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# Domestic Hot Water Heaters - Sizing

Domestic hot-water systems supplies taps in houses or buildings with hot water.

### Output Heating Capacity

Output heating capacity of a domestic hot-water system:

hout = q ρ dt cp                                (1)

where

hout = heating capacity, output (Btu/h)

q = flow rate (gph)

ρ = 8.34 - density of water (lbs/gal)

dt = temperature rise (oF)

cp = 1.0 - specific heat of water (Btu/lboF)

### Input Heating Capacity

Input heating capacity of a domestic hot-water system:

hin = q ρ dt cp / μ                             (2)

where

hin = heating capacity, input (Btu/h)

μ = efficiency

### Recovery Rate

Recovery rate for an electrical heater in a domestic hot water system:

RR = hin μ / dtρ

= P (3413 Btu/kW) / dtρ                              (3)

where

RR = recovery rate (gal/h)

P = electrical power supplied by heater (kW)

### Temperature Rise

Temperature rise in an electrical heater in a domestic hot-water system:

dt = hinμ / qρ

= P (3413 Btu/kW) / qρ                                (4)

### Electrical Power Supply

Electrical power supply to a domestic hot-water systems:

P = q ρ dt cp / (3413 Btu/kW)                                (5)

## Related Topics

### • Water Systems

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

## Related Documents

### Domestic Service Water Supply Systems

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

### Fixtures - Hot Water Content

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

### Hot Water Circulation System - Return Pipes

Hot water can be circulated through a return pipe if it's instantly required at the fixtures.

### Hot Water Consumption vs. Occupants

Consumption of hot water per person or occupant.

### 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.

### Water - Human Activity and Consumption

Activity and average water consumption

### Water Supply Systems - Online Design Application

Free online tool for designing water supply systems in buildings.

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