A limit switch is a temperature safety control on a furnace that shuts off the burners when the heat exchanger or cabinet gets too hot. The high-limit switch most often trips because of restricted airflow — a clogged filter, closed vents, or undersized ducts. Rollout switches are related controls that detect flames escaping the burner area.
Why it matters to you
A tripping high-limit switch is a frequent reason a Fraser Valley furnace blows cold air between short bursts of heat. Repeated overheating also stresses the heat exchanger, shortening the furnace's life.
Practical guidance
Start with the free check: replace the filter if you cannot see light through it, and make sure supply and return vents are open. If the furnace keeps cycling off, or a rollout switch has tripped, call a technician — never bypass a safety switch.