Electrical Water Heaters Power Rating Calculations – Part One

Most Domestic and commercial buildings need a service hot water system. Depending upon the type of building, this system could range from as small as an under sink water heater for washing hands to a 10,000 gallon hot water storage tank system used in a hospital laundry.


This Article is intended to help designers to choose the appropriate type and calculate the required power rating for thee chosen type of Electrical Water Heater.

Before going on with the calculations, we need to give 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 types and construction of common Water Heaters.

1-      Hot Water System Components

A Hot water heating system has (4) major components (see Fig.1): Heat energy source, Heat transfer equipment, Distribution system, Terminal hot water usage devices. Fig (1): Hot Water System Components The most important components are the first and second ones.

1.A- Heat energy sources, they may be: Fuel combustion, Solar energy collection, Electrical conversion, Recovered waste heat.

1.B- Heat transfer equipment, they may be: Direct heat transfer (see Fig.2) is from the combustion of fuels or direct conversion of electrical energy into heat, Fig (2): Direct Heat Transfer Indirect heat transfer (see Fig.3) uses heat energy originating from remote heat sources, such as boilers, solar heat collectors, cogeneration refrigeration or waste heat. Fig (3): Indirect Heat Transfer

2- Types of Water Heaters in Domestic And Commercial Buildings

Common types of commercial and industrial water heating equipment include: Storage water Heaters (Tank-Type), Instantaneous water heaters (Tankless-Type), Hybrid water heaters.

2.1- Storage Water Heaters (Tank-Type) This type of heaters incorporates the burner, storage tank, outer jacket, insulation and controls in a single unit and is normally installed without dependence on other hot water storage equipment (see Fig.4). Fig (4): Typical Storage Water Heater (Tank-Type). They are available in electric, liquid propane (LP) and natural gas models. Natural gas and LP water heaters normally use less energy and are less expensive to operate than electric models of the same size. There are two types of Storage water heaters (Tank-Type) which are: Volume Storage water heaters (Tank-Type), Small Storage water heaters (Tank-Type).

2.1.A Volume Storage Water Heaters (Tank-Type) They are typically vertical, cylindrical tanks, usually standing on the floor or on a platform raised a short distance above the floor (as in Fig.4). In houses they can be mounted in the ceiling space over laundry-utility rooms. Typical sizes for household use range from 75 to 400 liters (20 to 100 US gallons).

2.1.B Small Storage Water Heaters (Tank-Type) Small storage tank water heaters, known as point of use (POU), utility or mobile home water heaters, are good choices for adding hot water to small buildings, shops or garages (see fig.5). Fig (5): Small Storage Water Heaters (Tank-Type) These water heaters usually range in size from 2.5 to 19 gallons. The largest of these miniature units can also be used to provide hot water to secondary bathrooms that may be situated far from your home's main water heater. Tiny point-of-use (POU) electric storage water heaters with capacities ranging from 8 to 32 liters (2 to 6 gallons) are made for installation in kitchen and bath cabinets or on the wall above a sink. They typically use low power heating elements, about 1 kW to 1.5 kW, and can provide hot water long enough for hand washing, or, if plumbed into an existing hot water line, until hot water arrives from a remote high capacity water heater.

Storage water heaters (Tank-Type) have a special type which is the Solar water heaters. 2.1.C Solar Water Heaters Solar powered water heaters have two main components (see Fig.6): Solar collectors, Storage tank. Fig (6): Solar Water Heater Components 1- Solar Collectors Solar collectors are installed outside dwellings, typically on the roof or walls or nearby, 2- Storage Tank The potable hot water storage tank is typically a pre-existing or new conventional water heater, or a water heater specifically designed for solar thermal. Types Of Solar Water Heaters Solar powered water heaters have two main types: The direct-gain type, The Indirect or closed-loop type. A- The Direct-Gain Type: In this type (see Fig.7), the potable water is directly sent into the collector. Many such systems are said to use integrated collector storage (ICS), as direct-gain systems typically have storage integrated within the collector. Fig (7): Direct-Gain Solar Water Heater Heating water directly is inherently more efficient than heating it indirectly via heat exchangers, but such systems offer very limited freeze protection (if any), can easily heat water to temperatures unsafe for domestic use, and ICS systems suffer from severe heat loss on cold nights and cold, cloudy days. B- The Indirect Or Closed-Loop Type: This type does not allow potable water through the panels, but rather pump a heat transfer fluid (either water or a water/antifreeze mix) through the panels (see Fig.8). After collecting heat in the panels, the heat transfer fluid flows through a heat exchanger, transferring its heat to the potable hot water. Fig (8): Closed-Loop Solar Water Heater When the panels are cooler than the storage tank or when the storage tank has already reached its maximum temperature, the controller in closed-loop systems will stop the circulation pumps.

2.2- Instantaneous Water Heaters (Tankless-Type) They can called also on-demand water heaters, this type of heaters has minimal storage capacity, they do not store hot water; rather they Heat water as it passes through a series of coils in the unit (see Fig.9). They are available in electric, LP and natural gas models. Most tankless units can provide up to 3.5 gallons of heated water per minute. Fig (9): Instantaneous Water Heaters (Tankless-Type) They usually include a flow switch as part of the control system. Tankless, instantaneous water heaters are best used for a steady, continuous supply of hot water. Tankless heaters may be installed throughout a household at more than one point-of-use (POU), far from a central water heater, or larger centralized models may still be used to provide all the hot water requirements for an entire house. The main advantages of tankless water heaters are: A continuous flow of hot water as compared to a limited flow of continuously heated hot water from conventional tank water heaters, Since the unit only heats water as you use it, a tankless heater is usually more energy efficient than a traditional storage tank water heater. The main disadvantages of tankless water heaters are: Tankless water heaters can provide an unlimited amount of hot water, but it can only provide a limited number of POU, Common types of water heaters can't supply hot water at more than two points at a time.

A famous type of Instantaneous water heaters (Tankless-Type) is Electric shower heads 2.2.A Electric Shower Heads An electric heating element is incorporated into such shower heads to instantly heat the water as it flows through (see Fig.10). Fig (10): Electric Shower Heads Electric showers have a simple electric system, working like a coffee maker, but with a larger water flow. A flow switch turns on the device when water flows through it. Once the water is stopped, the device turns off automatically. An ordinary electric shower often has three heat settings: low (2.5 kW), high (5.5 kW) or cold (0 W) to use when a central heater system is available or in hot seasons.

2.3 Hybrid Water Heaters A hybrid water heater (see Fig.11) is a water heating system that integrates technology traits from both the tank-type water heaters and the tankless water heaters. It Heats cold water via an electrical heating element and heat pump that pulls in ambient air and extracts the available heat. Fig (11): Hybrid Water Heaters They are also called Heat Pump Water Heaters (HPWH), they have small storage tanks that temper incoming cold water; This means hybrids only have to increase water temperature from warm to hot as opposed to tankless which has to raise completely cold water to hot. The defining characteristics of a "hybrid water heater" are: A combination of water flow of tank and efficiency of tankless of water heater, Built-in small storage water reservoir as part of heat exchanger (typically between two gallons to 20 gallons), Dual activation: flow sensing and thermostat control. Hybrid water heaters can be gas-fired (natural gas or propane), or be electrically powered using a combination of heat pump and conventional electric heating element.

3- How to choose the best type of water heater?

Many factors determine which water heater is best for your home. The three main factors to consider when choosing your water heater are:   Water Storage Capacity, Water Heating Method, water heating system structure, water heating system Fuel Type, Recovery Rate, Space Limitations, Energy Efficiency.

3.1 Water Storage Capacity (by gallons or liters) An undersized water heater will work harder and have a shorter lifespan.  So make sure to select a hot water heater that provides enough hot water for your home. The typical capacities for different types of water heaters are as follows: Volume Storage water heaters (Tank-Type): Typical sizes for household use range from 75 to 400 litres (20 to 100 US gallons). Small Storage water heaters (Tank-Type:  range in size from 2.5 to 19 gallons. Tiny point-of-use (POU) electric storage water heaters with capacities ranging from 8 to 32 liters (2 to 6 gallons). Instantaneous water heaters (Tankless-Type): Most tankless units can provide up to 3.5 gallons of heated water per minute. Hybrid water heaters: Typical sizes between two gallons to 20 gallons.

3.2 Water Heating Method According to the user requirements, the selected method of water heating method can be determined as follows: Storage water Heaters (Tank-Type), Instantaneous water heaters (Tankless-Type), Hybrid water heaters. Each offers unique advantages, and you can compare features and benefits in the table below. Type Method of operation Factors to be Considered Conventional Tank Stores hot water regularly in a tank sized to suit the users requirements. Economical, Can be positioned in closet, basement or garage Capacity ranges from 20 to 100 gallons, Efficiency varies between models, brands and  fuel sources. Tankless They do not store hot water; rather they Heat water as it passes through a series of coils in the water heater. Require a larger up-front investment, best used for a steady, continuous supply of hot water, A continuous flow of hot water, Hang on wall and frees up floor space, Excellent option for residences occupied part-time, usually more energy efficient by as much as 30%, can provide an unlimited amount of hot water, but for only limited number of POU, Requires ventilation. Hybrid Heats cold water via an electrical heating element and heat pump that pulls in ambient air and extracts the available heat. Require a larger up-front investment, Magnesium anode rod extends life of the tank, Heat pump delivers more hot water, up to 33 percent faster than standard electric water heater.

3.3 Water Heating System Structure Also, according to the user requirements, the structure or distribution of hot water system can be one of the following types: Centralized (Whole House) system, Point of Use Systems. Each type is explained in detail in above paragraphs.

3.4 Water Heating System Fuel Type As per the user requirements, the selected fuel for the water heating system can be one of the following: Natural gas, LP (liquid propane gas), Electric, Solar. Most water heaters are fueled by gas or electricity. Refer to the table below for comparison: Type Factors to be Considered Gas Requires a slightly larger up-front investment, Must be vented outdoors for safety, Units with sealed combustion or power venting increase safety, Cost less to operate, Not affected by power outages (tank-style only). Electric Generally cost less than gas models, Easy to maintain, Requires no combustibles or venting, Heats water quickly, Offer high energy factor ratings. Hybrid (electric) Requires a larger up-front investment, Requires no combustibles or venting, Heats water up to 33 percent quicker than standard electric models, Lower operating costs saves hundreds annually, 8700 BTU/h compressor is the most powerful in its class. Liquid Propane (gas) Requires a slightly larger up-front investment, Must be vented outdoors for safety, Units with sealed combustion or power venting increase safety, Cost less to operate than electric models, Not affected by power outages (tank-style only). Solar Power relatively good payback period, in average between 5-10 years, low maintenance costs, relatively high upfront costs, in most areas they will require electrical or gas or other fuel backup during the winter period, they require excellent overheating and freeze protection, Environment friendly.

3.5 Recovery Rate It is the amount of gallons or liters of hot water that the water heater is capable of providing in a given period of time (hour or minute). So, The greater your demand for hot water, the higher recovery rate you need. The used units expressing the recovery rate for water heaters are: GPH (Gallons Per Hour): The amount of water, in gallons, that is used each hour by the plumbing fixtures and equipment, such as dish machines. GPM (Gallons Per Minute): The amount of water, in gallons, flowing through a plumbing fixture or through an instantaneous water heater per minute. Common Recovery Rates: Common recovery rates for Electrical water heaters for given input power are as in below table: Recovery Rates in Gallons per hour – Electrical Water Heater Common recovery rates for Gas water heaters for consumed BTU are as in below table: Recovery Rates in Gallons per hour – Gas Water Heater BTU (British Thermal Unit): The quantity of heat required to raise the temperature of one pound of water one degree F.

3.6 Space Limitations Once you know the capacity for your water heater, remember to take the unit’s dimensions into consideration. You can follow the following recommendations:   A new conventional storage replacement unit may be larger than the old one because more insulation is required to meet the latest strict federal energy standards. Keep this in mind where units are installed in closets or other close quarters, If you are upgrading to a larger unit, you may need to have plumbing run to it if it has to be relocated.  One way to avoid relocating the unit is to select a model in a non-standard size, such as a unit that is shorter but larger around, known as a “low boy” hot water heater. Lowboys vary between 30 to 49 inches and hold up to 50 gallons of water, Tall water heaters range from 50 to 76 inches and can hold up to 100 gallons of water. They're ideal for basements or garages where height isn't an issue, If you are purchasing a tankless water heater, be sure the location you choose for installation meets ventilation requirements, The ideal location for a tankless unit is on an exterior wall near a gas supply line, water supply line and electrical power source. This is also the easiest and most cost-effective way to run the venting, The unit should have ½-inch clearance on the sides, 12 inches on the front and 18 inches off the floor, Hybrids offer a narrow 21-inch diameter for access into smaller locations.

3.7 Energy Efficiency Whichever fuel source you use, a water heater can be the third largest energy user in your home, so you’ll want a unit that offers energy and cost savings. Fortunately, almost all water heaters offer increased efficiencies to meet increasingly strict federal energy standards. Energy factor EF and yearly operating costs can be found on the Energy Guide label on the unit (see Fig.12). Fig (12): Energy Guide Label Energy factor EF measures how efficiently a unit converts energy into heat as well as how much heat is lost during storage. The higher the energy factor, the more energy efficient the water heater is. Look for EF ratings as close to 1 as possible. Electric heaters tend to have the highest EF ratings.

In the next Article, I will explain in detail the Sizing and Power Rating Calculations for Electrical Water Heaters. So, please keep following.

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