated daily use of
hot water for different types of buildings as shown in Table4.
Table4: Hot Water Demand on Daily Use
Notes:
Example#4: Determine the peak hot water requirement for an apartment building housing 200 people? Solution: From the data in Table 4 above: Hot water required per person = 40 gal/day  (conservative assumption) Number of people = 200 Daily requirements = 200 × 40 = 8000 gal/day. Maximum hours demand = 8000 × 1 ⁄7 = 1140 gal/hour. Duration of peak load = 4 hr. Water required for 4hr peak = 4 × 1140 = 4560 gal. 
Method#5: Generic Curves Method
Another straightforward method of
sizing hot water system for large commercial and institutional applications
such as hotels, motels, hospitals, nursing homes, office buildings, food
service establishments etc is based on generic curves of “recovery rate v/s
usable storage capacity”.
Generic Curves of “recovery rate v/s usable storage capacity”.
These curves provide a straight
relationship, for example, for hot water demand based on the number of beds
in hospitals or hot water demand based on number of students in the school.
These curves are provided in the ASHARE applications handbook, chapter 45,
which showing the relationships between
recovery and storage capacity for various types of buildings. Samples are
depicted below (not to scale).
From these curves, a selection among the numerous
combinations of recovery rate and usable storage capacity for a given design
can be made.
Typically, selection of the minimum recovery capacity
and the maximum storage capacity on the curves will yield the smallest hot
water capacity capable of satisfying the building demand. Minimizing the
recovery capacities will place less demand on the heat source.
Notes:
The following examples, taken from the American Society of Heating, Refrigeration, and AirConditioning Engineers Applications Handbook (ASHRAE 2003), illustrate the use of the tables and curves for selecting storage and recovery capacities: Example#5: Determine the required water heater size for a 300student women’s dormitory using the following criteria: Storage system with minimum recovery rate. Storage system with recovery rate of 2.5 GPH per student.
Solution:
1. With a minimum recovery rate of 1.1 GPH per
student:
a. Recovery rate = 300 x 1.1 = 330 GPH. b. Storage = 12 gal. per student OR 300 x 12 = 3600 gallons c. Tank size = 1.43 x 3600 = 5150 gallons  [* On 70% net usable basis multiply by factor of 1.43 (1/0.7)] 2. At a recovery rate of 2.5 GPH per student: a. Recovery = 300 x 2.5 = 750 GPH b. Usable Storage Capacity = 5 gallons, OR 300 x 5 = 1500 gallons c. Tank size = 1.43 x 1500 = 2150 gallons  [On 70% net usable basis] Example#6: Determine the water heater size and monthly hot water consumption for an office building to be occupied by 300 people. Storage system with minimum recovery rate. Storage system with 1.0 gal per person storage. Solution:
1. With a minimum recovery rate of 0.10 GPH per
person:
a. Recovery rate = 300 x 0.1 = 30 GPH. b. Storage = 1.6 gal per person or 300 x 1.6 = 480 gallons c. Tank size = 1.43 x 480 = 690 gallons [* On 70% net usable basis multiply by factor of 1.43 (1/0.7)] 2. For storage @ 1 gal per person: a. Storage = 300 x 1 = 300 gallons b. Recovery capacity = 0.175 GPH per person c. Recovery = 300 x 0.175 = 52.5 GPH. d. Tank size = 1.43 x 300 = 430 gallons [* On 70% net usable basis multiply by factor of 1.43 (1/0.7)]
Example#7:
For a 2000student high school, determine: Storage system with minimum recovery rate. Storage system with a 4000gal maximum storage capacity. Solution:
1. With the minimum recovery rate of 0.15 GPH per
student:
Recovery rate = 2000 x 0.15 = 300GPH. The storage required is 3 gal per student, or 2000 × 3 = 6000gal storage. The tank size is 1.43 × 6000 = 8600 gal. 2. When maximum storage capacity is stated as 4000 gallons: Usable storage capacity = 0.7 x 4000 = 2800 gallons Storage capacity per student = 2800 / 2000 = 1.4 gal per student From the curve, the recovery capacity at 1.4 gal per student = 0.37 GPH per student Therefore, total recovery = 0.37 x 2000 = 740 GPH
Example#8:
Determine the required heater capacity for an apartment building housing 200 people, if the storage tank has a capacity of 1000 gal. What heater capacity will be required if the storage tank is changed to 2500gal capacity? Solution: Refer to Table4: Hot Water Demand on Daily Use in above, From the data in this table: Hot water required per person = 40 gal/day  (on the conservative side) Number of people = 200 Daily requirements = 200 × 40 = 8000 gal. Maximum hour demand = 8000 × 1 ⁄7 = 1140 gal. Duration of peak load = 4 hrs. Water required for a 4hr peak = 4 × 1140 = 4560. If a 1000gal storage tank is used, hot water available from the tank = 1000 × 0.70 = 700. Water to be heated in 4 hrs = 4560 – 700 = 3860 gal. Heating capacity per hour = 3860 ⁄4 = 965 gal. If instead of a 1000gal tank, a 2500gal tank had been installed, the required heating capacity per hour would be [4560 – (2500 × 0.70)]/4 = 702 gal.
Notes for Method#5:
Water heater selection is best made on the basis of hot water usage. However, calculations may lead to a combination of tank size and heat input which do not exist. In this case, the tank size and/or heat input must be balanced to achieve the desired result. Therefore, it is necessary to understand that heat input provides hot water, at the hourly recovery rate, hour after hour. The storage tank represents instant hot water at greater thanheater recovery. The following key features are: 1. Select maximum recovery and minimum storage if the hot water demand period is longer than 3 or 4 hours (long demand). Storage must be sufficient to handle any peaks within the demand period. 2. Select minimum recovery and maximum storage if the hot water demand period is less than 3 or 4 hours (short demand). Heater recovery must be sufficient to reheat the entire tank contents before the next demand period. 3. Equipment sizing calculations may lead to a combination of heater recovery and storage tank which are not made. If so, both factors may be “adjusted” to favor one or the other as desired. Here’s how:
4. For instantaneous use, heater recovery is most important for all practical purposes, i.e. it heats the water at the rate it is being used. If a tank type water heater is used, the tank size is minimum or just large enough to put the heat into the water. 5. Check for the possibility of any hot water needs occurring during the recovery period which could affect the reheating of the system. Add heater recovery and/or storage tank capacity as necessary to handle unusual conditions. 
Method# 6: Estimating Hot Water Demand on Building's operating
characteristics
Method#6 of calculating hot water usage is outlined in ASPE’s
Domestic Hot Water Heating Design Manual, which addresses specific
occupancies (see Table5 and Table6) and tailors the calculation process to
the type of building based on its individual operating characteristics.
Table6: Low, Medium, and High Guidelines: Hot Water Demand and
Use for Multifamily Buildings
Calculation procedures by using Method#6:
Advantages and disadvantages of Method#6:
Solution of Example#1 in above by Using Method# 6:
1. Determine occupant demographic classification: The apartment
is being marketed to middleincome people, so a lowdemand occupant
classification is selected from Table5 Reprinted from Domestic Hot Water
Heating Design Manual, American Society of Plumbing Engineers.
2. Determine peak usage and maximum storage volume times: Using
the 30/3 recommendation from the Manual (ASPE), a peak usage of 30 minutes
and a maximum 3hour storage volume are selected.
3. Determine peak demand and maximum storage volume: Using Table 6,
the 30 minute peak demand is determined to be 1.7 gal /person. And the
maximum 3hour usage (storage) is 6.1 gallons.
4. Perform calculations: the minimum recovery per hour = 80
people x 1.7 gallon per person = 136 gph
The recommended storage volume = 80 people x 6.1 gallon per person
= 488 gallons
5. Perform storage tank volume calculation: the actual minimum
storage tank size = recommended storage Volume / Tank Volume usage factor = 488
gallons /0.70 = 697 gallons.
Notes for the above example:

In the next Article, I will explain in detail the following:
 Calculation of The Total Demand of Water in GPH or GPM for existing buildings.
 Step#5: Calculate the power rating in KW for the required electrical water heaters.
So, please
keep following.
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