Furnace Repair & Service

Furnace repair and service is the work of diagnosing and fixing a gas or electric forced-air furnace that will not light, will not stay lit, blows cold air, or keeps shutting itself down. JC & JC HVAC Mechanical Contractors repairs furnaces across DC Metro, around the clock. Schedule a furnace repair or call (301) 509-2475 if you are without heat now.

Modern furnaces are built around safety checks, and a no-heat call can come down to one of those checks failing. That is good news: a methodical technician can usually trace a no-heat call to one specific part. This page walks through the main furnace failure points and how they are repaired. For boilers, heat pumps or general heating questions, our heating repair page covers the wider picture.

Why does my furnace light, then shut off after a few seconds?

A furnace that lights and then drops its flame within a few seconds usually has a dirty or failing flame sensor, a poor ground, or a weak flame that the control board cannot prove.

The flame sensor is a thin metal rod that sits in the burner flame. The control board sends a small voltage through it and reads a tiny current that only flows when flame is present. A light coating of oxidation on the rod is enough to weaken that signal, and the board shuts the gas valve as a precaution. It will typically retry a few times and then lock out.

Cleaning the sensor often restores operation, but we also measure the microamp signal and check the burner ground and flame pattern. If the sensor keeps fouling, the root cause may be dirty combustion air, a cracked porcelain insulator or burners that need cleaning and adjustment.

Why is my furnace running but blowing cold air?

Cold air from a running furnace usually means the blower is moving air but the burners are not lit, because of a failed ignitor, a gas supply problem, an open limit switch, or a condensate or pressure switch fault.

Start with the simple things: confirm the thermostat is set to heat rather than fan-on, and check that the gas supply valve to the furnace is open. Beyond that, the likely culprits are mechanical.

• Hot surface ignitor: these ceramic parts crack with age and stop glowing hot enough to light the gas
• High-limit switch: a clogged filter or closed registers can overheat the heat exchanger and trip the limit, which shuts the burners while the blower keeps running to cool things down
• Gas valve: a valve that receives its signal but does not open needs testing before replacement
• Rollout switch: a tripped rollout switch means flame escaped the burner area, which calls for a heat exchanger and venting inspection, not just a reset

What do the blinking lights on my furnace mean?

The blinking LED on a furnace control board is a diagnostic code; counting the flashes and matching them to the chart on the furnace door points to the failing circuit, such as the pressure switch, limit, ignition or flame sensing.

Write down the flash pattern before you cut power, because many boards clear the code once power is cycled. Common codes point to a pressure switch that will not close, which often traces back to a weak inducer motor, a cracked pressure hose, or water sitting in the inducer housing of a condensing furnace. Other codes indicate repeated ignition failure or a limit circuit that keeps opening.

On condensing furnaces, pressure switch codes are especially common because the switch also senses when condensate is not draining. A cracked trap, a kinked drain hose or a clogged condensate line can all produce the same code as a failing inducer motor, so we check the drain path before condemning more expensive parts.

A code tells us where to look. It does not tell us why the part failed, which is why we test the circuit rather than simply swapping the part the code names.

Is a cracked heat exchanger dangerous?

Yes, a cracked or rusted-through heat exchanger can let combustion gases, including carbon monoxide, mix with the air circulated through your home, so a furnace with a confirmed crack should not be operated.

Heat exchanger damage is often caused by years of overheating from restricted airflow, or by corrosion in units installed where moist or chemical-laden air reaches the burners. Signs include flame that wavers or changes when the blower starts, soot around the burner area, rollout switch trips and CO alarms. We inspect the heat exchanger visually and with combustion testing. If we find a crack, we will show you the evidence and explain the repair or replacement options.

Why does my furnace bang or boom when it lights?

A loud bang at startup usually means delayed ignition: gas builds up in the burner area before it lights, often because of dirty or misaligned burners, a weak ignitor, or low gas pressure.

When the ignitor is slow or the flame has trouble carrying across the burners, gas collects for a moment and then lights all at once. Each of those small explosions stresses the heat exchanger and can eventually crack it. It can also trip the rollout switch. The repair usually involves cleaning and aligning the burners and the crossover channel, testing the ignitor, and checking manifold gas pressure against the furnace's specifications. Because delayed ignition gets worse over time and damages the heat exchanger, it is a problem worth calling about promptly rather than living with through the winter.

What causes an electric furnace to stop heating?

Electric furnaces can stop heating because of a failed heating element, a stuck sequencer or relay, a tripped breaker, or burned wiring at the high-current connections.

Electric furnaces bring elements on in stages to avoid a huge inrush of current. When one sequencer fails, part of the heat disappears and the furnace runs constantly without keeping up. We test element resistance, sequencer operation and amperage on each circuit, and we look closely at wire terminals for heat damage. Because these circuits carry heavy loads, a loose connection is a fire concern as well as a comfort problem.

What to expect during your repair visit

A furnace repair visit follows the heating sequence from thermostat to supply registers, identifies the failed component, and ends with an explanation and a clear repair option before any parts are replaced.

• We ask what the furnace has been doing and when the problem started
• We read control board codes and inspect the filter, blower and burner compartment
• We run a heating cycle and test each stage: inducer, pressure switch, ignition, flame proving and blower
• We measure temperature rise and, on gas furnaces, check gas pressure and venting
• We explain the cause, present the repair, and complete it once you approve
• We retest a full heating cycle after the repair

We explain what failed and why before any part is replaced. Every repair is backed by our 1-Year Workmanship Warranty.

Repair now or start planning a replacement?

A single failed ignitor or flame sensor on a sound furnace is a repair. A cracked heat exchanger, or a third major part failing in two seasons, is the moment to compare repair against furnace replacement. We will give you the facts for both and let you decide.

Related services

If your home uses a heat pump with a furnace backup, our heat pump repair team can check both halves of the system. Gas supply problems that starve the furnace are handled through our gas line repair service. For local information, see furnace repair in Silver Spring, Bethesda and Washington, DC.

Get your furnace running again

No heat is not something to wait out. Call (301) 509-2475 for 24/7 emergency service or book a furnace repair online. If the repair turns into a replacement, financing is available, and you can ask about our VIP Comfort Club maintenance plan.