Furnace Not Heating or Blowing Cold Air? Causes and Fixes

What is your furnace doing?

Pick the option that matches. You'll see what's safe to check on your own, what it usually turns out to be, and when to stop and call.

Why is my furnace not heating? 7 checks before you call

A good share of no-heat calls end with one of these. They take about five minutes and none of them involve opening the burner area.

1. 1

   Thermostat mode and fan

   Set it to Heat, a few degrees above the room temperature. Set the fan to Auto. On On, the fan keeps blowing between heating cycles and the air feels cold even when nothing is broken.

2. 2

   Thermostat power

   If the screen is blank or dim, replace the batteries. Some thermostats run on furnace power: a blank screen there points back to the furnace.

3. 3

   Breaker and furnace switch

   Check the breaker labeled furnace or FAU. Then find the switch on the wall or the furnace itself. It looks like a light switch and is easy to flip by accident.

4. 4

   Front panel

   The door over the blower has a safety switch behind it. If the panel isn't pushed all the way in, the furnace won't run at all.

5. 5

   Filter

   Pull it out and hold it up to a light. If you can't see light through it, replace it. A choked filter is the most common reason a furnace overheats and shuts itself off.

6. 6

   Gas shutoff

   On the gas pipe next to the furnace there's a small valve, often with a yellow handle (number 7 on the photo above). Handle parallel to the pipe means open. Crossways means closed. Don't force it.

7. 7

   Vents and drain

   Open every supply vent and keep the return grille clear. On high-efficiency furnaces with white plastic pipes, a kinked or clogged condensate drain trips a float switch and stops heating.

Furnace blowing cold air or not starting: the usual causes

A gas furnace starts in a fixed order: the thermostat calls for heat, the inducer motor clears the exhaust path, a pressure switch confirms it, the ignitor heats up, the gas valve opens, the flame sensor confirms the flame, and only then the blower sends warm air to the house. When one step fails, everything after it stops. The photos below are parts our technicians replaced on Bay Area calls.

Worn or cracked hot surface ignitor

The ignitor is a thin ceramic element that glows orange-hot and lights the gas. It heats up and cools down thousands of times a season, and eventually it cracks. A cracked ignitor often fails on the first cold morning after a long mild stretch, when the furnace hasn't run for months.

What you notice: the inducer starts, you hear a few clicks, but no flame. The furnace tries two or three times, then locks out. If the fan comes on anyway, the air is cold.

You can: watch through the window: no orange glow within about a minute points here.

Call us: a replacement ignitor has to match your furnace, and it breaks if touched with bare fingers.

Dirty flame sensor

Right next to the burners sits a small metal rod that proves the flame is there. Over the years it gets a thin coating and stops reading the flame. The furnace then shuts the gas off as a safety measure, a few seconds after lighting.

What you notice: the burners light, run for a few seconds, and go out. This repeats until the furnace locks out.

You can: note how long the flame stays on before it goes out.

Call us: cleaning it means removing it from the burner area, which we handle during a visit or a tune-up.

Inducer motor and pressure switch

The inducer is the small fan near the top of the furnace that pushes exhaust out through the flue before the burners may light. A pressure switch, connected to it with a thin hose, confirms it's working. Worn bearings, a cracked hose or a blocked vent stop the sequence right here. It's one of the parts we replace most often.

What you notice: humming or grinding at startup, then nothing. Often a blinking code that points at the pressure switch. The one in the photo was replaced during a maintenance visit in San Francisco because it was getting loud, before it failed.

You can: listen at startup and check that the outdoor exhaust pipe isn't blocked.

Call us: a failing motor won't recover. Repeated restarts only add wear.

Overheating: clogged filter and the limit switch

The limit switch shuts the burners off when the furnace gets too hot inside. The usual reason is airflow: a filter like the one in the photo, closed vents, or a blocked return. Bay Area smoke seasons fill filters faster than the package says.

What you notice: it heats for a few minutes, stops, the fan keeps running, then it starts again. The house never quite gets warm.

You can: replace the filter and open every vent. Change filters every 1 to 3 months, more often in smoke season.

Call us: if it still short cycles with a clean filter. The limit switch or the blower may be failing.

Control board

The control board runs the whole startup sequence and watches every safety switch. Power surges, age and moisture take them out. A failed board can make a furnace do nothing at all, run the fan non-stop, or show codes that don't match what's happening.

What you notice: no response to the thermostat even with power on, or a code that keeps changing.

You can: read and photograph the blinking code before you call.

Call us: boards are tested before they're replaced. Swapping one on a guess is expensive.

Blower motor and capacitor

The blower moves warm air through the ducts. When the motor or its start capacitor weakens, the furnace heats up but can't push the heat out, so it overheats and shuts down. The motor in the photo came out of a Carrier Infinity in Palo Alto.

What you notice: burners light but little air comes out of the vents, a loud hum from the cabinet, or a burning smell.

You can: check that the filter and vents are open, since airflow problems look similar.

Call us: if airflow is weak with a clean filter, or you hear humming without the fan turning.

Thermostat and wiring

Thin low-voltage wires connect the thermostat to the furnace board. A loose or broken wire means the furnace never hears the call for heat. On one San Jose call the furnace was fine: the wires had simply come off. No parts, just a careful look at the connections.

What you notice: the thermostat says heating, but the furnace stays silent.

You can: new batteries, and check that the thermostat sits firmly on its wall plate.

Call us: if the furnace doesn't respond with power on and fresh batteries.

Gas supply, gas valve and standing pilots

If other gas appliances work and the shutoff by the furnace is open, the gas valve inside the furnace may not be opening. On older furnaces with a standing pilot light, the pilot can go out because of a draft or a worn thermocouple.

What you notice: everything starts, the ignitor glows, but there's no flame. Or on an old furnace, no pilot flame at all.

You can: check the shutoff handle position and relight a pilot exactly as the furnace label says, once.

Call us: for anything inside the gas valve. PG&E can also relight a pilot and check gas appliances.

Furnace blinking light codes

Heat pump blowing cold air?

A heat pump that blows cool air for a few minutes on a cold, foggy morning is usually defrosting. Moist Bay Area air puts frost on the outdoor coil when it's in the 30s and low 40s, and the system reverses for 5 to 15 minutes to melt it. Steam from the outdoor unit during defrost is normal.

Heat pump air also feels cooler than furnace air. It comes out of the vents around 90 degrees, while a gas furnace pushes air at 120 or more. That's why it feels lukewarm on your hand even when it's heating the house fine.

Not normal: cool air for more than 15 minutes, an outdoor unit that doesn't spin, ice that stays on the coil, or a system that only keeps up on Emergency Heat. On heat pump calls we most often find failed start parts or a fan motor in the outdoor unit, or a refrigerant leak at the line connections. Adding refrigerant without fixing the leak only buys a few weeks. More on heat pump repair.

Why Bay Area furnaces fail the way they do

• Long idle summers

  Furnaces here sit for months. Dust settles on the flame sensor and burners, and weak ignitors crack on the first cold night of the season. That's why no-heat calls spike with the first cold snap.

• Attics, crawlspaces and garages

  Many Peninsula and South Bay homes from the 1940s to 70s have the furnace in the attic, a low crawlspace or a garage closet. Dust, rodents and moisture there are hard on wiring and motors.

• Smoke season

  Wildfire smoke loads filters in days, not months. A clogged filter in September is a common reason for an overheating furnace in November.

• Coastal air

  Near the bay and the coast, damp salty air corrodes outdoor heat pump parts and connections faster.

Repair or replace?

What a no-heat visit looks like

1. 1

   You call or book

   Phone +1 (650) 542-8555, 8am - 8pm every day, or book online any time.

2. 2

   We test before we touch

   The technician reads the codes, checks gas pressure, voltages and every safety switch, and finds the actual cause.

3. 3

   You get the price first

   You hear what failed and the price before any work. The $85 diagnostic is credited toward the repair if you go ahead.

Furnace not heating: questions