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Engineering ToolBox > Sizing Domestic Hot-Water Heaters
A domestic Hot-Water System supplies the taps in a house or a building with hot water.
Output heating capacity of a domestic hot-water system can be expressed as
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 capacity of water (Btu/lboF)
Input heating capacity of a domestic hot-water system can be expressed as
hin = q ρ dt cp / μ (2)
where
hin = heating capacity, input (Btu/h)
μ = efficiency
Recovery rate in an electrical heater in a domestic hot water system can be expressed as
q = hin μ / dt ρ = P 3413 (Btu/kW) / dt ρ (3)
where
P = electrical power supplied by heater (kW)
Temperature rise in an electrical heater in a domestic hot-water system can be expressed as
dt = hin μ / q ρ = P 3413 (Btu/kW) / q ρ (4)
Electrical power supply to a domestic hot-water systems can be expressed as
P = q ρ dt cp / 3413 (Btu/kW) (5)