CO Alarm and Chimney Spillage on a Natural-Draft Boiler
A technician arrives at a 1920s house with a natural-draft oil boiler vented into an unlined masonry chimney. The basement CO detector has alarmed twice in the past week, usually when the clothes dryer and bathroom exhaust fan run. The boiler draft overfire measures -0.01 in. w.c. with the burner running, and the barometric damper is nearly closed. A match test at the draft hood shows flue gases spilling into the room. The chimney has visible efflorescence, and the flue pipe has a long horizontal run with two elbows before it enters the chimney. The basement has been tightened with weatherstripping and a new solid door. The technician must decide immediate action, the test sequence, and the corrective measures.
1. What immediate action should the technician take, and why is it unsafe to simply adjust the barometric damper and leave?
Read the worked answer
The technician should treat this as a CO and venting emergency. Shut down the burner, ventilate the basement, and keep the boiler off until the venting problem is corrected. Confirm the CO detector is functioning and advise the occupants to leave if CO readings are elevated. Do not simply open or close the barometric damper to make the match test look better. The damper is a draft regulator, not a cure for a blocked or undersized chimney, a long horizontal connector, or a depressurized basement. Closing the damper further can increase spillage; opening it may reduce draft overfire and make combustion worse. The -0.01 in. w.c. overfire reading is inadequate for most oil appliances, and spillage at the draft hood confirms that flue gases are not safely leaving the building. The technician must find the cause, not mask the symptom, because CO can accumulate even when the flame appears normal.
2. Describe a logical test sequence to identify the cause of the spillage.
Read the worked answer
First, inspect the chimney and flue connector visually for blockage, soot, collapsed liner, or bird nests. Then measure draft with the burner running and with the worst-case exhaust fans operating, including the dryer and bathroom fan. Check draft at the overfire port and at the breech, and perform a smoke or match test at the draft hood. Verify the barometric damper operates freely and is not stuck or mislocated. Measure the basement pressure relative to outdoors with a manometer; a tightly sealed basement with exhaust fans can depressurize the space and reverse draft. Check the flue connector size, length, slope, and number of elbows against the appliance and code requirements. Inspect the chimney for an adequate liner and correct height. Do not rely on a single draft reading; the failure occurs under worst-case conditions, so reproduce those conditions during testing.
3. What corrective measures are likely needed, and how should the technician prioritize them?
Read the worked answer
Priority one is to restore safe venting. The unlined masonry chimney is unsuitable for an oil appliance because flue gases are acidic and can deteriorate mortar, and the efflorescence suggests moisture and deterioration. A listed stainless steel liner sized to the appliance is likely required, along with a proper cap and support. The long horizontal flue pipe with two elbows should be shortened or rerouted to maintain adequate draft. Combustion air must be provided to the basement, either by a correctly sized outdoor air opening or a ducted combustion air system, especially because the basement has been tightened. The barometric damper should be set only after the chimney and air supply are corrected. Finally, perform a combustion test and worst-case draft test, verify CO levels in the living space, and document the results. The technician should not return the boiler to service until spillage is eliminated under worst-case conditions.