Some typical water heater configurations:
The water is heated and stored in the same storage tank at the same temperature as supplied to the consumers.
The water is heated and stored in the same storage tank at higher temperature than supplied to most of the consumers. The hot water is mixed down to consumer temperature with cold water before supplied to the fittings.
The water is heated and stored at consumer temperature before distributed to normal consumers. Water from this store is supplied to an other heater and storage tank where the water is heated to higher temperatures before distribution.
The quantity of hot water is determined by number of occupants and their consumption habits. Timing is very important since consumption varies over the day.
A hot water accumulator - tank volume - will reduce required maximum heat supply. The heat supply to a system with an accumulator can be calculated as:
H = cp V (q2 - q1) / t (1)
H = heat (power) supply (kW)
V = accumulator volume stored (liter)
cp = specific heat water (4.19 kJ/kgoC)
q1 = temperature of the cold feed water (oC)
q2 = temperature of the hot water (oC)
t = available time for the accumulated volume to be heated (sec)
An accumulator with 200 liter is filled with cold water with temperature 5 oC. The electric power required to heat the water to 50 oC in 5.5 hours can be calculated as:
H = (4.19 kJ/kgoC) (200 liters) ((50 oC) - (5 oC)) / ((5.5 hours) (3600 s/hour))
= 1.9 kW
Which is close to the typical power of electric heating elements in hot water accumulators for normal consumption.
Eq. (1) can be modified to express heated accumulated volume if heat supply capacity and available time for the heating is known:
V = Ha ta / (cp (q2 - q1)) (1b)
Ha = heat supply available (kW)
ta = heating time available (sec)
With an instantaneous heater with no accumulating calorifier - the heat supply can be calculated as:
H = cp v (q2 - q1) (2)
v = required volume flow (liter/s)
A shower consumes 0.05 liter/s of hot water. There is no storage tank and the water is heated continuously from 5 oC to 50 oC. Required power to heat the water can be calculated as
H = (4.19 kJ/kgoC) (0.05 liter/s) ((50 oC) - (5 oC))
= 9.4 kW
This high power demand is in general to much for common domestic electric systems and the main reason for the widely use of electric hot water accumulators.
A benefit with an accumulator is a stable hot water temperature. Modulating a large power supply may create unacceptable temperature variations - especially sensible in showers.
Typical hot water storage volume for electric or gas heated systems vs. number of occupants in household:
The required heating surface of an heat exchanger can be calculated as:
A = 1000 H / k qm (3)
A = heating surface (m2)
H = rate heating (kW)
k = overall heat transmission coefficient (W/m2K)
qm = logarithmic mean temperature difference (K)
The heat transmission coefficients depends on
A boiler with correct rating must be selected from manufacturer catalogs where
Boiler rating = Heating capacity of calorifier + safety margin (normally 10 - 20%)
Maximum volume flow through connection pipes to fittings and other equipment is determined by the maximum demand of each consumer.
Maximum volume flow through main pipes is determined by the maximum demand of the fittings and statistic demand based on the number and types of fittings and equipment supplied.
The diagram below indicates typical minimum collector area and storage volume vs. occupants in a household for solar hot water production.
Hot and cold water service systems - design properties, capacities, sizing and more.
Cold water storage for occupants in common types of buildings as factories, hospitals, houses and more
Introduction to general design of domestic service water supply systems - with pressurized or gravity tanks.
Lime deposited vs. temperature and water consumption.
Content of hot water in some common used fixtures - basins, sinks and baths.
Recommended dimensions hot and cold water pipes.
Hot water can be circulated through a return pipe if it's instantly required at the fixtures.
Consumption of hot water per person or occupant.
Hot water heating temperatures adapts to outdoor temperatures.
Free online design tool for designing hot water heating systems - metric units.
Online design tool for hot water heating systems.
Dimensions and capacities of hot-water storage tanks.
Hot water consumption of some common equipment as basins, sinks, baths and showers.
Design hot water consumption of fixtures - basins, showers, sinks and baths.
The Legionella Pneumophila bacterium thrives in water supply systems and air conditioning systems - and may cause the Legionnairs disease.
Final mass and temperature when mixing fluids.
Activity and average water consumption
Calculating expected demand for water supply service lines.
It is fundamental to keep the potable water in the water supply systems uncontaminated.
Converting WSFU - Water Supply Fixture Units - to GPM.
WSFU is used to calculate water supply service systems.
Required water supply to public buildings.
Free online tool for designing water supply systems in buildings.
Water velocities in pipes and tubes should not exceed certain limits.
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