HVAC Systems Encyclopedia

A comprehensive encyclopedia of heating, ventilation, and air conditioning systems

Boiler Selection & Sizing for HVAC Engineers

Boiler Selection & Sizing for HVAC Engineers

Boiler selection impacts energy costs, reliability, and system control. Proper sizing and type selection optimize efficiency while meeting load requirements across all operating conditions.

Boiler Types

Fire-Tube Boilers

Hot combustion gases flow through tubes surrounded by water.

Characteristics:

  • Capacity: up to 25,000 lb/h steam or 25 MMBtu/h hot water
  • Pressure: typically < 150 psig
  • Efficiency: 75-84% (non-condensing)
  • Advantages: Simple, low cost, compact
  • Disadvantages: Slower response, thermal shock sensitivity

Water-Tube Boilers

Water flows through tubes surrounded by hot combustion gases.

Characteristics:

  • Capacity: 10,000 - 500,000+ lb/h steam
  • Pressure: up to 1,500+ psig
  • Efficiency: 80-85% (non-condensing)
  • Advantages: High capacity, high pressure capable, fast response
  • Disadvantages: Higher cost, larger footprint

Condensing Boilers

Recovers latent heat from flue gas condensation.

Efficiency: 90-99% (HHV basis)

Key requirement: Return water temperature < 130°F for condensing

Applications: Low-temperature systems (radiant floor, baseboard), outdoor reset control

Boiler Sizing

Heating load:

$$Q_{boiler} = \frac{Q_{design}}{n_{boilers} - 1} \times SF$$

Where:

  • $Q_{design}$ = design heating load (Btu/h)
  • $n_{boilers}$ = number of boilers
  • $SF$ = safety factor (typically 1.15-1.25)

Piping loss allowance: Add 10-15% for distribution losses

Pickup allowance: Historical practice (20-30%) no longer recommended for modern systems

Efficiency Metrics

Combustion Efficiency:

$$\eta_{comb} = 100 - %{loss,dry} - %{loss,moisture}$$

AFUE (Annual Fuel Utilization Efficiency): Seasonal efficiency including cycling losses, jacket losses.

Thermal Efficiency:

$$\eta_{thermal} = \frac{Q_{output}}{Q_{input}} \times 100%$$

Turndown Ratio

Ratio of maximum to minimum firing rate.

$$Turndown = \frac{Q_{max}}{Q_{min}}$$

Typical values:

  • Single-stage: 1:1 (on/off only)
  • Two-stage: 2:1 or 3:1
  • Modulating: 5:1 to 10:1
  • Condensing: 10:1 to 20:1

Benefits of high turndown:

  • Better part-load efficiency
  • Reduced cycling
  • Improved comfort

Practical Applications

  1. Multiple boilers: Size for N+1 redundancy
  2. Lead-lag control: Sequence boilers based on load
  3. Outdoor reset: Lower supply temperature when outdoor temperature rises (10-20% energy savings)

Related Technical Guides:

References:

  • ASHRAE Handbook of HVAC Systems and Equipment, Chapter 32: Boilers
  • ASME Boiler and Pressure Vessel Code