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Sizing Domestic Hot-Water Heaters

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

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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 / dt ρ

    = P (3413 Btu/kW) / dt ρ                              (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 = hin μ / q ρ

    = P (3413 Btu/kW) / q ρ                                (4)

Electrical Power Supply

Electrical power supply to a domestic hot-water systems:

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

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