Hydronic System Fundamentals and Heat Load
Domain 1, Hydronic System Fundamentals and Heat Load, tests design heat loss, design conditions, Btu/h-to-flow conversions, and basic system layout. Expect calculation items, not just definitions.
Core rules
- Design heat loss uses Q = U × A × ΔT for each assembly, then adds infiltration and ventilation. Use the local winter design temperature and 70°F indoor unless the project states otherwise.
- Boiler load includes pickup for piping, warm-up, and domestic hot water. A common residential pickup is 1.15–1.25; it does not replace a real load calculation.
- Hydronic flow: GPM = Btu/h ÷ (500 × ΔT). The 500 factor comes from 8.33 lb/gal × 60 min/h × 1 Btu/lb·°F.
- A 20°F design ΔT is common for fin-tube baseboard; panel radiators, radiant floors, and snow melt may use different ΔT values.
- Connected radiation output is not the same as design heat loss. The boiler and circulator are selected from the load and the required temperature difference.
Worked example
A room-by-room load totals 60,000 Btu/h. With a 20°F ΔT, flow = 60,000 ÷ (500 × 20) = 6 GPM. Add 15% pickup: 69,000 Btu/h and 6.9 GPM. The circulator must deliver about 7 GPM at the calculated head, not simply match the boiler size.
Traps
- Using 180°F supply as the design temperature instead of the actual operating curve.
- Omitting infiltration, duct losses, or outdoor reset effects.
- Oversizing because the old boiler was 100,000 Btu/h.
- Mixing net and gross boiler ratings, or ignoring altitude derate.