The short answer
Check the thermostat, power switch, filter and furnace door before calling, since any of them can stop heat. If those are fine, the cause is usually an ignition, venting or blower fault, and repairing a working furnace is still allowed under Bay Area Air District rules.
A Fremont furnace usually gets its first real workout of the season on a damp, gray morning when the house feels colder than the thermometer suggests. If it has been sitting since spring, that first call for heat is when a cracked igniter, a dusty flame sensor or a sticking pressure switch finally shows itself. ReliableAir HVAC Services repairs gas furnaces throughout Fremont in Alameda County, roughly six miles north of our San Jose base, including homes in 94536, 94538, 94539 and 94555. This page gives you a short pre-call checklist, explains the failures we most often find, and clears up a common worry about whether new Air District rules mean an older furnace should be replaced instead of repaired.
A pre-call checklist
Several no-heat problems have simple causes. Working through this list first can save a visit, or at least make the visit faster.
- Thermostat set to Heat, setpoint above room temperature, and fresh batteries if it uses them.
- The furnace service switch, which often looks like a light switch on or beside the unit, is on.
- The breaker for the furnace is not tripped.
- The blower compartment door is fully closed; most furnaces will not run with it open.
- The air filter is not clogged.
- On high-efficiency furnaces, the white plastic vent pipes outside are not blocked by leaves or debris.
Failures we find most often
Ignition parts top the list. Hot surface igniters are fragile and crack with age. Flame sensors build up a thin coating that keeps them from proving the flame, so the burners light and then shut off. Pressure switches and their small rubber hoses can fail or clog, especially on high-efficiency furnaces where condensate drains through the same area.
Blower problems come next. A failing capacitor or motor can leave the burners firing with no air moving, which trips the high-limit switch. Control boards fail less often, but power surges and heat inside a tight closet can take them out. Finally, airflow issues such as blocked returns or a filter that is never changed cause many overheating shutdowns.
Read the status light before you call
Most furnaces built in the last few decades have a small light on the control board visible through a window in the blower door. It flashes in a pattern that matches a fault code printed on a label inside the panel. Counting the flashes and telling us the pattern gives the technician a head start, though we still verify the fault with testing before replacing anything.
Signs that mean shut it off
If you smell gas, leave the home and call your gas utility from outside. If a carbon monoxide alarm sounds, get everyone out and call for emergency help. Turn the furnace off and leave it off if you see a yellow, wavering flame, scorch marks near the burner area, soot around the vent, or water pooling under a high-efficiency furnace. Booming at ignition is another reason to stop using it until it is inspected.
Repair or replace under the new rules
Fremont is within the Bay Area Air District, and its appliance rules get a lot of attention. Here is what they actually say. Under Rule 9-4, natural gas furnaces manufactured from 2029 must meet a zero-NOx emission standard, which in practice points homeowners toward heat pumps when a furnace is replaced after that date. The rules do not require replacing working equipment. They apply only when an existing furnace or water heater fails or is replaced.
So repairing your furnace is a normal choice. When the failed part is a major one, such as a cracked heat exchanger, or when the furnace is old and needing frequent repairs, we will tell you directly and explain what a heat pump would involve. Fremont homeowners can check BayREN and their local clean energy provider for current electrification incentives; amounts change by funding cycle, so we confirm the current offer before quoting it.
What we check before we leave
After the repair, we run the furnace through several full cycles. We confirm smooth ignition and a stable flame, measure temperature rise, check that the limit and pressure switches respond correctly, and inspect the venting. You will get a clear summary of the repair and anything else we noticed.
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