The short version. Most furnace trouble falls into a handful of patterns. A flame sensor cleaning solves a lot of “no heat, the blower runs” calls and is often a do-it-yourself fix. A filter change solves most short-cycling and weak-airflow issues. Call us if you smell gas, the blower will not stop after the thermostat is set to Auto, or an older furnace has a real component failure.
The most common furnace problems we diagnose
| Problem | Most likely cause | Can you fix it yourself? |
|---|---|---|
| No heat, blower runs | Dirty or failed flame sensor or igniter | Maybe (a flame sensor cleaning) |
| No heat, blower silent | Thermostat, control board, or no power | Maybe (thermostat batteries, a tripped breaker) |
| Short cycling (on and off rapidly) | Dirty filter, oversized furnace, or a safety switch | Try the filter; the rest needs a pro |
| Blower runs constantly | Thermostat set to “on” instead of “auto” | Yes (check the thermostat) |
| Weak airflow | Dirty filter, blocked vents, or duct issues | Filter and vents yes; ducts need a pro |
| Odd burning smell | Dust burning off after summer, or something more | Wait one cycle, then call if it continues |
| Banging or rumbling | Delayed ignition, blocked burner, or ductwork | No |
| High utility bill | Aged filter, an aging system, or duct losses | Try the filter first |
No heat but the blower runs
The blower running without heat means the furnace is “trying” but failing to ignite or sustain the flame.
Most likely causes:
- A dirty flame sensor, the most common cause. The flame sensor is a small metal rod that confirms the burner lit. When it gets coated, the furnace shuts the gas off for safety. Cleaning it often restores normal operation.
- A failed igniter, the part that lights the burner. Igniters wear out and eventually crack.
- A dirty or failed pilot tube on older furnaces.
What you can check yourself:
- With the power off, pull the front panel and look at the burner area. If the flame sensor (the small metal rod in front of the burner) is coated with white or gray residue, gently clean it with fine steel wool or sandpaper, reinstall, and test.
- This is a no-cost fix that resolves a large share of “no heat, the blower runs” calls.
When to call:
- Cleaning the sensor doesn’t fix it
- You smell gas (always call immediately)
- Older furnace with pilot light not staying lit
No heat, blower silent
The blower not running means the furnace isn’t getting the signal to start, or isn’t getting power.
Most likely causes:
- Dead thermostat batteries
- Tripped breaker at the panel
- Failed control board on the furnace
- Failed transformer
- Furnace switch turned off (yes, this happens)
DIY check:
- Verify thermostat displays a temperature and is set to heat mode
- Replace thermostat batteries if applicable
- Check the breaker labeled “furnace” at the panel; reset if tripped
- Check the wall switch near the furnace; sometimes turned off during summer maintenance
When to call:
- All of the above check out and the furnace still won’t start
- Breaker trips again after reset (electrical issue requiring diagnosis)
Short cycling
The furnace turns on, runs for 3 to 8 minutes, shuts off, then starts again 2 to 5 minutes later.
Most likely causes:
- Dirty air filter restricting airflow
- Oversized furnace heating the home too quickly
- Failed limit switch shutting the system down for safety
- Failed flame sensor (intermittent, comes back on after cooling)
DIY check:
- Replace the air filter. If it’s the dark gray “I haven’t changed this in 6 months” filter, that’s likely the cause.
- Wait one heating cycle and observe.
When to call:
- Filter replacement doesn’t resolve
- Furnace is over 12 years old (signals component failure)
- You notice the home heating unevenly (cold rooms while others are warm)
Blower runs constantly
The blower runs nonstop, even after the burner shuts off. The home isn’t getting hotter; the blower just won’t quit.
Most likely cause:
- Thermostat set to “on” instead of “auto.” The blower runs continuously, blowing room-temperature air. This is the most common cause and the easiest fix.
Other causes:
- A failed fan limit switch (less common)
- A wiring short on the blower circuit (less common)
DIY check:
- Open your thermostat and look for “Fan: On / Auto” setting. Switch to “Auto.”
When to call:
- Thermostat is set to Auto and blower still runs continuously
- Blower runs even when system is set to “Off”
Weak airflow from vents
Air comes out of the vents but feels weak or not very warm.
Most likely causes:
- Dirty air filter restricting flow
- Closed or blocked vents (furniture, rugs over floor registers)
- Ductwork leaks (common in Seattle homes with old, brittle duct mastic)
- Ductwork sizing issues (undersized for current furnace)
- Failed blower motor capacitor (motor running at reduced speed)
DIY check:
- Replace filter
- Verify all supply registers are open
- Walk through the home and feel each register; if some are strong and some are weak, you have a duct issue. If all are weak, you have a system-wide issue.
When to call:
- Filter replacement doesn’t fix
- You see disconnected ducts in the basement or attic
- Blower motor sounds different than normal (humming, grinding)
Weird burning smell from vents
Smell coming from the vents when the furnace first runs each fall.
Most likely cause:
- Dust burning off the heat exchanger after a summer of disuse. Completely normal. Open windows for 10 to 15 minutes during the first burn. Smell should be gone after one heating cycle.
More serious causes (if smell persists):
- Electrical burning (wiring insulation failing): turn off the furnace at the switch, call us
- Plastic or chemical burning: turn off at the switch, call us
- Mold smell: usually a duct or condensate issue, call us
Carbon monoxide concern:
- CO has no smell. It’s not a smell issue. If your CO detector goes off, evacuate and call 911.
Banging or rumbling
Loud bang on startup, or continuous rumbling during operation.
Most likely causes:
- Delayed ignition: gas builds up briefly before lighting, then ignites with a bang. Usually a dirty burner or a weak igniter.
- Sediment in the heat exchanger
- Loose ductwork expanding and contracting (common in older homes)
Always call:
- A loud bang on startup is a safety issue. We prioritize it for the next available visit.
High utility bill
Heating costs are noticeably higher than previous winters.
Most likely causes:
- A dirty filter forcing the system to work harder
- An aging furnace that has lost efficiency over the years
- A thermostat that is not reading correctly
- Duct losses, especially in an unconditioned attic or crawl space
- Insulation that has settled or degraded in the attic or walls
DIY check:
- Replace filter
- Verify thermostat is reading correctly (use a separate thermometer in the same room)
- Check attic or crawl space ducts for visible disconnects
When to call:
- DIY checks don’t explain the increase
- System is over 15 years old (consider replacement; see when to replace furnace Seattle)
Schedule a furnace diagnostic
Call (206) 200-9134. Our hours are Monday through Friday, 7 AM to 5 PM. The diagnostic is $129 plus tax, credited toward equipment replacement.
