Furnace Short-Cycling in Foggy SF Weather: What Is Usually Going On

San Francisco winters are not brutally cold, but the constant damp and fog put real wear on older furnaces, especially in homes where the unit is original to the house. A furnace that clicks on, runs for a couple of minutes, and then shuts off is one of the most common cold-season calls we get in the city. This behavior is called short-cycling, and it is always worth diagnosing before the weather gets worse.

In most cases it traces back to one of three things: a dirty flame sensor, a clogged filter restricting airflow, or a thermostat miscalibrated for the home's insulation level. None of these are safe to keep running through. A furnace that short-cycles repeatedly is working harder than it should and wearing out faster. Learn about how furnaces work from Wikipedia's overview of heating systems.

Older SF housing stock also means older duct or radiator setups that were never designed for today's digital thermostats. These older systems can throw off temperature readings and cause the exact short-cycling pattern homeowners describe to us most often during foggy winter mornings.

A dirty flame sensor is one of the most common and cheapest fixes when caught early. A thermostat calibration issue can often be resolved in a single visit. But a furnace that continues to short-cycle because the underlying cause was ignored will eventually fail at the worst possible time. Schedule a check before that happens. Our San Francisco HVAC team is available 24 hours a day at (628) 900-2928. Also see our full SF HVAC maintenance guide for seasonal care tips that prevent issues like this from developing.