Heating Systems, Equipment and Design
Heating system design is judged on three things: the correct load, correctly corrected terminal capacity, and a hydraulically stable circuit. Chinese practice fixes design temperature differences (95/70 °C for high-temperature hot water, 85/60 or 75/50 °C for low-temperature, 45/35 °C for radiant floor), so most marks come from translating catalogue ratings into actual conditions.
Load and terminal selection
- Heating load includes transmission through the envelope, infiltration or cold-air ingress, and any heat needed to raise cold supply air; pipe losses are added only where the code requires it.
- Radiator and convector ratings are quoted at standard conditions (95/70/18 °C). Correct with Q = Q_std (Δt/Δt_std)^n, where Δt is mean water temperature minus room temperature and n is the manufacturer's exponent (about 1.25–1.35 for panel radiators).
- Radiant floor: surface temperature limits are 29 °C in occupied zones (35 °C at the perimeter), which caps supply water near 45 °C.
Hydraulics
- Two-pipe systems with thermostatic valves need a differential-pressure control valve or a variable-speed pump; single-pipe vertical risers need a cross-over bypass at each radiator so lower floors are not starved.
- Size mains for 0.5–1.2 m/s and verify the index circuit; balancing valves are chosen on surplus pressure, not on pipe diameter.
- Closed expansion vessels are sized on system water volume and the pressure band between cold fill and safety-valve set pressure.
Worked example
A room needs 1500 W. Radiators were rated at 95/70/18 °C, but the design is 85/60/20 °C. Δt = (85+60)/2 − 20 = 52.5 K; Δt_std = 82.5 − 18 = 64.5 K. Ratio = 0.814; raised to n = 1.3 gives 0.760. Required catalogue capacity = 1500/0.760 = 1974 W. Selecting from the uncorrected 1500 W figure undersizes the room by roughly a quarter.
Traps
Using logarithmic mean temperature difference where the exam expects the arithmetic mean; forgetting that the exponent changes with radiator type; ignoring boiler and fitting pressure loss when setting the safety valve; applying two-pipe balancing logic to a single-pipe riser; omitting water treatment and make-up water limits, which are code items rather than options.