Boiler Stack Economizer Payback Calculator

Stack economizers recover 5–15% of boiler fuel via flue gas heat exchange.

Boiler MMBTU/hr

Annual run hours

Fuel $/MMBTU

Efficiency gain %

Installed cost

Incentive

Simple payback (yrs)

3.79

Annual savings

$39,600

Net capex

$150,000

How the math works

Annual MMBTU = capacity × hours. Savings = MMBTU × $/MMBTU × gain %.

10 × 5,500 = 55k MMBTU × $9 × 8% = $39,600/yr. ($180k − $30k) / $39.6k = 3.79 yr.

How to Use

Enter boiler MMBTU/hr capacity.
Enter annual run hours.
Enter fuel $/MMBTU.
Enter expected efficiency gain %.
Enter installed cost.
Enter incentive.
Read simple payback.

Frequently Asked Questions

How economizers work?

Flue gases exit boiler at 350-500°F. Economizer = stack-mounted heat exchanger that recovers heat to preheat: feedwater (most common), combustion air, or domestic hot water. Result: gas leaves stack at 200-300°F. Boiler efficiency improves from 80-82% to 85-90%. Recovers 5-15% of fuel cost depending on application.

Cost + sizing?

Equipment $5,000-30,000 per MMBTU/hr boiler capacity. 100 BHP boiler (~3.3 MMBTU/hr): $15-100k. Installation: $25-150k for retrofit (stack work, piping, controls). Permitting: $5-20k (mechanical permit, gas line revision). Total project: $50-300k for mid-size commercial boiler. Larger industrial boilers: $200k-2M projects.

Best applications?

Constant high-load steam boilers (industrial, hospital): excellent fit. 5,000+ run hours/yr. High fuel cost markets (NE, CA): faster payback. Small intermittent boilers: poor fit (cycle losses negate gains). High-temperature flue gas: more recoverable heat. Older boilers: bigger relative gain. Modern condensing boilers: already capture this internally.

Risk + maintenance?

Acid condensation in flue gas if exhaust drops below dew point: corrosion. Stainless steel construction: $$$, but 25+ year life. Maintenance: annual inspection of heat exchanger, periodic cleaning. Backflow prevention valves: protect water supply. Failure modes: tube bundle leaks (major), controls failure (minor). 15-20 year economic life.