When the heat cuts out, the instinct is to panic or start flipping switches at random. Neither helps. A calm, ordered check of the most common culprits takes about ten minutes, and it often ends with warm air coming back on its own.
Work through the steps below in order. Each one rules out a simple cause. If you reach the end and the furnace still will not run, you will at least be able to tell the technician exactly what you found, which makes the repair visit faster.
Start with the thermostat
The thermostat causes more no-heat calls than most homeowners expect. Make sure it is set to heat, not cool or off, and that the temperature setting is a few degrees above the current room temperature. If the screen is blank or dim, swap in fresh batteries. A thermostat with a dead battery cannot call for heat, and the furnace will sit silent.
If your thermostat runs on a schedule, check that it is not sitting in an overnight setback or a vacation hold. Our thermostat repair page covers the failures we see most often in Herndon homes.
Make sure the furnace has power
Furnaces have their own shutoff switch, usually mounted on the wall near the unit, and it looks exactly like an ordinary light switch. Guests, kids, and even homeowners bump it off by accident. Flip it on if you find it off.
Next, check the breaker panel. A tripped breaker for the furnace can be reset once. If it trips again right away, leave it off and call a technician, because something in the system is overloading the circuit and forcing it is not safe.
Check the filter and the vents
A filter choked with dust can overheat a furnace until its safety limit switch shuts the whole system down. Pull the filter out and hold it up to the light. If you cannot see light through it, replace it and give the furnace a few minutes to reset.
Walk through the house and confirm that supply vents are open and not blocked by rugs, furniture, or closed doors in unused rooms. A furnace starved of return air behaves a lot like a broken one.
Tell us what you checked when you call. The details help our technician zero in on the real problem faster.
Look and listen at the furnace
Stand near the unit while someone turns the thermostat up. Listen for the sequence: a click, the inducer fan, the igniter, then the main burners. If the blower runs but you never hear ignition, or you hear repeated clicking with no flame, the problem is in the ignition or gas supply and needs a professional. Note any blinking error light on the control board, since the blink pattern tells a technician exactly where to start.
When to stop and call
Stop troubleshooting and call if you smell gas or anything like rotten eggs, if the breaker trips twice, if you see scorch marks or melted wire insulation, or if the furnace short-cycles and shuts down repeatedly. Those are signs of problems that are unsafe to poke at. For anything else on this list that did not fix it, a blower motor or ignition issue may be the cause, and a technician can diagnose it in one visit.
Frequently asked questions
How do I know if the thermostat is the problem?
If the screen is blank, the batteries are the first suspect. If the display works but nothing happens when you raise the setpoint well above room temperature, try fresh batteries and confirm the mode is set to heat. If it still will not call for heat, the thermostat or its wiring may have failed.
Should I keep resetting the breaker if it keeps tripping?
No. Reset it once. A breaker that trips again immediately is doing its job, protecting the circuit from an overload or short. Leave it off and call a technician rather than forcing power back to the furnace.
Is it safe to run the furnace if I smell gas?
No. Do not touch any electrical switches, do not light anything, and do not try to find the leak yourself. Leave the house and call your gas utility from outside, then call a heating technician once the utility clears the home.
Why does my furnace turn on but shut right back off?
That is called short cycling, and a clogged filter or an overheating limit switch is the most common homeowner-level cause. Replace the filter and make sure vents are open. If it keeps happening, the flame sensor or another ignition part likely needs professional attention.
What should I tell the technician when I call?
Describe exactly what the furnace does when you call for heat: nothing at all, blower only, clicking with no flame, or starts then stops. Mention any error-light blink pattern and everything you already checked. Good details shorten the diagnostic visit.
Sources and Further Reading
Last updated: October 19, 2026
No heat tonight? Talk to a heating technician.
Call or email Emberline Heating and describe what your furnace is doing. We will take it from there.