The short answer
We repair standard-efficiency, high-efficiency condensing and older gas furnaces, plus electric furnaces, in Sunnyvale homes. Each type has typical failure points, and we test the whole unit, including its safety controls, before recommending a repair.
Furnace problems in Sunnyvale usually arrive with the first damp, chilly evenings of late fall, when a system that has been silent for months is switched back on. ExpertFlow HVAC Repair Service is about 4 miles southwest of Sunnyvale in Los Altos, and we repair furnaces throughout 94085, 94086, 94087 and 94089. The type of furnace you own says a lot about what is likely to go wrong, so this page is organized around the three kinds we work on most.
Standard-Efficiency Furnaces
These are the furnaces with a metal flue pipe running up through the roof, common in garages and hall closets across Sunnyvale. They are relatively simple and long-lived, which is why many are still in service. The parts that wear out are predictable: hot surface igniters crack, flame sensors build up residue, inducer motors get noisy and fail, and pressure switches stick. Blower motors and their capacitors weaken over time, leaving the burners running while little warm air reaches the rooms.
With these units, we also look closely at the metal flue and its connection to the furnace, because a corroded or separated vent pipe is a safety issue rather than a comfort one.
High-Efficiency Condensing Furnaces
You can recognize these by the white plastic vent pipes, often exiting through a side wall. They extract more heat from the gas by condensing water vapor, and that water has to go somewhere. Most of the trouble we see with them in Sunnyvale traces back to the condensate system. After a dry summer, traps and drain lines can be clogged with sludge, and the furnace shuts down on a pressure switch fault because water cannot drain.
Other common issues include intake or exhaust pipes blocked by debris or nesting insects, and secondary heat exchangers that restrict airflow when dirty. Variable-speed blowers in these units sometimes fail at the motor module, which we can diagnose and replace.
- Water around the furnace base or in the drain pan
- Repeated ignition attempts followed by lockout
- Gurgling sounds from the drain trap
- A flashing code pointing to the pressure switch
Older and Electric Furnaces
Some Sunnyvale homes still run very old furnaces with standing pilot lights or electric furnaces with heating elements in place of burners. Pilot furnaces commonly fail at the thermocouple or gas valve, and parts for the oldest models can be hard to find. Electric furnaces fail at elements, sequencers and limit switches; those repairs are usually straightforward, but high operating cost is worth discussing if the unit runs a lot.
For very old equipment we are candid about whether a repair is a sensible use of money or simply buys a short delay.
Reading the Status Light
Most furnaces built in the last few decades have a small LED on the control board that blinks a code when something goes wrong. You can usually see it through a sight glass in the lower panel. Before you call, count the flashes and check the chart printed inside the furnace door, or simply take a photo. That code often points us to a part before we arrive, which makes a same-visit repair more likely. Please do not remove panels or bypass safety switches to get the furnace running; those switches exist to stop unsafe operation.
A few other details help too. Tell us roughly where the furnace sits, whether it shares a closet or garage wall with a water heater, and whether anyone has worked on it recently. If the problem comes and goes, note when it happens: only on the first call for heat in the morning, after long run times, or at random. Intermittent faults are the hardest to catch on a single visit, and a pattern you have noticed can lead us straight to a sensor or switch that only fails under certain conditions.
Repair, Replace, and the Air District Rules
Most furnace failures are worth repairing. We start talking about replacement when the heat exchanger is cracked, when the furnace is very old and needs a major component, or when repairs are coming one after another. If you do replace, keep in mind that Bay Area Air District Rule 9-4 requires gas furnaces manufactured from 2029 to meet a zero-NOx standard, which in practice points future replacements toward heat pumps. The rule does not require replacing a furnace that still works. In Sunnyvale's mild winters, a heat pump is a practical option, and we can lay out both choices side by side.
More services in Sunnyvale
- AC Repair in Sunnyvale
- Heating Repair in Sunnyvale
- Heat Pump Installation in Sunnyvale
- AC Installation in Sunnyvale
