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Hot Water Systems

Reference data and engineering information about hot water systems for water systems applications.

hotwatersystems

Overview

Engineering reference data for Hot Water Systems in water systems.

Key Formulas

Hydrostatic Pressure

P=ρghP = \rho g h

Pressure due to water column.

Flow Rate

Q=AvQ = A v

Area × velocity.

Variables

SymbolDescriptionUnit
PPPressurePa
QQFlow ratem³/s
hhHead/depthm

Hot Water Circulation Systems

Hot water can be circulated through a return pipe if it's instantly required at the fixtures. This eliminates the wait time for hot water to reach distant fixtures and reduces water waste.

Key Design Considerations:

  • Return pipes must be properly insulated to minimize heat loss
  • Circulation pump sizing depends on total pipe heat loss and system volume
  • A check valve prevents reverse flow when the pump is off
  • Balancing valves ensure proper flow distribution to all branches

Domestic Hot Water Sizing

The basic formula for water heater sizing:

Q=m⋅c⋅ΔTtQ = \frac{m \cdot c \cdot \Delta T}{t}

Where:

  • QQ = heating power required (kW)
  • mm = mass of water (kg)
  • cc = specific heat of water (4.186 kJ/kg⋅°C4.186 \text{ kJ/kg·°C})
  • ΔT\Delta T = temperature rise (°C)
  • tt = heating time (seconds)

Recovery rate for continuous heating:

R=Pρ⋅c⋅ΔTR = \frac{P}{\rho \cdot c \cdot \Delta T}

Where:

  • RR = recovery rate (L/h)
  • PP = heater power input (W)
  • ρ\rho = water density (1 kg/L at standard conditions)

Hot Water Demand Estimation

Daily hot water demand can be estimated using:

D=N⋅qD = N \cdot q

Where:

  • DD = total daily demand (liters/day)
  • NN = number of occupants
  • qq = per-person consumption rate (liters/occupant/day)

Typical consumption values vary by building type:

  • Residential: 50–80 L/person/day
  • Hotels: 80–150 L/person/day
  • Hospitals: 100–200 L/person/day

Hot Water Fixture Flow Rates

Common fixture flow rates for hot water supply design:

Fixture TypeTypical Flow Rate
Basin (lavatory)0.10–0.15 L/s
Shower0.15–0.20 L/s
Kitchen sink0.15–0.20 L/s
Bath0.20–0.30 L/s

Design Procedure Overview

For domestic hot water service systems, the general design procedure includes:

  1. Determine peak demand based on number and type of fixtures
  2. Calculate required heater capacity using demand and recovery formulas
  3. Size storage tank to buffer between peak demand and heater output
  4. Size distribution pipes per recommended velocity limits (typically ≤ 2 m/s)
  5. Design circulation loop if instant hot water is required at fixtures
  6. Select appropriate insulation to minimize standby heat losses

References

Source: engineeringtoolbox.com