Yes. A gas furnace burns fuel, and anything that burns fuel can produce carbon monoxide. In a furnace that is sealed and venting properly, that gas goes up the flue and outside. The danger starts when the heat exchanger cracks or the flue is blocked, because then the gas has somewhere else to go.
A furnace has a heat exchanger, a metal chamber that keeps the burning gas separate from the air your blower pushes through the house. Combustion happens on one side. Your air passes over the other side, picks up the heat, and never touches the flame. That separation is the whole safety design.
When the heat exchanger cracks, the two sides are no longer separate. Combustion gas, carbon monoxide included, can enter the air stream and get distributed through every supply register in the house. Metal expands when it heats and contracts when it cools, and after enough of those cycles a crack opens. Age, oversizing and restricted airflow all speed it up.
The other route is the flue. If the vent is blocked by a bird nest, a collapsed liner or snow at the termination, exhaust that should leave the building backs up into it instead.
The symptom pattern matters more than any single one of them. Carbon monoxide poisoning is often mistaken for a cold, and the detail that separates it is that everyone in the house feels it and everyone feels better outside.
If an alarm is sounding or people feel unwell, leave the building first and make the phone calls from outside. Do not stay inside to investigate.
An active alarm is an emergency call, not a contractor call. Washington Gas runs an emergency line and the fire department will meter the house.
Stay out until the responders say it is safe to enter. The CPSC is explicit about this. Do not go back in to open windows, fetch anything or shut the furnace off. Ask the responders or the utility to isolate the equipment.
Leave the furnace off and have the heat exchanger and the venting tested before it runs again. Heat is not worth the risk for one night.
A visual look at the burner flame is a screening step, not a diagnosis. Most of a heat exchanger cannot be seen without pulling the burner assembly, and a crack that opens only once the metal is hot at operating temperature can look closed on a cold inspection.
Testing means running the system and measuring. Combustion analysis at the flue reads what the furnace is actually producing. A camera goes where eyes cannot. Ambient carbon monoxide readings get taken at the registers with the blower running, because that is the path the gas would take into the living space.
We write down what the instruments read and show you the numbers. If the exchanger is sound, you should be told that plainly and not sold a system. If it is cracked, you should see the evidence before you decide anything.
Yes. A detector is a threshold device, and a slow leak can sit below its trigger level for a long time while still affecting people in the house. Detectors also expire, usually between five and ten years depending on the model, and an expired one can read normal while sensing nothing at all. Check the date printed on the back.
On every level and outside every sleeping area. Maryland requires them in new and substantially renovated homes, and they are worth having regardless. Put one near the furnace, but do not rely on that one alone, because the gas travels wherever the ductwork goes.
No. Carbon monoxide comes from burning fuel. An electric furnace or an electric heat pump has no combustion and produces none. If the house also has a gas water heater, a gas range, a fireplace or an attached garage, those remain sources and detectors are still needed.
Almost never in a way worth doing. The exchanger is the core of the furnace, and on most residential units replacing it costs enough relative to the machine that replacing the furnace is the sensible answer. Before deciding, it is worth asking whether the manufacturer still covers the part on that model and serial, because that changes the arithmetic.
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