HVAC Hydronic Systems
Hydronic (water-based) heating and cooling systems use pumps to circulate hot or chilled water through coils, heat exchangers, and terminal units. Proper pump sizing ensures comfort, energy efficiency, and equipment longevity.
Three main hydronic circuits:
- Chilled water (CHW): Chiller → AHU coils / fan coils (typically 6-12°C supply)
- Hot water heating (HHW): Boiler → heating coils / radiators (typically 60-82°C supply)
- Condenser water (CW): Chiller condenser → cooling tower (typically 29-35°C)
Step 1: Determine Water Flow Rate
Flow rate is determined by the cooling or heating load:
For water (ρ = 1000 kg/m³, cp = 4.186 kJ/kg·K):
Design Temperature Differences
| System | Typical ΔT |
|---|---|
| Chilled water | 5-6°C (6° supply / 12° return or 7°/13° common) |
| Heating hot water (low temp) | 11-20°C (82°/60° old standard; 50°/40° for condensing boilers) |
| Condenser water | 5-6°C (32° supply / 37° return typical) |
Worked Example
Chiller plant cooling load = 1,000 kW, chilled water ΔT = 6°C:
Q = 1000 × 0.86 / 6 = 143 m³/h (≈ 630 gpm)
Step 2: Calculate System Head Loss
Pipe Friction Loss
Size pipes for 1-2 m/s velocity (CHW/HHW), 1.5-2.5 m/s (condenser water):
- Target friction loss: 200-400 Pa/m (2-4 ft H₂O per 100 ft)
- Higher velocities save pipe cost but increase pump energy
Component Losses (in order of flow path)
| Component | Typical Head Loss |
|---|---|
| Chiller/boiler evaporator/condenser | 3-8 m (per manufacturer) |
| Cooling coil (AHU) | 2-5 m |
| Control valves (2-way, at design) | 2-5 m |
| Strainer | 1-3 m |
| Pipe fittings (elbows, tees) | 20-50% of pipe loss |
| Check valve, isolation valves | 0.5-2 m |
Closed vs Open Systems
Closed systems (CHW, HHW): No static lift; pump only overcomes friction. Static pressure changes with elevation but cancels around the loop. Pressurized to ~1-2 bar at the pump suction.
Open systems (condenser water, cooling tower): Must lift water to tower distribution height + overcome friction on both supply AND return. Static lift is a one-time elevation head (pump must lift water from basin to top of tower).
Step 3: Select Pump
Common HVAC pump types:
| Pump Type | Application |
|---|---|
| End-suction (base-mounted) | Most common; 5-500 m³/h; 10-60 m head |
| Vertical in-line | Small systems; space saving; 5-100 m³/h |
| Split-case (double-suction) | Large systems (>200 m³/h); high efficiency |
| Vertical multi-stage | High-rise buildings; high head requirements |
Oversizing Rule
Add 10-20% safety margin on flow and head:
- Flow: Add 10% for load uncertainty and future
- Head: Add 10-15% for fouling (scaling increases resistance over time)
Do NOT oversize by more than 20% — oversized pumps run left of BEP, short-cycle, waste energy.
Step 4: Variable Flow and VFDs
Modern HVAC systems use 2-way control valves and VFDs for major energy savings.
Constant vs Variable Flow
| System | Pump Control | Energy at Low Load |
|---|---|---|
| Constant volume, 3-way valves | Constant speed, full flow always | 100% power regardless of load |
| Variable volume, 2-way valves | VFD, pressure control | ~30-50% power at 50% flow (cube law) |
VFD Control Strategy
Install differential pressure (DP) sensor across the system or at the most remote/farthest coil:
- DP at pump: Simple but over-pressurizes near coils
- DP at end-of-line (critical zone): Best energy savings; resets to minimum needed
- DP reset by most-open valve (best): adjusts setpoint so one valve is 90% open
Primary-Secondary vs Variable Primary
For large chiller plants:
- Primary-secondary: Constant low-head primary pumps through chillers; variable secondary pumps to building. Simple, decoupled, but uses more pumps.
- Variable primary flow (VPF): One set of variable-speed pumps through chillers and building. Fewer pumps, lower energy, but requires careful chiller flow control and minimum-flow bypass.
VPF is modern standard for new plants.
Pipe Sizing for HVAC
Use these velocity guidelines:
| Pipe Size | Max Velocity (CHW/HHW) | Max Velocity (CW) |
|---|---|---|
| DN50 (2") | 1.0 m/s | 1.2 m/s |
| DN100 (4") | 1.5 m/s | 2.0 m/s |
| DN200 (8") | 2.0 m/s | 2.5 m/s |
| >DN300 (>12") | 2.5 m/s | 3.0 m/s |
Expansion Tanks and Pressurization
Closed hydronic systems require expansion tanks because water expands ~4% when heated from 10°C to 80°C:
- Diaphragm/bladder tank: Most common; pre-charged with air
- Compression tank: Open or closed; older systems
- Pump suction pressurization: Must have positive NPSH; minimum ~1 bar at highest point + vapor pressure
Size expansion tank for ~6% system volume, set fill pressure for ~1 bar above static head at pump suction.
Air Elimination
Air causes noise, corrosion, and flow problems:
- Install air separators (microbubble) at the hottest point in the system (boiler discharge or chiller return)
- Automatic air vents at all high points
- Run pumps during fill/purge cycle to push air to vents
- Use glycol for outdoor/freezing applications (reduces heat capacity; must derate pumps)
Condenser Water System Special Notes
- Open cooling tower systems: pump from tower basin to condenser to tower top
- Add strainers and side-stream filtration (towers collect dirt)
- Water treatment essential (scale, corrosion, biological control)
- Freeze protection in cold climates (tower basin heaters, drain-down)
- Size for 0.05-0.07 l/s per kW of chiller capacity (per ton of refrigeration: ~3 gpm)
Parallel Pump Operation
For redundancy and turn-down, install multiple pumps in parallel:
- 2 × 50% pumps (one standby) most common
- 3 × 33% pumps allow better turn-down
- Pumps operating in parallel each deliver less than rated (system curve intersection shifts)
- Staging pumps on/off based on load; VFD on lead pump
Summary
Size HVAC pumps from actual load and system resistance — flow from kW/(4.186×ΔT) and TDH from pipe friction plus equipment losses (chillers 3-8m, coils 2-5m, valves 3-5m). Use VFDs with 2-way control valves for 40-60% energy savings vs constant flow. Don't add more than 10-20% margin. Install air elimination at the high point and expansion tanks in closed systems. Condenser water systems are open — add elevation lift and specify robust water treatment.