Boiler & Hydronic Sizing Calculator

Size a boiler for a given heat load, calculate design flow rate, pipe velocity, expansion tank, and radiator output correction per ASHRAE / CIBSE / EN 442.

ASHRAE / CIBSE / EN 442 · SI units

Heat Load & Design Conditions

kW
°C
Typical: 11°C (low-temp), 20°C (conventional)
×
Typical 1.10–1.20 per CIBSE

Boiler & Fluid

Pipe Velocity Check

m

Expansion Tank

L
°C
°C
bar

Radiator Output Correction (EN 442)

W
°C
°C
°C

Boiler Sizing Results

Required boiler output (with safety factor)92 kW
Required fuel input97.9 kW
Boiler efficiency used94.0 %
Design flow rate67.67 L/min
Design flow rate17.88 GPM

Pipe Velocity

Velocity in 22 mm (¾") (ID 20.2 mm)3.52 m/s
Excessive (>2.5 m/s — erosion risk, select larger pipe)
Velocity (imperial)11.548 ft/s

Expansion Tank

Water expansion fraction2.12 %
Minimum expansion tank volume6.4 L

Closed diaphragm tank — pre-charge to system cold-fill pressure (typically 0.5–1.0 bar). Add 10–15% for safety.

Radiator Output (EN 442 Correction)

Log-mean temperature difference (actual)49.83 K
Output correction factor99.6 % of rated
Actual output at operating conditions995.7 W

Output is 0% below the EN 442 rating. Low-temperature systems typically need 20–40% larger radiators than the catalogue value.

Engineering Reference

Hydronic Flow Rate — ASHRAE

The fundamental hydronic sizing equation (ASHRAE HVAC Systems & Equipment, Ch. 12):

Q̇ = ṁ × Cp × ΔT   →   V̇ = Q̇ / (ρ × Cp × ΔT)

Where Q̇ is heat transfer rate (kW), ρ is fluid density (kg/m³), Cp is specific heat (kJ/(kg·K)), and ΔT is the temperature differential. Lower ΔT systems (underfloor heating at ΔT 5–11°C) require much higher flow rates for the same heat output — size pumps and pipes accordingly.

Radiator Derating — EN 442

EN 442 rates radiators at ΔT50K standard conditions: mean water 70°C (flow 75 / return 65), room 20°C. LMTD is used, not arithmetic mean.

ΔT_LM = (T₁ − T₂) / ln(T₁/T₂)   where T₁ = T_flow − T_room, T₂ = T_return − T_room
Output = Rated × (ΔT_actual / ΔT_rated)^1.3

The exponent 1.3 is from EN 442 — it accounts for mixed convection/radiation. At flow 55°C / return 45°C / room 20°C, the LMTD ≈ 27.5K (vs rated 50K), so correction factor = (27.5/50)^1.3 ≈ 0.49 — the radiator only delivers ~49% of its catalogue output. Sizing for heat pump low-temperature systems typically requires 2× the radiator area.

Expansion Tank — ASHRAE Fundamentals §13.4

Closed (diaphragm/bladder) expansion tanks must accommodate the thermal expansion of the entire system volume as water heats from cold-fill to operating temperature:

V_tank = V_sys × (ρ_cold/ρ_hot − 1) / (1 − p_atm/p_sys)

Pre-charge the tank to the system cold-fill pressure (typically 0.5–1.5 bar gauge). Add 10–15% to the calculated volume. Under-sized tanks result in pressure relief valve lift on every heating cycle.

Worked Example — 80 kW System, ΔT 20°C

Heat demand: 80 kW  →  With 1.15 safety: 92 kW output
Condensing gas η = 0.94  →  Input = 92 / 0.94 = 97.9 kW

Flow rate: V̇ = 92 / (972 × 4.196 × 20) = 0.00113 m³/s = 1.13 L/s = 67.7 L/min

28 mm pipe (ID 26.2 mm): v = 0.00113 / (π × 0.0131²) = 2.1 m/s — elevated, try 35 mm
35 mm pipe (ID 33.2 mm): v = 0.00113 / (π × 0.0166²) = 1.3 m/s — acceptable

Expansion tank (system 200 L, cold 10°C, hot 80°C, 3 bar):
ρ_cold = 999.8 kg/m³, ρ_hot = 971.8 kg/m³
Expansion fraction = 999.8/971.8 − 1 = 0.0288 (2.88%)
Acceptance = 1 − 1.013/3 = 0.662
V_tank = 200 × 0.0288 / 0.662 = 8.7 L → specify 10 L tank

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