Short answer
Check the thermostat fan setting first: set to ON, not AUTO, the blower runs continuously and delivers room-temperature air between burns. If the fan setting is right, the usual causes are a burner failing to light, a flame sensor that needs cleaning, or a clogged filter tripping the high-limit switch. The fan setting costs nothing to check and explains a meaningful share of these calls, which is why it is worth ruling out before anything is booked. If the furnace does light but stops shortly afterwards and blows cool air until it restarts, that pattern points at overheating on a safety control rather than at the burner, and the filter is the first thing to look at.
Start with the free one
Thermostat fan settings are ON and AUTO. On AUTO the blower runs only when the furnace is producing heat. On ON it runs constantly, which means that between heating cycles it is circulating room-temperature air past you, and air at room temperature feels cold when it is moving.
This is not a fault and it is a deliberate setting some people prefer for even temperatures and continuous filtration. But it is switched on by accident often enough that it is worth ruling out before anything else, and it takes two seconds.
The pattern tells you the rest
| What happens | Likely cause | Homeowner fix? |
|---|---|---|
| Cold air all the time, blower never stops | Fan set to ON | Yes, set it to AUTO |
| Burner lights, runs briefly, shuts off, repeats | High-limit trip from restricted airflow | Yes, change the filter |
| Burner lights then goes out after a few seconds | Dirty flame sensor or ignition fault | Sometimes |
| Blower runs, burner never lights at all | Ignition, gas supply or control board | No |
| Warm at first then cold late in each cycle | Overheating and cutting out | Start with the filter |
| Cold air plus a burning smell or soot | Stop, combustion problem | No, shut it off |
Why a dirty filter produces cold air
It is worth understanding because it is the most common real fault here and the cheapest to fix. A furnace transfers heat from combustion into the air moving across its heat exchanger. If a clogged filter restricts that airflow, heat builds in the exchanger instead of being carried away, and the high-limit switch shuts the burner off to protect it.
The blower keeps running to clear the residual heat, so what you feel at the register is a short burst of warm air followed by a longer run of cool air. Then it fires again and repeats. That cycle, burner on, brief heat, burner off, fan continues, is the signature, and the repeated overheating it involves is a recognised contributor to heat exchanger cracking. Which is why the cheapest component in the system causes the most expensive failure in it.
The checks, in order
- Set the thermostat fan to AUTO. Rule out the setting before anything else, and confirm the thermostat is set to heat and above room temperature.
- Change the filter. Not inspect: change, or at least hold it up to a light. If you cannot see light through it, it is restricting airflow. A one-inch filter in a Vermont heating season often needs replacing every one to three months.
- Open every register and return. Closed registers and furniture over return grilles restrict airflow in exactly the same way as a filter, and closing vents in unused rooms does not save money on most systems: it starves the furnace.
- Check the breaker and the service switch. Most furnaces have a wall switch nearby that looks like a light switch. The switch gets turned off by mistake constantly.
- Check the condensate drain on a high-efficiency unit. Condensing furnaces produce water, and a blocked condensate trap or a full pump shuts the unit down on a safety switch. It looks like a serious fault and it is often a clogged tube.
- Watch one full cycle. Stand and observe: does the burner light, how long does it stay lit, when does the blower start and stop. Being able to describe that sequence accurately is worth more to whoever comes out than any guess about the cause.
Stop immediately and shut the furnace off if you smell a sharp or acrid odour, see soot, see a yellow or lifting burner flame instead of a steady blue one, or if any carbon monoxide alarm sounds. Those point at combustion problems, and a cracked heat exchanger produces no symptom in how the furnace runs.
The Vermont detail: filters and woodstoves
Filter intervals here are shorter than the packaging suggests for two local reasons. Houses run their heating for a long season, so the filter accumulates far more hours than the same filter would in a mild climate. And a large share of Vermont houses also run a woodstove or pellet stove, which puts fine ash into the indoor air that the furnace filter then collects.
A household with a woodstove and pets can blind a one-inch filter in under a month during heating season. If you have been changing yours quarterly on general advice and your furnace short cycles, that is a plausible cause on its own.
When the furnace is short cycling instead
A furnace that runs and delivers cold air is one fault. One that starts and stops repeatedly without the cold-air complaint is another, and the related article on short cycling goes deeper on causes beyond airflow.
Common questions
What is a high-limit switch?
It is a safety device that shuts the burner off when the heat exchanger gets hotter than it should. It is doing its job, not failing, and the cause is nearly always insufficient airflow: a clogged filter, closed or blocked registers, an undersized return, or a weakening blower. A furnace that fires, runs briefly, shuts off and repeats is short cycling on this limit, and the correct response is to restore airflow rather than to look for something wrong with the switch.
Can I fix a furnace blowing cold air myself?
Often, yes. Correcting the fan setting, changing the filter, opening registers and returns, resetting the breaker or service switch, and clearing a blocked condensate drain are all reasonable homeowner actions and between them resolve a large share of cases. Cleaning a flame sensor is within reach of a confident owner with the power off. What is not homeowner territory is the gas valve, the control board, or anything to do with the heat exchanger.
What happens if I keep running a furnace that short cycles?
You put the heat exchanger through repeated overheating and cooling. That is a recognised contributor to cracking, and a cracked heat exchanger can let combustion products including carbon monoxide into the air the furnace is circulating through your house. It also wears the ignition components and wastes fuel, since the least efficient part of any burn is the start. The disproportion is the point: the fix is usually a filter, and the consequence of not fixing it is the most serious failure the appliance has.
How do I know if my heat exchanger is cracked?
You generally cannot confirm it yourself. That is exactly why it is dangerous. But there are signals that mean stop: a yellow, flickering or lifting burner flame, not a steady blue one; soot around the furnace; a sharp acrid smell when it runs; visible rust or scaling; a carbon monoxide alarm activating; or household members with headaches, nausea or drowsiness that improve when they leave the house. Any of those means shut the furnace off, and an alarm means leave the building and call from outside.
How often should I change the furnace filter?
For a standard one-inch pleated filter during a Vermont heating season, every one to three months, and at the shorter end if there are pets, a woodstove or pellet stove, or recent construction dust. Deeper media filters in a proper cabinet run six to twelve months. Rather than following a calendar blindly, hold the filter up to a light, if you cannot see light through it, it is restricting airflow regardless of how long it has been in.
Should I close vents in rooms I do not use?
Generally no. It feels like it should save money, but most residential duct systems are not designed for it: closing registers raises static pressure across the whole system. That reduces airflow through the furnace, pushes it toward the high-limit trip that shuts it off, and can increase duct leakage. If several rooms genuinely do not need heat, the right answer is zoning or a properly balanced system, not closed registers.