How to Troubleshoot a Furnace That Will Not Start

How to Troubleshoot a Furnace That Will Not Start

Medium 20 minutes

A furnace that refuses to start on a cold New York City winter night is an urgent problem that needs fast attention. Before calling for emergency service, there are several checks you can perform that resolve the issue surprisingly often. Many furnace failures are caused by simple problems like a tripped breaker, a clogged filter, a thermostat issue, or a pilot light that has gone out. Walking through these common causes systematically can save you the cost of an emergency service call and get your heat restored in minutes. This guide covers gas and electric furnaces commonly found in NYC homes.

Safety Warnings

  • If you smell natural gas, do not flip any switches or create sparks. Leave the building immediately and call your gas company from outside.
  • Never bypass or disable safety switches on a furnace. They exist to prevent fires and carbon monoxide exposure.

Tools & Materials Needed

  • Flashlight
  • Replacement air filter
  • Long-reach lighter for pilot light
  • Screwdriver

Step-by-Step Instructions

Step 1: Check the Thermostat

Make sure the thermostat is set to heat mode with the temperature set above the current room temperature. Replace the batteries if it has them. Try raising the temperature 5 degrees above room temperature to trigger the furnace to start.

Step 2: Check the Power Supply

Verify the furnace power switch, usually a standard light switch on or near the unit, is in the on position. Then check the circuit breaker panel for a tripped breaker. Reset it by flipping it fully off and then back on. For gas furnaces, verify the gas valve is open.

Step 3: Inspect the Air Filter

A severely clogged filter can cause the furnace to overheat and trigger the high-limit safety switch. Remove and inspect the filter. Replace it if dirty, then try restarting the furnace by turning it off for 30 seconds and back on.

Step 4: Check the Pilot Light or Ignitor

For older furnaces with a standing pilot light, look through the viewing window to see if it is lit. Follow the manufacturer instructions to relight it. For newer furnaces with electronic ignition, listen for clicking sounds indicating the ignitor is attempting to fire.

Step 5: Reset the Furnace

Most furnaces have a reset button, usually red, on the unit. Press it once and wait for the furnace to attempt a start cycle. If it fails to start after two reset attempts, do not keep pressing it. The issue likely requires professional diagnosis.

When to Call a Professional

If the furnace will not start after checking power, filter, and pilot light, or if you smell gas at any point, call a professional immediately. Repeated lockouts indicate a failing component that requires expert diagnosis.

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