Furnace Turning On and Off Every Few Minutes? (Toronto)
A furnace that lights, runs for a minute or two and shuts off is either overheating or failing to prove its flame, and the most common cause is a filter so blocked the furnace cannot move air through itself. Change the filter first; if it still cycles, it needs a technician, because every failed cycle wears out the ignition parts.
What to do, in order
Pull the filter and look at it
If it is grey, matted or you cannot see light through it, replace it. A blocked filter starves the furnace of airflow, the heat exchanger overheats, and a safety limit shuts the burners down. This is the single most common cause and it costs a few dollars to rule out.
Check that vents and returns are open
Walk the house and open every supply register, and make sure no return grille is behind a sofa, under a rug or blocked by boxes. Closing off rooms to save money is a common cause of exactly this fault.
Check the thermostat's position
A thermostat in direct sunlight, above a lamp or television, or near a supply vent reads warmer than the room and shuts the furnace off early. If it has been moved or something new sits near it, that may be your answer.
Watch one full cycle and note what happens
Stand at the furnace and watch. Does it light and then go out, or does the blower run on while the burners stop? Does it try three times and lock out? Count how long it runs. Those details tell a technician a great deal before they arrive.
Look at the exhaust vent outside
On a high-efficiency furnace, find the white plastic pipes on the outside wall and check they are not blocked by snow, ice or a nest. A blocked intake or exhaust makes the pressure switch shut the furnace down, and clearing it can fix the problem entirely.
Call us
Our assistant confirms what the furnace is doing and your postal code and connects you live to a technician who is available now. Tell them what you already checked and what you observed during a cycle.
Call a professional immediately if…
- You smell gas at any point. Leave the building and call Enbridge Gas at 1-866-763-5427 from outside.
- A carbon monoxide alarm has sounded. Leave and call 911.
- There is a bang or a whoomph when the burners light, which can be delayed ignition.
- It still cycles after a new filter and with every vent open.
- You can see soot around the burners or the vent, or the flames are yellow and lazy rather than blue.
What happens once you call
The technician will find why the cycle is being cut short. The usual suspects are a dirty flame sensor, which stops the furnace confirming its flame; a limit switch tripping from restricted airflow; a failing pressure switch or a blocked vent; or a control board fault. Most are same-visit repairs.
They should also ask why airflow was restricted in the first place. A furnace that short cycles because the return air is undersized will do it again with a clean filter, and that is a ducting conversation rather than a furnace one. If the diagnosis is simply a new filter every month, ask what the underlying restriction is.
What short cycling is doing to the furnace
Every start is the hardest moment in a furnace's life: the igniter glows, the gas valve opens, the burners light. A unit short cycling does that several times an hour instead of a few times a day, so components rated for years of normal cycling wear out in months.
The cause is usually one of three, and they are not equally urgent. A blocked filter or restricted airflow makes the unit overheat and trip its own limit switch — cheap, and often fixable by you. A dirty flame sensor stops it confirming the flame, so it shuts down seconds after lighting — cheap, but needs a technician. A blocked or restricted flue also trips the safeties, and that one is a combustion problem rather than a comfort one.
Because you cannot tell which from the outside, the honest advice is: change the filter, and if it continues, have it looked at within a few days rather than living with it through the winter.
Questions
Is short cycling actually harmful?
Yes, in two ways. Each start is the hardest moment for the igniter and the blower, so frequent cycling wears them out far faster than steady running. And the underlying cause is often a safety device doing its job because something is overheating or venting badly, which is not a condition to leave running for weeks.
How often should I change my filter?
A one-inch filter typically every one to three months, more often with pets or during renovation. A four or five-inch cabinet filter usually lasts six to twelve months. Write the date on the edge when you fit it; nobody remembers otherwise, and a forgotten filter causes exactly this fault.
Could the furnace simply be too big for the house?
It could. An oversized furnace heats the space to the thermostat setting before it has run long enough to settle, so it stops and starts all day. It is common in houses where a replacement was sized by matching the old unit rather than by calculating the heat loss. A technician can tell you if that is what you are seeing.
The blower keeps running after the burners stop. Is that normal?
Yes. The blower runs on to move the remaining heat out of the exchanger, which is both efficient and protective. What is not normal is the burners going out while the blower runs and then relighting a few minutes later, over and over. That is the pattern worth calling about.