HVAC and Electrical Experts
Written by Peter
Master Electrician at PRO Electric plus HVAC, serving Fairfax, Loudoun, Arlington, and Prince William Counties. Virginia License #2705181607.
No Heat? Start with the Simple Things.
Heating repair across Fairfax, Loudoun, Arlington, and Prince William Counties.
Hi, I am Peter, the Master Electrician at PRO Electric plus HVAC. A furnace that will not turn on is one of those problems that always seems to happen on the coldest night. The good news is that some causes are simple enough to check yourself, and knowing them can save you a cold evening or a service call. Let me walk through what to look at, and where the line is for calling a professional.
A furnace needs a few basic things to fire: power, a thermostat calling for heat, fuel or working heating elements, and its safety systems giving the all clear. When one of those is missing, it stays off.
Simple things to check first
- The thermostat. Make sure it is set to heat, the temperature is above the room temperature, and the batteries are good. It sounds obvious, and it is often the answer.
- The power switch and breaker. Furnaces have a switch nearby that looks like a light switch, and they run on a circuit that can trip. A tripped breaker is a common cause, which connects to why a breaker keeps tripping.
- The filter. A badly clogged filter can make the furnace overheat and shut down on its safety control. A fresh filter is worth trying.
- The furnace door panel. Many furnaces will not run if the front panel is not fully closed, because a safety switch keeps them off.
When it is a job for a professional
- Ignition problems. A furnace that tries to start but will not light may have a failing igniter, flame sensor, or gas supply issue. These are not do it yourself repairs.
- It runs but blows cold. That is a different problem, and I cover it in why a furnace blows cold air.
- Repeated shutdowns or odd noises. A furnace that starts and quits, or makes new noises, is signaling a fault that needs a look.
- Any smell of gas. Stop and treat that as urgent.
A gas smell is never a troubleshooting step
If you ever smell gas around the furnace, do not try to start it or hunt for the cause. Leave the area and get help. Everything else on this page can wait until that is ruled out, because gas is the one furnace issue where there is no margin for poking around.
How we help
If the simple checks do not bring the heat back, we diagnose the furnace properly, from ignition and safety controls to the blower and the electrical side, and get it running safely. Because we handle both the heating and the electrical, we can sort out whether the problem is the furnace itself or the circuit feeding it, which often saves a second visit. For a heat pump rather than a furnace, the heating path is a bit different, covered in heat pump and furnace heating.
Frequently asked questions
Why will my furnace not turn on?
Common reasons are a thermostat set wrong or with dead batteries, a tripped breaker or the furnace power switch turned off, a clogged filter tripping a safety shutdown, or a furnace door panel that is not fully closed. Beyond those, ignition or safety part failures need a technician.
What should I check first if my furnace has no heat?
Start simple: confirm the thermostat is set to heat and above room temperature with good batteries, check that the furnace switch is on and its breaker has not tripped, replace a dirty filter, and make sure the front panel is fully closed. These four checks solve a surprising number of no heat calls.
Why does my furnace try to start but not light?
A furnace that attempts to start but will not light usually has an ignition issue, such as a failing igniter or flame sensor, or a problem with the gas supply. These are not do it yourself repairs and should be handled by a technician, since they involve fuel and safety components.
Is it safe to keep resetting my furnace?
Resetting once after checking the basics is reasonable, but repeatedly resetting a furnace that keeps shutting down is not a fix and can be unsafe. The safety controls are shutting it off for a reason. If it will not stay running, it needs to be diagnosed rather than forced.
What should I do if I smell gas near my furnace?
Treat it as urgent. Do not try to start the furnace or look for the source. Leave the area and get help. A gas smell is the one furnace situation where troubleshooting is never the right first step, and it should always come before any other checks.
Furnace will not turn on?
Heating repair across Northern Virginia.