Electrical Water Heaters Power Rating Calculations – Part Three

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:

1. The Minimum number of plumbing facilities for a given type of building/occupancy,
2. The total demand of Water in GPH or GPM,
3. The size (volume) in gallons or liters for the required electrical water heaters,
4. 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: International Codes Methods. Second: Methods used only for existing buildings: Actual Water Meter History Method, Clock Recording Method, Comparison with Similar 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# 2: Estimating Hot Water Demand on Fixture Units Method #3: Estimating Hot Water Demand on Occupants/Units Method #4: Estimating Hot Water Demand on Daily Use Method #5: Generic Curves Method Method# 6: Estimating Hot Water Demand on Building's operating characteristics

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. Table-1: 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: If a particular fixture or a specific building type is not listed above, the flow rate can be assigned based on engineering judgment, best practices historical data, or supplier’s instructions. A Demand Factor is applied to calculate the Maximum Probable Demand, which is the rate at which the heater will generate hot water and is also termed as “the recovery rate or heater capacity”. A high demand factor will mean a higher recovery rate or bigger heater size. The storage volume of the tank needs adjustment for usable volume to account for the drop in temperature resulting from withdrawal of hot water and continuous entry of cold water in storage tank. The “maximum probable demand” is thus factored by the “storage capacity factor” to determine the “storage tank capacity” Calculation procedures by using Method#1: Count the fixtures, Extract the gallons per hour for each type of fixtures for that type of building by using the sizing chart from ASPE Data Book: Vol.4 which is shown in Table 1. Multiply the number of fixtures by the gallons per hour for that type of building to get the total gallons/hour for this type of fixtures, Add the total gallons/hour for all the fixtures. Multiply this total by the simultaneous usage factor (demand factor) to get the maximum hourly demand (Total Demand). Calculate the minimum recommended storage volume by multiplying the total demand by the storage factor. Advantages and disadvantages of Method#1: It is a simple method his is a simplified approach that saves the effort of first estimating the fixture units, and then estimating flow against the fixture units, as explained above. But this method has the following limitations: It can only be applied to the types of facilities listed. It is to be used only for the sizing of storage tank systems. It does not consider the type of occupants. It does not address high-use or high-volume fixtures. Example#1:  A new apartment building will have 16 three-bedroom 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 middle-income 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 Table-1, Demand factor = 0.30 Total demand (minimum recovery per hour) = Demand factor x Subtotal = 0.30 x 1104 gallons/hour = 331gph From Table-1, 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: The Fixture rate of water consumption; The length of time it is normally in use, The average period between successive uses. All the above factors together determine the maximum probable rate of flow. Table-2 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 table-2 to determine the probable water demand. Figure-2 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: Calculate The Minimum Number of fixture units from step#2 (if it isn't known) Sum up the fixture units for your application; Use the above sum value to enter from the bottom on the X-axis; read up to the curve that best fits the application. Then read to the left for the corresponding gallons per minute (gpm) requirement. 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 Table-2 Fixture Type Qty. Fixture demand weight Hot Water Cold Water Total (Hot & Cold) WC (flush valve) 52 @ 10 - 520 520 Urinals 30 @ 5 - 150 150 Lavatories 40 @ 2 - - 80 Lavatories 40 @ 1.5 60 60 Total 60 f/u 730 f/u 750 f/u Since the hot water is required only at lavatories, the total fixture load is 60 f/u. Step 2: Using Hunter Curves (Figure-2), 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: The fixture count method is based on theory of probability. This method is considered accurate for large groups of fixtures but for smaller applications, this may yield erroneous results. The reader is advised to use discretion and refer to the local codes and standards. the flow probability as a function of fixture units will also vary with the type of facility and it depends on the usage time duration and other specific requirements. A 100% simultaneous draw-off may occur in buildings, such as factory wash-rooms, hostel toilets, showers in sports facilities, places of worship, and the like. In these cases, all fixtures are likely to be open at the same time during entry, exit and recess.

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 (or hourly peak demand), Working load (influenced by duration of use). 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. Table-3 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. Table-3: Peak Hot Water Demands and Use for Various Types of Buildings  We can use also the chart in below for buildings not included in Table-3: Example#3: Determine the monthly hot water consumption for a 2000-student high school. Solution: Refer to Table-3, 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 Table-4. Table-4: Hot Water Demand on Daily Use Notes: Daily hot water requirements and demand characteristics vary with the type of building; for instance the commercial hotel will have a lower daily consumption but a high peak load. A better class 4 or 5 star rated hotel has a relatively high daily consumption with a low peak load. For residences and apartments, the increasing use of dishwashers and laundry machines will require additional allowances of 15 gal/dishwasher and 40 gal/laundry washer. 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 4-hr 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: Let’s consider an example of a medical facility to illustrate the concept of ‘fixture count’ and ‘number of beds’ procedure. The later should be applied during preliminary stage and should be detailed out based on fixture count method. Note that the data available in various handbooks is generic. No facility is alike; for instance a 100-bed hospital or 100-room hotel may be categorized in 3-star, 5-star or 7-star category each offering different level of luxuries and equipment. There may be certain specific design features for each facility that must be accounted for in your design. Some of these are listed below and must be carefully evaluated when applying safety margins or demand factors. The best way to capture the specific conditions is to take a reference historic data from an already running facility and applying sound judgment. The following examples, taken from the American Society of Heating, Refrigeration, and Air-Conditioning 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 300-student 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 2000-student high school, determine: Storage system with minimum recovery rate. Storage system with a 4000-gal maximum storage capacity. Solution: 1. With the minimum recovery rate of 0.15 GPH per student: Recovery rate = 2000 x 0.15 = 300-GPH. The storage required is 3 gal per student, or 2000 × 3 = 6000-gal 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 2500-gal capacity? Solution: Refer to Table-4: 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 4-hr peak = 4 × 1140 = 4560. If a 1000-gal 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 1000-gal tank, a 2500-gal 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 than-heater 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: Where it is important that hot water temperature be maintained (as opposed to “within a 30°F drop” being o.k.), increase recovery capacity in preference to increasing tank size. This will aid in maintaining system temperature. Also, assume 10% less draw efficiency than if the 30°F drop was acceptable. Where it is important to maintain water volume (for demands possibly in excess of heater recovery), increase tank size in order to provide “instant” hot water. 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 Table-5 and Table-6) and tailors the calculation process to the type of building based on its individual operating characteristics.  Table-5. Occupant Demographic Classifications No occupants work Public assistance and low income (mix) Family and single-parent households (mix) High number of children Low income High Demand Families Public assistance Singles Single-parent households Medium Demand Couples High population density Middle income Seniors One person works, 1 stays home All occupants work Low Demand Table-6: Low, Medium, and High Guidelines: Hot Water Demand and Use for Multifamily Buildings Calculation procedures by using Method#6: Determine occupant demographic classification by using Table-5 Determine peak usage and maximum storage volume times by using Table-6 and the recommendation from the Manual (ASPE). Determine peak demand and maximum storage volume Calculate the minimum recovery per hour Calculate the recommended storage volume Calculate the actual minimum storage tank size = recommended storage Volume / Tank Volume usage factor Advantages and disadvantages of Method#6: Not all types of facilities are addressed, but those that are can be accurately calculated using this method. It can be used to establish the sizing for systems using a storage tank or instantaneous or semi-instantaneous heaters. It also addresses additional concerns such as high-usage or high-volume fixtures. Solution of Example#1 in above by Using Method# 6: 1. Determine occupant demographic classification: The apartment is being marketed to middle-income people, so a low-demand occupant classification is selected from Table-5 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 3-hour 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 3-hour 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 two solutions for example#1 in above, using the methods#1 & 6 led to calculation of different recovery rates and storage volumes. Method#1 determines recommended size based on a general population. While method#6 is more specific as to population, and it yielded results indicating that using smaller equipment was a possibility. If the occupancy had been classified as medium demand, the recovery rate would have been the same using both methods. Again, based on information and experience, it is up to the engineer to determine the amount of storage most suitable to the application.

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.

Back To  Electrical Water Heaters Power Rating Calculations Course