You turn the thermostat up, wait for that first wave of warm air — and it never comes. Either the blower runs but the air stays cool, or the furnace sits there silent and does nothing at all. A no-heat furnace is the classic call of the first cold snap across the Peninsula and older East Bay neighborhoods, and it's usually a single failed part or a simple airflow problem, not a dead system.
This page covers what keeps a furnace from heating, what you can safely check yourself, and when it's time for a licensed tech.
How it usually shows up:
- The thermostat calls for heat and the blower runs, but the air is cool or only lukewarm.
- The furnace lights for a moment, then shuts down — sometimes over and over.
- Nothing happens at all: no blower, no click, no glow behind the panel.
- It heated fine yesterday and quit overnight, often after running hard on a cold night.
Why your furnace isn't heating
When a furnace won't make heat, the cause is almost always one of these — roughly in the order we work through them:
- A dirty air filter. The cheapest and most common culprit — a clogged filter chokes airflow, the furnace overheats, and a safety switch shuts the burners down before the house warms up. Always rule it out first.
- A failed hot-surface igniter. This brittle element glows red-hot to light the gas; when it cracks or burns out, there's no ignition and no heat. One of the most common no-heat failures.
- A dirty or misaligned flame sensor. Very common. This thin rod confirms the burner lit; coated in residue, it makes the furnace shut the gas off within seconds because it "can't see" the flame. Cleaning or realigning it clears many no-heat calls.
- A thermostat problem. Wrong mode, a low setpoint, dead batteries, or loose wiring can stop the call for heat from reaching the furnace.
- Gas supply off or a failed gas valve. A closed shutoff, an empty tank, or a valve that won't open means the igniter glows but no gas arrives.
- A tripped high-limit switch. When the furnace overheats — usually from that dirty filter or blocked vents — the limit opens to protect the heat exchanger and cuts the burners.
- An ignition lockout. After a few failed light attempts, the control board locks out on purpose and won't retry until it's reset.
- A blower motor or capacitor problem. If the blower can't move air, the furnace overheats and shuts down, or never delivers the warm air it's making.
The usual fix
Our tech follows the furnace's sequence of operations to find exactly where it breaks: the thermostat calls for heat, the inducer starts, the igniter glows, the gas valve opens, the flame sensor proves the flame, and the blower delivers it. Whichever step fails points to the part. Most no-heat calls end with a cleaned or replaced flame sensor, a new hot-surface igniter, or restored airflow; gas-valve and control-board work come up less often. You get a firm, flat-rate quote before any part goes in.
DIY vs. calling a pro — the honest version
Safe to check yourself first (free):
- Check the thermostat. Set it to HEAT, push the setpoint several degrees above room temperature, and replace the batteries if it takes them.
- Change the air filter if it's dirty. This alone clears a large share of no-heat calls, and it costs nothing.
- Confirm the gas is on. Make sure the gas shutoff is open and that other gas appliances — your stove or water heater — are working, which tells you gas is reaching the house.
- Reset the breaker once and make sure the furnace's power switch (it looks like a light switch on or near the unit) is on.
Call a pro when:
- The furnace won't ignite, or lights and immediately quits.
- It's short-cycling — starting and stopping every few minutes.
- You smell gas or a burning odor, or the furnace keeps locking out after repeated tries.
If you smell gas that's strong or won't go away, don't hunt for it — leave the house and call your gas utility or 911 first. Everything past the basics involves the gas train and line voltage, where a licensed tech keeps you and your warranty safe.
What it costs in the Bay Area
Furnace repairs vary by the part. In our pricing, a hot-surface igniter runs about $120–$350 and a flame sensor $90–$250 — the common, affordable no-heat fixes — while a control board is roughly $200–$600 if the board itself has failed. Cleaning a flame sensor or clearing an airflow restriction sits at the low end. We charge a diagnostic fee that's credited toward the repair when you approve the work, always with a firm, upfront price first.
Get your heat back on
Triton repairs gas and electric furnaces across the Peninsula and East Bay as part of our full HVAC repair service — San Mateo, Burlingame, Redwood City, Alameda, San Leandro, and Hayward — where coastal damp and older homes are hard on igniters and flame sensors that only run a few months a year. Same-week and often same-day appointments, upfront pricing, licensed techs.
Book online or call (888) 993-5699. We'll find why the heat quit and get it running again.
Related pages
- HVAC Repair in the SF Bay Area — our full heating & cooling hub
- Furnace Won't Turn On / No Power — total no-start diagnosis
- Furnace Short Cycling (On and Off) — lights then quits, over and over
- Carrier Furnace Code 33 (Limit Circuit Fault) — the overheating/airflow code
- Furnace Blowing Cold Air — blower runs but the air isn't warm
- HVAC & Furnace Repair in San Mateo — Peninsula service area
Frequently asked questions
Why is my furnace running but blowing cold air? The blower has power and runs, but the burners aren't staying lit — usually a dirty flame sensor that shuts the gas off within seconds of ignition, a failed hot-surface igniter, or a tripped high-limit from a clogged filter. Start with a fresh filter and confirm the thermostat is on HEAT; if the air is still cool, it needs a tech to check the igniter and flame sensor.
Why won't my furnace turn on at all? No blower, glow, or click usually points to power or the thermostat: a tripped breaker, the furnace's power switch left off, dead thermostat batteries, or a blown low-voltage fuse on the control board. Check those first. If everything has power and the furnace still won't start, the control board or ignition control may have failed. Our no-power page walks through it.
What's the most common reason a furnace stops heating? A dirty air filter and a dirty flame sensor are the two you'll hit most. The filter chokes airflow and trips a safety; the flame sensor gets coated and makes the furnace shut the gas off seconds after lighting. Both are inexpensive fixes, which is why changing the filter is always the first free thing to try.
Is it safe to keep resetting my furnace? Resetting once after a power blip is fine. But if the furnace keeps locking out and you keep resetting it, you're forcing repeated ignition attempts on a system that's telling you something is wrong — a failing igniter, no gas, or a flame it can't sense. After one or two resets with no lasting heat, stop and call.
I smell gas or a burning smell from my furnace — what should I do? A strong or persistent gas smell is an emergency: leave the house and call your gas utility or 911 before anything else. A faint, brief "burning dust" smell on the first run of the season is normal. A hot, electrical, or plastic burning odor means shut the furnace off and have it checked before running it again.
How much does furnace repair cost in the Bay Area? It depends on the part: a hot-surface igniter is about $120–$350 and a flame sensor $90–$250, while a control board runs roughly $200–$600. Cleaning a flame sensor or clearing an airflow restriction sits at the low end. We credit the diagnostic fee toward the repair and give you a firm price before any work begins.