DIY Furnace Checkup: 8 Steps to Do Before Winter

A clogged filter is the #1 cause of reduced furnace efficiency and early breakdowns. During peak heating season, North NJ homes with pets or dust accumulation can clog a filter in as little as 4–6 weeks.

Find your filter slot — typically a rectangular slot on the side or bottom of the furnace unit, or inside the return air duct near the air handler. Remove the old filter and check the size printed on the cardboard frame (e.g., 16x25x1).

Insert the new filter with the airflow arrow pointing toward the furnace blower, not away from it. Write the installation date on the frame with a marker so you remember when to change it next.

Before assuming your furnace has a problem, verify the thermostat is working correctly. Switch it to Heat mode, set it 5°F above the current room temperature, and listen for the furnace to engage within 1–2 minutes.

If nothing happens, replace the thermostat batteries — this solves the problem more often than you'd think. Also confirm the fan setting is on Auto (not On), which runs the blower only when the furnace is actively heating.

If your thermostat is more than 10 years old or still uses a dial, upgrading to a programmable or Wi-Fi thermostat (like the Honeywell T10) can reduce your heating bill by up to 10%.

Older furnaces (pre-2000) use a standing pilot light that burns continuously. Open the furnace access panel and look for a small blue flame. A steady blue flame is healthy. If the flame is orange, yellow, or flickering, there may be a dirty burner or a failing thermocouple.

Modern furnaces use intermittent electronic ignition — no standing flame. When the thermostat calls for heat, you'll hear a clicking sound (the igniter) followed by the burner firing. If the furnace clicks repeatedly without igniting, the igniter may be failing.

If the pilot light won't stay lit: turn the gas valve to Off, wait 5 minutes, then try relighting according to the label on your furnace door. If it goes out again within a few minutes, the thermocouple likely needs replacement — a $50–$100 repair a tech can handle in under an hour.

Let your furnace run through a complete heat cycle (until the thermostat is satisfied and the blower shuts off). Different noises mean different problems:

Banging or booming at startup: Often delayed ignition — gas builds up before finally igniting. This can stress the heat exchanger over time.

Squealing or whining: Usually a worn blower motor belt or dry motor bearings. Easily fixed if caught early — ignored, it can lead to a burned-out motor.

Rattling or vibrating: Check that the access panels are fully secured. Loose panels are harmless, but rattling from inside the unit may indicate a loose heat exchanger component — which can cause CO leaks.

Rumbling after shutdown: Likely dirty burners causing combustion to continue briefly after the gas shuts off. Burner cleaning is part of a professional tune-up.

Walk through your home and physically inspect every supply vent (warm air blows out) and return vent (pulls air in). Ensure they're open — it's common for homeowners to close vents in unused rooms thinking it saves energy, but it actually increases static pressure and strains the system.

Look for blocked return vents. Large return registers are often located low on a wall or on the floor. Make sure rugs, furniture, and curtains aren't covering them.

Feel for airflow at each supply vent while the furnace is running. Rooms with very weak airflow may have a partially collapsed or disconnected duct — worth investigating before winter.

If your furnace has an AFUE rating of 90% or higher — usually identifiable by a white PVC flue pipe instead of metal — it produces condensation as a byproduct of combustion and needs a drain line.

Locate the condensate drain: a clear or white 3/4" PVC pipe running from the furnace to a floor drain, utility sink, or condensate pump. Pour a small amount of water into the drain pan and confirm it flows freely.

A clogged condensate drain will trigger the furnace's float switch safety shutoff — a common reason high-efficiency furnaces unexpectedly stop working in winter. You can clear minor clogs with a wet/dry vacuum attached to the drain outlet.

The flue pipe carries combustion exhaust (including carbon monoxide) safely out of your home. For standard-efficiency furnaces, this is a metal B-vent or single-wall pipe running to the chimney. For high-efficiency furnaces, it's white PVC exiting through a sidewall.

Visually trace the entire flue pipe from the furnace to its exit point. Look for: visible rust spots or holes, joints that have separated or are only loosely connected, and any dark discoloration or soot streaks on the outside of the pipe (indicates leakage).

Any damage to the flue pipe is a serious safety hazard. Carbon monoxide from a leaking flue can accumulate in your home silently. If you find damage, do not run the furnace — call an HVAC contractor immediately.

Carbon monoxide is odorless and colorless — CO detectors are your only warning system. Press the test button on every CO and smoke detector in your home. If the alarm sounds weak, replace the batteries. If the unit is more than 5 years old, replace the entire detector.

You should have at least one CO detector on every floor of your home, and one within 10 feet of each gas appliance including your furnace, water heater, and oven.

If your CO detector goes off during normal furnace operation, leave your home immediately, call 911, and then your gas utility. Do not re-enter until emergency services clear the building.