Hydronic System Fundamentals for HVAC Engineers

Hydronic systems distribute thermal energy via water circulation. Proper hydraulic design ensures adequate flow to all loads while minimizing pump energy and maintaining control stability.

System Configurations

Primary-Secondary Pumping

Decouples production (chillers/boilers) from distribution (building loads).

Components:

Primary loop: Constant flow through production equipment
Secondary loop: Variable flow to building loads
Common pipe: Hydraulic separator (low pressure drop connection)

Benefits:

Chillers/boilers see constant flow (manufacturer requirement)
Variable speed secondary pumps save energy
Easy to add/remove production equipment

Variable Primary Flow

Eliminates secondary pumps; single variable speed pumps serve both production and distribution.

Requirements:

Minimum flow through chiller (typically 30-50% of design)
Bypass valve or dedicated minimum flow loop
Advanced controls to prevent low flow

Benefits: Eliminates secondary pumps, reduces complexity, lower first cost

Pump Sizing

Flow rate:

$$GPM = \frac{Q}{500 \times \Delta T}$$

Where:

$Q$ = heat transfer rate (Btu/h or tons × 12,000)
$\Delta T$ = temperature difference (typically 10-20°F for chilled water, 20-40°F for hot water)
500 = constant for water (8.33 lb/gal × 60 min/h × 1.0 Btu/(lb·°F))

Head requirement:

$$H_{total} = H_{static} + H_{friction} + H_{equipment} + H_{control}$$

Pump power:

$$hp = \frac{GPM \times H_{ft}}{3,960 \times \eta}$$

Differential Pressure Control

Maintains system $\Delta P$ at remote sensor location.

Control strategies:

Fixed $\Delta P$: Simple but energy waste at part load
Reset $\Delta P$: Reduces setpoint as valves open (saves 20-40% pump energy)

Practical Applications

Chilled water: 10-12°F $\Delta T$, 40-45°F supply
Hot water: 20-40°F $\Delta T$, 140-180°F supply
Condenser water: 10°F $\Delta T$, 85°F design leaving

Related Technical Guides:

References:

ASHRAE Handbook of HVAC Systems and Equipment, Chapter 13: Hydronic Heating and Cooling System Design
ASHRAE Design Guide: Variable Primary Flow Chilled Water Systems