Two Boilers on a Common Chimney
Two identical oil-fired boilers, each rated 1.00 gph at 100 psi, share an 8 in. by 12 in. exterior masonry chimney. Each boiler has its own 7 in. breeching with a barometric draft damper and a manual damper. With only boiler A running, the breach draft is -0.04 in. w.c., CO2 is 11.8%, smoke is 0, and stack temperature is 410°F. When boiler B starts, boiler A CO2 falls to 8.5%, smoke rises to No. 2, and the breach draft at A becomes +0.02 in. w.c. Boiler B shows similar but less severe changes. The chimney is 28 ft high with a 6 in. rain cap and no liner. The mechanical room has a 24 in. by 36 in. louvered combustion air opening with a gravity damper. Outdoor temperature is 25°F. The barometric dampers are set wide open. The boilers are in a basement with a tight-fitting door.
1. Explain why boiler A CO2 drops and smoke rises when boiler B starts. Include the roles of draft, excess air, and common venting.
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When boiler B starts, the common chimney must carry both flue-gas volumes. If the chimney is undersized, oversized, or unlined, the added flow can raise the pressure at boiler A breeching instead of pulling a steady negative draft. The +0.02 in. w.c. at A shows that: products are not being removed cleanly, and A may be spilling or recirculating. CO2 at A falls because excess air and dilution air increase, while smoke rises because draft is insufficient for clean combustion. Wide-open barometric dampers make this worse: they admit basement air, cool the stack, and reduce draft. The problem is a venting and combustion-air interaction, not a nozzle problem.
2. What measurements would you take to confirm the cause before changing the venting?
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Measure each boiler alone and both together: breach draft, overfire draft, CO2 or O2, smoke, stack temperature, and barometric damper position. Perform a spillage test at the breeching and check the combustion air opening for actual airflow, not just its size. Verify chimney height, liner condition, and cross-sectional area against the combined input. Observe whether A draft reverses only when B starts. Check the barometric dampers for proper setting and free operation. These measurements separate a chimney capacity problem from a combustion-air shortage or a damper problem.
3. Recommend corrective actions. Address combustion air, barometric dampers, and venting, and explain why simply opening the barometric dampers wider is not a fix.
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Corrective actions: provide adequate combustion air to the mechanical room, verify the louver and gravity damper actually open, and set each barometric damper per the appliance listing. If the chimney cannot handle both boilers, line it to the correct size for the combined input or provide separate venting. Do not open the barometric dampers wider; that increases dilution air, lowers draft, and can worsen spillage. After changes, recheck draft, CO2, smoke, and stack temperature with one boiler and with both running. A chimney liner or power-venting solution must be listed for the application.