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Hydronic Heating System Simulator

Closed-loop hydronic system — boiler output, flow rate, pump TDH, expansion tank, and terminal units. Animated loop diagram shows temperature gradient from hot supply to cooler return as heat is released at each terminal.

System Parameters

Design Heat Load120k BTUH

Supply Water Temperature180°F

System ΔT (supply − return)20°F → Return 160°F

Terminal Unit Type

Total Loop Length400 ft

Pump Efficiency65%

System Fill Pressure12 psi

Relief Valve Setting30 psi

System Design Results

System Flow Rate

12.0 GPM

BTUH ÷ (500 × ΔT)

Recommended Pipe Size

ID 1.049" · 4.45 fps

Pump TDH

19.2 ft

8.32 psi

Pump BHP

0.04 HP

at 65% efficiency

Terminal Units Required

15 units

Fan Coil Unit @ 8,000 BTUH/unit

Unit Flow Rate

2 GPM/unit

Total 30.0 GPM

System Water Volume (est.)

240 gal

Approx from loop length & GPM

Expansion Tank Size

0.2 gal

Fill 12 psi → Relief 30 psi

✅Velocity Check: 4.45 fps in 1" — ACCEPTABLE (1.5–4.5 fps)

ASHRAE recommendation: 2–4 fps for closed hydronic systems. Minimum 0.5 fps to prevent air binding. Maximum 4 fps to prevent erosion in copper.

Velocity by Pipe Size at 12.0 GPM

¾"

7.22 fps

4.45 fps

1¼"

2.57 fps

1½"

1.89 fps

1.15 fps

2½"

0.80 fps

0.52 fps

0.30 fps

Formula: GPM = BTUH ÷ (500 × ΔT). Pump TDH from Darcy-Weisbach friction + 5 psi fitting allowance. Expansion tank per ASME Section VIII: V_t = V_s × (e_f − e_i) / (1 − P_a/P_f). Standard ΔT: 20°F for conventional boilers, 30–40°F for condensing boilers (lower return temp maximizes condensing efficiency).