Sizing Domestic Hot-Water Heaters

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

A domestic Hot-Water System supplies the taps in a house or a building with hot water.

Output Heating Capacity

Output heating capacity of a domestic hot-water system can be expressed as

h_out = q ρ dt c_p         (1)

where

h_out = heating capacity, output (Btu/h)

q = flow rate (gph)

ρ = 8.34 - density of water (lbs/gal)

dt = temperature rise (^oF)

c_p = 1.0 - specific heat capacity of water (Btu/lb^oF)

Input Heating Capacity

Input heating capacity of a domestic hot-water system can be expressed as

h_in = q ρ dt c_p / μ         (2)

where

h_in = heating capacity, input (Btu/h)

μ = efficiency

Recovery Rate

Recovery rate in an electrical heater in a domestic hot water system can be expressed as

RR = h_in 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 can be expressed as

dt = h_in μ / q ρ

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

Electrical Power Supply

Electrical power supply to a domestic hot-water systems can be expressed as

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

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