Low-Level CO Alarms in a Home With a Shared Chimney
A family of four calls because their listed low-level CO alarm has sounded twice in one week, both times in the early morning. No one reports headaches when the alarm is silent. The house is a 1970s ranch with a natural-draft 100,000 BTU/h furnace and a natural-draft water heater sharing a closet off the hallway; both vent into one masonry chimney with a metal liner. The furnace has a standing pilot and a fan/limit control. The technician's meter reads 9 ppm in the master bedroom and 0 ppm outdoors. The furnace's flue gases read 6 ppm CO air-free; the water heater's read 220 ppm air-free. With the closet door open, neither draft hood spills. With the door closed and the bathroom fan running, the water heater's draft hood spills a steady stream. The closet's louvered door has its lower louver painted shut, and the furnace return grille sits in the hallway three feet from the closet door.
1. Where is the carbon monoxide coming from, and why does the alarm reach 70 ppm when the flue readings look acceptable?
Read the worked answer
The flue readings describe what each appliance is producing, not what is entering the house. The water heater's 220 ppm air-free is within the range a draft-hood appliance can produce normally, and the furnace's 6 ppm is clean, so combustion quality is not the headline problem. The problem is that the products are not leaving. The draft hood is spilling, and the spilled gas is being drawn into the home. The closet is being depressurized from two directions: the closed door with a painted-shut lower louver starves the space, the running bath fan pulls on the house, and the return grille three feet away pulls closet air into the return and distributes it through the supply registers. Early morning combines a cold chimney with weak draft, a first furnace cycle, and bathroom fan use. The alarm is reporting accumulated low-level exposure over hours, not a single catastrophic release, which is exactly why low-level alarms are useful and why a clean flue reading does not clear the appliance.
2. What is your order of operations, from arrival through diagnosis, and what safety actions do you take?
Read the worked answer
On arrival, treat the alarm history as real. Ask about symptoms; if anyone has headache, nausea, or dizziness, have the occupants leave and call the emergency number, and ventilate the house. Do not operate switches or create ignition sources while CO may be present. Take ambient readings in each room and at the supply registers before changing anything, then read CO at each draft hood and analyze both appliances. Then do a worst-case combustion appliance zone test: windows and doors closed, all exhaust fans, dryer, and range hood running, the closet door closed, and the furnace operating. Use a smoke pencil at each draft hood and a manometer for the closet's pressure relative to the house. Inspect the chimney and liner, the connector slope, the heat exchanger, and the closet louvers, and measure the return's effect on the closet. Shut off or tag the water heater if it spills under worst-case conditions.
3. What repairs and follow-up would you specify, and how would you protect yourself and the customer on paper?
Read the worked answer
Restore combustion air to the closet, which means freeing or replacing the painted-shut louver and installing correctly sized high and low openings to outdoors per the code for the combined input. Correct the return air arrangement so the return does not depressurize the combustion appliance zone. Verify the chimney liner is intact and correctly sized for the appliances still connected to it, with the water heater's connector properly sloped and inserted. Strongly recommend replacing the 1970s natural-draft furnace with a closed-combustion unit and, if the water heater stays atmospheric, adding a spill switch or converting it to power venting. Install CO alarms per the manufacturer's instructions. Then repeat the worst-case test and show the customer the before-and-after numbers. On paper, record every CO reading with the conditions, photograph the louvers and the draft hood, list the repairs performed and those declined in writing, obtain a signature, and schedule a follow-up visit rather than declaring it fixed over the phone.