Energy Efficiency, Load Analysis, and Project Economics
This domain tests combustion efficiency, load analysis, and economic comparison of gas projects.
Core rules
- Combustion efficiency rises as excess air falls, but too little air produces CO and smoke. Find the knee of the curve, not the lowest possible O2.
- Stack temperature and excess air drive stack loss. Condensing equipment recovers latent heat only below the flue-gas dew point.
- Load analysis uses a load-duration curve, not a single peak. Diversity and simultaneity factors prevent oversizing.
- Compare fuels on cost per delivered MMBtu: fuel price ÷ (heating value × efficiency). Include maintenance, capital, and emissions costs.
- Simple payback = capital cost ÷ annual savings. For longer-lived projects, use NPV or life-cycle cost with a discount rate.
Worked example
Natural gas costs $8.00/MMBtu and the boiler is 85% efficient: delivered cost = 8.00/0.85 = $9.41/MMBtu. Propane costs $2.50/gal with 91,500 Btu/gal and 80% efficiency: 0.0915 MMBtu/gal × 0.80 = 0.0732 MMBtu delivered per gallon; 2.50/0.0732 = $34.15/MMBtu. Natural gas is far cheaper per delivered MMBtu, but the comparison is incomplete without connection cost and standby requirements.
A $50,000 burner upgrade saves $12,500/year: simple payback = 50,000/12,500 = 4.0 years.
Traps
Comparing price per gallon or per cubic foot without heating value and efficiency is meaningless. Do not claim condensing savings when return water is too warm. Also avoid sizing on connected load alone.