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)
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