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# Hot Water Circulation System - Return Pipes

A return circulation pipe is sometimes provided in a hot-water system where it is desirable to have hot water available continuously at the fixtures. Typically for systems where the distance from the hot water heater to the consumption fixtures exceeds 25 - 30 m.

### Time for Hot Water to reach Fixture without Circulation Pump

• 1 US gpm = 0.0630 liter/sec
• 1 feet = 0.305 m

### Hot Water Circulation Pump

A smaller pipe with an inline pump is connected to a point close to the most distant fixture and to a point close to the hot water heater. The pump can run continuously or intermittent circulating enough water to keep the temperature drop in the pipeline during low or no consumption within an acceptable limit.

The required circulated water flow can be calculated

Q = q / (ρ cp dt)                                        (1)

where

Q = pump capacity (m3/s)

q = heat loss from the pipe line (W)

ρ = density of water (kg/m3) (988 kg/m3 at 50 oC)

cp = specific heat of water (J/kgoC) (4182 J/kgoC at 50 oC)

dt = temperature drop (oC)

Typical heat loss from an insulated pipe line is in the range 30 - 60 W/m. Acceptable temperature drop can be 10 oC.

### Example - Required Circulation Volume in a Hot Water Return Pipe Line

The length of a pipeline inclusive the circulation line is 100 m. With water at temperature 50 oC the average specific heat loss from the pipe line is estimated to 30 W/m. The total heat loss from the whole pipe line can be calculated as

q = (100 m) (30 W/m)

= 3000 W

Required water flow to limit the temperature drop to 10 oC can be calculated as

Q = (3000 W) / ((988 kg/m3) (4182 J/kgoC) (10 oC))

= 7.2 10-5 m3/s

= (7.2 10-5 m3/s) (1000 liter/m3)

= 0.072 liter/s

## Related Topics

### • Water Systems

Hot and cold water service systems - design properties, capacities, sizing and more.

## Related Documents

### Design of Hot Water Heating Systems

Calculate gravity and forced circulating hot water heating systems.

### Domestic Hot Water Heaters - Sizing

Domestic Hot-Water sizing equations - heating capacity, recovery rate and power supply.

### Domestic Hot Water Service Systems - Design Procedures

Design procedures for domestic hot water service systems.

### Domestic Service Water Supply Systems

Introduction to general design of domestic service water supply systems - with pressurized or gravity tanks.

### Energy Accumulated in Heated Water - kWh

The amount of thermal energy stored in heated water.

### Fixtures - Hot Water Content

Content of hot water in some common used fixtures - basins, sinks and baths.

### Hot and Cold Water Pipes Sizing

Recommended dimensions hot and cold water pipes.

### Hot Water Consumption vs. Occupants

Consumption of hot water per person or occupant.

### Hot Water Heating System Temperatures vs. Outdoor Temperatures

Hot water heating temperatures adapts to outdoor temperatures.

### Hot Water Heating Systems - Online Design Application

Free online design tool for designing hot water heating systems - metric units.

### Hot Water Heating Systems - Online Design Application, Imperial Units

Online design tool for hot water heating systems.

### Hot Water Storage Tanks - Capacities vs. Dimensions

Dimensions and capacities of hot-water storage tanks.

### Hot Water Supply - Flow Rates to Fixtures

Hot water consumption of some common equipment as basins, sinks, baths and showers.

### Hot Water Supply vs. Fixtures Consumption

Design hot water consumption of fixtures - basins, showers, sinks and baths.

### Pipes - Insulated Heat Loss Diagrams

Heat loss (W/m) from 1/2 to 6 inches insulated pipesĀ - ranging insulation thickness 10 to 80 mm and temperature differences 20 to 180 degC.

### Water Distribution Pipes - Materials

Materials used in water distribution pipes.

### Water Supply due to Fixtures Water Requirements

Supply requirements for common fixtures.

### Water Supply Fixture Units - WSFU vs. GPM and Liters/sec

Converting WSFU - Water Supply Fixture Units - to GPM.

### Water Supply Fixture Units WSFU

WSFU is used to calculate water supply service systems.

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