Furnace Not Working or Blowing Cold Air?
A furnace not working usually comes down to one of five things: a thermostat set wrong, a tripped breaker or a switched-off power switch, a blocked filter tripping the limit switch, a failed igniter, or a safety lockout the furnace has entered deliberately. Some of that is safe to check yourself. Some of it, particularly anything involving a smell of gas or a carbon monoxide alarm, means leave the house first. Shemtov Systems services heating across Silver Spring and Montgomery County, MD. Call 240-355-3333.
Key takeaways
- If you smell gas or a CO alarm sounds, leave the house first and call from outside. Nothing below applies until that is ruled out.
- Check the furnace power switch. It looks like a light switch on or near the unit and is switched off by mistake more often than you would expect.
- A furnace that starts then shuts down after a minute or two is usually going into a safety lockout, not failing at random.
- Blowing cold air on a heat pump is often normal defrost, not a fault.
- A furnace that has sat unused since March commonly fails on the first genuinely cold night, which is why the tune-up is booked in autumn.
First: is this a safety situation?
If you smell gas, or a carbon monoxide alarm is sounding, do not troubleshoot. Leave the building, take everyone with you, and call from outside. Gas smell is your utility's emergency line; a CO alarm is 911. A cracked heat exchanger can put combustion gases into the air a house circulates, which is precisely why the annual heating inspection includes checking it. That check is part of the Maximum Performance Membership.
Everything below assumes there is no gas smell and no alarm.
Check the thermostat, then the power switch
Confirm the thermostat is set to HEAT, that the setpoint is above room temperature, and that the fan is set to AUTO rather than ON. Fan set to ON blows unheated air continuously and is very often reported as a furnace blowing cold.
Then find the furnace power switch. It is an ordinary-looking light switch on or beside the unit, frequently at the top of basement stairs, and it gets switched off by someone who thought it was a light. Check the breaker too, and if it has tripped, reset it once. If it trips again, stop: that is a fault, and repeatedly resetting it is a fire risk.
Check the filter
A blocked filter restricts return air, the heat exchanger runs hotter than designed, and the high-limit switch shuts the burner down to protect it. The classic symptom is a furnace that fires, runs for a minute or two, then stops, sometimes cycling like that repeatedly. Replace the filter and give it a full cycle before deciding anything else is wrong. See how often to service an HVAC system for intervals.
What the blinking light on the furnace means
Most modern furnaces have a small LED visible through a sight glass on the control board, and it blinks a diagnostic code. A legend is usually printed inside the access panel. That code tells a technician in seconds what the furnace thinks is wrong, so photographing it before you call is genuinely useful. It commonly points at flame sensing, pressure switch, or limit-switch lockouts.
Do not clear codes by cycling power repeatedly. A furnace that has locked out has done so for a reason, and a lockout that keeps returning is information worth keeping.
Gas furnace versus electric versus heat pump
The three fail differently. A gas furnace has ignition, gas valve and combustion safeties, so its faults tend to be lockouts and no-ignition. An electric furnace has heating elements and sequencers, so its faults tend to be partial heat rather than none. A heat pump is a different machine again: it moves heat rather than making it, so its supply air is cooler than a furnace's even when working perfectly, and it periodically runs a defrost cycle that genuinely does blow cool air for a few minutes.
If you are not sure which you have, our guide comparing a furnace against a heat pump in Maryland describes how to tell them apart.
Why does it always fail on the first cold night?
Because that is the first time it is asked to do real work since March. A component that was marginal in the spring, a weak igniter, a dirty flame sensor, a sticking pressure switch, gets through a mild October and fails when the system runs for hours instead of minutes. It is not coincidence and it is not bad luck; it is the reason heating tune-ups are scheduled in autumn rather than in December. See central heating services.
What is safe to do yourself, and what is not
Safe: thermostat settings and batteries, the furnace power switch, one breaker reset, the air filter, clearing anything blocking return or supply registers, and photographing the diagnostic code.
Not safe: opening the burner compartment, cleaning a flame sensor if you are unsure what it is, anything involving the gas valve, bypassing a safety switch, or repeatedly resetting a breaker that keeps tripping. Those safeties exist because the failure modes are combustion-related.
When to call
Call once the thermostat, the power switch and the filter are ruled out. Call immediately for a gas smell, a CO alarm, a burning smell, water pooling at the base of the unit, or a breaker that will not stay on. Shemtov Systems has serviced heating in Montgomery County since 1981 and advertises a 24-hour answering service. See furnace repair and service or call 240-355-3333.
Frequently asked questions
My furnace turns on then shuts off after a minute. What is that?
That pattern is called short cycling, and the most common benign cause is a blocked air filter tripping the high-limit switch. Replace the filter first. If it continues with a clean filter, the furnace is entering a safety lockout for another reason, commonly flame sensing or a pressure switch, and it needs a technician to read the diagnostic code.
Why is my furnace blowing cold air?
Check whether the thermostat fan is set to ON rather than AUTO, which circulates unheated air continuously. If the fan is on AUTO and the air is still cold, the burner is not firing or is locking out shortly after ignition. On a heat pump, cool supply air during a defrost cycle is normal and lasts a few minutes.
Is it safe to reset the furnace breaker?
Once, yes. If it trips again, stop and call. A breaker that trips repeatedly is doing its job against a genuine fault, usually a shorted component or a seized motor, and repeatedly forcing it back on is a fire risk.
What does the blinking light on my furnace mean?
It is a diagnostic code, and the legend is usually printed inside the access panel. Count the flashes and photograph it before calling. It will not tell you the repair, but it narrows the fault considerably and saves diagnostic time.
How do I know whether the heat exchanger is cracked?
You generally cannot tell visually from outside the unit, which is why it is part of a professional heating inspection. Warning signs worth acting on are a carbon monoxide alarm, soot around the unit, a persistent sharp smell when the heat runs, or repeated unexplained lockouts. Treat a CO alarm as an emergency regardless of what the furnace appears to be doing.
Talk to Shemtov Systems
If your furnace is not working, not turning on, or blowing cold air, call Shemtov Systems at 240-355-3333. Family-owned in Silver Spring since 1981, serving Montgomery County and the Washington DC metro, with a 24-hour answering service.
Services related to this guide
Shemtov Systems has served Montgomery County and the Washington DC metro since 1981. Call 240-355-3333 to talk to a technician.