Hydronic Heating System Fundamentals
Hydronic Heating System Fundamentals
Hydronic fundamentals tests the physics behind moving heat with water: heat loss, Btu/h, temperature difference, flow, pressure, expansion, and air control. Expect definition and calculation items plus layout recognition of series loops, zone valves, circulators, and primary/secondary piping.
Core rules
- Heat carried by water: Btu/h = 500 × gpm × ΔT°F for typical water; the 500 constant comes from 8.33 lb/gal × 60 min/h × 1 Btu/lb·°F.
- A design 20°F ΔT is common; lower ΔT requires more flow, higher ΔT less flow but may affect comfort and boiler protection.
- Water expands when heated; a closed system needs an expansion tank, relief valve, and air elimination.
- Pressure must exceed the highest point plus a safety margin; static pressure is about 0.433 psi per foot of height.
- Air causes noise, poor circulation, and corrosion; use air scoops, vents, and proper pump location.
Worked example
A zone must deliver 48,000 Btu/h at a 20°F ΔT. GPM = 48,000 / (500 × 20) = 4.8 gpm. At 16°F ΔT, GPM = 48,000 / (500 × 16) = 6.0 gpm. The lower ΔT needs a larger circulator or wider piping, so check pump head and pipe velocity before changing design.
Traps
Do not mix Btu/h with Btu; do not ignore the 500 constant; remember 1 gpm is about 8.33 lb/min; and do not confuse system static pressure with pump head.