Overfired Boiler After Nozzle Replacement
A technician responds to a 1990s cast-iron boiler rated 175,000 Btu/hr input. The homeowner reports that the boiler was serviced last week by another company and now the basement smells oily and the boiler rumbles on startup. The technician finds a new 1.25 GPH, 80°B nozzle and a pump pressure gauge reading 140 psi. The burner is a typical retention-head oil burner. A combustion test shows 10.0% CO2, #2 smoke, 560°F net stack temperature, and -0.03 in. w.c. draft overfire. The air shutter and band are set about one-third open. The heat exchanger was cleaned, the combustion chamber is intact, and the flue passages are clear. The boiler nameplate input is 175,000 Btu/hr. The technician must decide whether to adjust the existing nozzle and pressure, replace the nozzle, or look for another fault, and must explain the combustion readings.
1. Calculate the actual firing rate and determine whether the boiler is correctly fired. Show the reasoning behind the calculation.
Read the worked answer
Nozzle ratings are stated at 100 psi, so a 1.25 GPH nozzle at 140 psi does not deliver 1.25 GPH. Flow varies with the square root of the pressure ratio: 140/100 = 1.4, and the square root of 1.4 is about 1.183. Actual flow is therefore 1.25 × 1.183 = about 1.48 GPH. At roughly 140,000 Btu per gallon, that is about 207,000 Btu/hr input, versus a 175,000 Btu/hr nameplate rating. The boiler is overfired by about 18 percent. That explains the high stack temperature, the rumble, and the oily smell. The correct fix is not to leave the nozzle at 140 psi and chase the air settings. Either reset the pump pressure to 100 psi if the appliance and burner manufacturer allow it, which returns the input to about 175,000 Btu/hr, or select a smaller nozzle for 140 psi. For example, a 1.05 GPH nozzle at 140 psi would deliver about 1.24 GPH, or about 174,000 Btu/hr. After correcting input, recheck draft and readjust combustion air.
2. The technician corrects the firing rate. What combustion targets should be used, and in what order should the final adjustments be made?
Read the worked answer
Establish draft first, because draft affects the entire combustion process. For a typical natural-draft oil appliance, verify the manufacturer’s specified overfire and breech draft; a common overfire target is about -0.02 in. w.c., but the appliance data plate and manual govern. Then adjust the air shutter and band for a zero smoke reading, or the lowest smoke number the manufacturer permits, usually a trace or zero. After smoke is acceptable, read CO2 and stack temperature. A well-adjusted oil burner commonly runs around 11.5 to 12.5 percent CO2 with zero smoke, but the exact target depends on the burner and appliance. Do not increase CO2 by closing air until smoke appears. The initial readings of 10.0 percent CO2 with #2 smoke were not a normal excess-air condition; they reflected an overfired burner with a mismatched air setting. Recheck draft after the air adjustment, because changing the air band changes the draft. Record the final nozzle size, pump pressure, smoke, CO2, stack temperature, and draft.
3. The homeowner asks why the technician cannot simply open the air shutter more and leave the 140 psi pressure alone. How should the technician explain the problem?
Read the worked answer
Opening the air shutter may reduce smoke temporarily, but it does not correct the root cause. The burner is still delivering about 207,000 Btu/hr into a heat exchanger rated for 175,000 Btu/hr. The extra heat raises stack temperature, reduces steady-state efficiency, and can distort the flame or cause rumbling. Adding air also lowers CO2 and increases the mass of flue gas, which can lower efficiency further. The combustion chamber and flue passages may be clear now, but continued overfiring can crack the chamber, warp baffles, and shorten the life of the boiler. The correct approach is to match input to the nameplate rating first, then set draft and air to the burner manufacturer’s specifications. The technician should also confirm the nozzle type and spray angle match the original specification, because a wrong spray pattern can cause smoke even at the correct firing rate. The customer should receive the final combustion test results in writing.