TL;DR

Excess fuel use is almost always excess air in the combustion process or degraded heat transfer surfaces. Run a flue gas analysis to set the air-fuel ratio correctly, then check for scale and soot.

What you might see

fuel use above the baseline for the same steam output
flue gas O2 above 4% at rated firing rate
flue gas CO above 100 ppm on a gas-fired boiler
stack appearing darker or sooty on oil-fired units

Likely causes

Tap to mark as ruled out

Excess combustion air from an incorrect air-fuel ratio setting diluting the flame and wasting heat up the stack
Waterside scale or fireside soot degrading heat transfer and requiring more fuel to produce the same steam
Burner nozzle wear or contamination changing the fuel spray pattern
Preheated combustion air temperature lower than design reducing thermal efficiency

Required tools

Calibrated combustion analyzer (O2, CO, CO2, temperature)
Burner adjustment tools (damper linkage, cam access)
Flue gas temperature probe

Safety first

Never reduce combustion air below the point where CO rises. Incomplete combustion produces carbon monoxide, an odorless and immediately dangerous gas.
Combustion adjustments must be performed by a qualified boiler technician. Incorrect settings can cause flame instability and lockout or a delayed ignition event.

Procedure

7 steps · ~60 min

1
Connect a combustion analyzer to the flue gas sample port downstream of the heat exchanger. Measure O2, CO, CO2, and flue gas temperature at rated firing rate.[1]
2
Compare O2 to the Cleaver-Brooks target for this boiler model and fuel type. Gas-fired targets are typically 2-4% O2 at full fire; oil-fired targets are 3-5%. Excess air above these values indicates an over-air condition.[1]
3
If O2 is high, reduce combustion air by adjusting the air damper linkage or combustion air blower speed. Reduce incrementally: 0.5% O2 at a time and re-measure CO after each adjustment.
4
Stop adjustment if CO rises above 100 ppm (for gas) or 200 ppm (for oil). This is the lean-limit boundary.
5
Record the final set points (damper position, fuel valve cam setting) on the burner data card.
6
Check flue gas exit temperature and compare to baseline. If elevated beyond what air-fuel adjustment explains, inspect for soot and scale (see the low-steam-pressure procedure on this machine).
7
Schedule combustion analysis at least annually and after every major maintenance event.

Sources

Cleaver-Brooks CB Firetube Boiler Operation, Service, and Parts Manual, Cleaver-Brooks
Cleaver-Brooks CB Boiler Operation, Maintenance and Parts Manual, combustion adjustment and efficiency optimization (general)
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