Furnace keeps shutting off

First, time one cycle

Almost every short cycling fault is identifiable from the run time alone, and this is a thirty second job. Set the thermostat to call for heat, and note how long from the burners lighting to them going out.

One detail worth noting while you watch: does the blower keep running after the burners stop? A blower that continues is the furnace deliberately dumping residual heat, which points at the overheating case. Everything stopping at once points elsewhere.

If it dies after a few seconds: the flame sensor

This is the single most common short cycling fault, and the pattern is very distinctive. The furnace goes through its sequence properly, the burners light, and then four to ten seconds later everything shuts down. Often it will try two or three times and then lock out.

What is happening: a furnace proves that it actually lit by passing a tiny current through the flame itself, read by a thin metal rod sitting in the burner flame. Over a season that rod builds up a film of oxide and combustion residue. The flame is there, but the control board cannot read it through the coating, so it does what it is designed to do with unburnt gas present and closes the gas valve immediately.

It is one of the cheapest repairs on a furnace, and it is a maintenance item rather than a failure. The part is usually fine and only needs cleaning. It does sit in the burner assembly, next to the gas valve, which is why it belongs to a technician rather than a weekend. If your furnace does this every autumn, ask for the flame sensor to be cleaned as part of an annual service and the problem generally stops recurring.

If it dies after a few minutes: airflow

Here the furnace runs properly for three to seven minutes, the burners cut out, and the blower carries on for a while afterwards. That is the high limit switch doing its job. The furnace has got hotter inside than it is allowed to, so it shut the gas off and left the fan running to cool the heat exchanger down.

The furnace is not the problem. It is not getting enough air across the heat exchanger to carry the heat away. In order of how often it is the answer:

1. A blocked filter. By a wide margin the most common cause. Hold it up to a light. If you cannot see light through it, it is the answer. Replace rather than clean, unless it is a washable type.
2. Closed or blocked supply vents. Closing vents in unused rooms feels efficient and is the opposite. The system needs a minimum number of open registers to move its design airflow. Furniture and rugs over floor registers do the same thing.
3. A blocked return. The return grille is usually the large one, often in a hallway. It needs to be clear.
4. A dirty blower wheel, or a failing blower motor. Both reduce airflow without anything obvious being blocked, and both need a technician.
5. Collapsed or disconnected flexible duct. Common in older crawlspace duct runs, and it usually shows up as one part of the house being cold as well.

Start with the filter and the vents, because between them they account for most of these calls and neither costs anything. If a fresh filter and open registers do not fix it, the remaining causes are inside the cabinet or under the floor.

Do not respond to this by turning the thermostat up. A furnace shutting down on its limit switch is protecting the heat exchanger, and repeatedly overheating it is how a heat exchanger cracks.

If it heats the house and restarts constantly: sizing or the thermostat

The third pattern looks different. Nothing shuts down early, the house gets warm, and the furnace simply starts and stops far more often than seems right, sometimes several times an hour.

Two usual causes. The first is a thermostat in the wrong place: above a supply register, in direct sun, on an exterior wall or near a lamp. It reads a small pocket of air rather than the room, satisfies quickly and calls again as soon as that pocket cools. Moving a thermostat is a real fix and a modest job.

The second is an oversized furnace, which is extremely common in retrofits. A furnace with far more capacity than the house needs heats the air around the thermostat quickly, shuts off, and repeats. It is less comfortable, harder on the equipment because most wear happens at start up, and it is the reason a correctly sized replacement often feels better than a bigger one. If you are being quoted for a replacement, this is worth raising, because the temptation to fit something larger than the load is exactly how the problem gets rebuilt.

What is different about East Bay furnaces

The first cold week is when this appears. A flame sensor that has been slowly filming up since spring will not announce itself until the furnace runs again in October. Running the heat for fifteen minutes in early autumn finds it while a technician can still come out the same week, rather than during the first cold snap when everyone in the county is calling.

Wall and floor furnaces do not do most of this. Much of Alameda, San Leandro and the older parts of Oakland runs gravity wall or floor units. They have no blower and no filter, so the entire airflow section above simply does not apply. A gravity unit that lights and goes out is usually a thermocouple or pilot problem rather than a flame sensor, which is a different part doing a similar job on an older design.

Oversizing is common in the older stock here. Houses that have been insulated, had windows replaced or been partly weatherised since the furnace went in now need materially less heat than they did. The furnace that was right for the house in 1995 can be substantially oversized for the same house today, and short cycling is how that shows up.