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Furnace Pressure Switch in Las Vegas: Why the Draft Fan Runs but Heat Never Starts
Short answer: If your furnace's draft motor hums but you never see a glow, hear a click, or get flame, the pressure switch is often not proving that the venting is drawing. Then the control will not let the furnace light. The usual cause is a blocked vent or intake, a cracked or water-filled tube, a clogged condensate drain on high-efficiency units, or a weak inducer motor, not just a "bad switch." Do the safe checks below, try one restart, and if it still will not light, leave it off and book a weekday visit, Monday–Friday 8am–5pm. If you smell gas, leave the house and call 911 or Southwest Gas from outside.
A pressure switch and a limit switch are different safeties. The pressure switch proves the inducer is pulling combustion gases out through the vent before the furnace is allowed to light. The limit switch shuts the burners off when the furnace overheats, usually from poor airflow; see furnace limit switch. Colton Air Conditioning schedules these calls Monday–Friday, 8am–5pm.
Where it fits in the start-up sequence
Thermostat calls for heat → inducer (draft) motor starts → pressure switch closes to prove draft → ignitor glows or sparks → gas valve opens → flame sensor confirms flame → blower starts.
If the switch never closes, the sequence stops right there. That is why the classic symptom is "fan runs, nothing else happens."
Symptoms at a glance
| Symptom | Likely cause | Check yourself or call |
|---|---|---|
| Draft motor runs, no glow or click, no flame | Pressure switch not closing: vent blockage, tube, drain, or inducer | Check filter and outdoor vent; one restart, then call |
| Draft motor loud, rattling, or slow | Inducer motor wearing out | Call |
| Lights, then shuts down and retries several times | Switch opening during the run, or another fault; see furnace short cycling | Call |
| Glow or click but burners never light | More likely ignitor or gas; see furnace pilot light | Check gas valve handle, one restart, then call |
| Blinking light, no heat | Lockout; many boards have a pressure-switch code | Write down the blink pattern and call |
| Gurgling, or water near a high-efficiency furnace | Condensate drain backing up | Call |
| Smell of gas or CO alarm | Possible gas leak or carbon monoxide | Leave the house; call 911 or Southwest Gas from outside |
Why the pressure switch won't close
| Cause | Homeowner clue |
|---|---|
| Blocked vent or intake pipe outside | Debris, a nest, or something stacked against the pipes |
| Cracked, loose, or water-filled sensing tube | A small rubber tube that is off or kinked (look, don't pull) |
| Clogged condensate drain or trap (high-efficiency units) | Gurgling, or water around the furnace |
| Weak or failing inducer motor | Motor sounds strained, slow, or noisy |
| Failed switch (less common first guess) | Faults continue after venting and inducer test good |
Swapping the switch without finding why it would not close usually means the same no-heat call again.
What not to do
- Do not jumper, tape, or bypass the pressure switch. It is there to keep combustion gases from backing into the house.
- Do not keep restarting a furnace that will not prove draft.
- Do not run unvented heaters indoors as a workaround.
- Do not ignore a CO alarm. Leave, call 911 from outside, and stay out until it is cleared.
Safe home checks (only with no gas smell)
- Thermostat on Heat, a few degrees above the room.
- Furnace switch on, and the blower door seated (many furnaces will not run with it off).
- Filter not packed. Replace it if it is gray or collapsed.
- Look at the vent and intake pipes outside. Clear anything blocking them only if you can reach them safely from the ground.
- Power the furnace off and on once. Listen: draft motor, then glow or click, then flame.
If the draft motor runs and nothing follows, leave it off and call.
A burning dust or oil smell when the furnace first runs is normal and can linger for a couple of days. If you smell gas, leave the house and call 911 or Southwest Gas from outside. If a CO alarm sounds, get everyone outside and call 911.
What a proper call usually includes
Checking the vent and intake for blockage, the sensing tube and port, the condensate drain and trap on high-efficiency units, the inducer motor's operation, and whether the switch closes when draft is actually present. The switch gets replaced last, after the reason it would not close is found.
Related: furnace repair Las Vegas, heating repair, furnace flame sensor, HVAC second opinion.
When to call
Call when the draft motor runs but the furnace never lights after the safe checks, the inducer sounds strained, water is backing up at a high-efficiency unit, or the board flashes a lockout code. Have ready if you can: the blink pattern, roughly how old the furnace is, and whether it is a high-efficiency unit with white plastic vent pipes. Hours are Monday–Friday, 8am–5pm.
FAQ
Is a pressure switch the same as a limit switch?
No. The pressure switch proves the venting is drawing before the furnace lights. The limit switch shuts the burners off if the furnace overheats. See furnace limit switch.
Can I bypass the pressure switch to get heat back?
No. Bypassing it can let combustion gases into the home. Leave it off and call when we are open.
Does a pressure switch fault always mean a new switch?
No. Most often something is stopping it from closing: a blockage, a tube, a drain, or the inducer.
What hours can I reach Colton?
Monday through Friday, 8am to 5pm.
Draft fan runs but the furnace never lights? Call Colton Air Conditioning at (702) 738-6976 Monday–Friday, 8am–5pm. Tell us what you heard and any blink code. We will tell you if we can see it.
Call (702) 738-6976 or Book Online. License 0092120. Hours Monday through Friday, 8am to 5pm. Saturday and Sunday are closed.
Services
Furnace Repair Las Vegas
Furnace repair for Las Vegas homes when the desert night finally drops and the heat will not stay on.
Heating Repair
Gas furnace repair for Las Vegas homes, plus heat pumps, with the safety checks a gas system needs.