What it is

These are the two temperature-actuated safety switches that protect a gas furnace from overheating and from fire:

  • High-limit switch — mounted on the heat exchanger or in the supply plenum. It opens the gas-valve circuit if the furnace gets too hot, which happens when airflow is lost (dirty filter, dead blower, closed-up ducts). It protects the heat exchanger from cracking and the cabinet from overheating. Most are auto-reset bimetal discs.
  • Flame-rollout switch — mounted near the burner box. It opens if flame "rolls out" of the burner area — which happens when the heat exchanger is blocked/cracked or the venting fails and combustion products spill forward instead of going up the flue. Rollout is a fire and CO hazard, so these are almost always manual-reset and they latch hard.

This article covers what each protects, reset behavior, setpoints, and how to test them properly — without jumpering a safety.

How it works

Both are simple thermal snap switches. A bimetal disc (two metals bonded together that flex at different rates with temperature) snaps its contacts at a calibrated temperature.

  • The high-limit is normally closed and wired in series with the gas valve / control circuit. If the air around the heat exchanger exceeds its setpoint (because the blower isn't carrying heat away), the disc snaps open, kills the gas valve, and the furnace shuts down on a limit fault. As the furnace cools, an auto-reset limit closes again. The blower usually keeps running to cool things down.
  • The flame-rollout is also normally closed and in the safety string, but it's positioned to feel heat where flame should NEVER be — at the front of the burners. If flame rolls out to that spot, the switch opens and locks out. Because rollout means something is seriously wrong (blocked exchanger, vent failure), it's manual-reset so a tech has to investigate before it can run again.

In the field

Read the setpoint. Limit switches are stamped with an open temperature (e.g., "L160," "L200" — the number is roughly the open temp in °F). Rollouts are similar, stamped with their trip temp. Match the number on replacement.

Test by continuity at temperature. Both are normally closed when cool. Ohm across the switch: it should read closed (continuity) at room temperature. If a limit reads OPEN at room temperature, it's either tripped/latched (rollout, manual reset) or failed open — a failed-open limit will prevent the furnace from firing.

Diagnose a tripping high-limit by chasing AIRFLOW, not the switch. A limit that trips during a heat cycle is almost always reporting an airflow problem: plugged filter, failed/slow blower, blocked return, closed registers, a too-low blower speed tap, or an oversized furnace. Measure temperature rise across the furnace — if the rise is above the data-plate range, the furnace is overheating for real and the limit is correct. Fix the airflow.

Diagnose a tripped rollout with extreme seriousness. A rollout trip means flame went somewhere it shouldn't. Before you EVER reset it, inspect: the heat exchanger for cracks or blockage, the burners for blockage/lint, the venting for obstruction or backdraft, and the inducer/draft. Resetting a rollout without finding the cause can leave a CO-producing, fire-prone furnace running. This is a stop-and-investigate fault.

Confirm the switch itself only after ruling out the cause. If airflow/temp rise is fine and a limit still trips, or venting/exchanger is fine and a rollout still trips, then test the switch against temperature (heat it and watch it open at roughly its stamped setpoint) and replace if it trips early or won't close.

Normal values & targets

  • High-limit open temperature: commonly in the range of about 150–200°F (e.g., L160, L170, L180, L200), stamped on the disc. Auto-reset closes after cooling, often ~25–40°F below the open point.
  • Flame-rollout open temperature: often higher, commonly in the 190–350°F range depending on location and design; manual-reset (latches until the button is pushed).
  • Temperature rise (the real diagnostic for limit trips): the furnace's allowable rise is on the data plate, commonly a 40–70°F band (e.g., "35–65°F rise"). Running above the top of the band overheats and trips the limit.
  • Contact state when healthy and cool: closed (continuity). Open at room temp = tripped or failed.
  • Reset type: high-limit usually auto-reset; flame-rollout almost always manual-reset.

Common faults & what they mean

  • Furnace fires, runs a few minutes, shuts off on limit, blower keeps running, repeats → classic overheat from low airflow: dirty filter, weak blower, blocked return/supply, low blower speed, or oversized furnace. Measure temp rise.
  • Furnace won't fire at all, limit reads open cold → failed-open limit switch (or it's stuck open); replace after confirming it's not just hot.
  • Rollout switch tripped (button popped) → flame rolled out: blocked/cracked heat exchanger, plugged burners, vent obstruction, or backdraft. INVESTIGATE before reset. Do not assume it's a bad switch.
  • Rollout trips again right after reset → the cause is still present; do NOT keep resetting. Find the blockage/crack/vent problem.
  • Limit trips with normal temp rise and good airflow → the limit itself is weak/failed (tripping early); test against temperature and replace.

Tech tips & gotchas

Never jumper a limit or rollout switch to "get the furnace running." These are the switches that prevent a cracked heat exchanger from cooking the house and flame from starting a fire. A jumpered limit can let a furnace overheat to the point of a cracked exchanger and CO. There is no acceptable reason to bypass one in service.

A tripping high-limit is an airflow message 95% of the time. The cheapest, most common cause is a filthy filter. Check the filter, the blower, and the temperature rise before you touch the switch.

A tripped rollout is a red-flag, stop-everything fault. It's the furnace telling you combustion is escaping the burner box. The most serious cause is a blocked or cracked heat exchanger — which is a condemn/replace situation, not a reset situation. Treat every rollout trip as a potential CO event until you prove otherwise.

Match the replacement's open temperature exactly. A limit with a higher setpoint than the original lets the furnace run hotter than designed before tripping — defeating the protection margin even though it "works."

If you reset a manual switch as part of testing, you still owe the customer a root-cause. A switch that latched did so for a reason.

Safety / code notes

The high-limit and rollout switches are required safety controls — bypassing either is never acceptable and creates fire and CO hazards. A flame-rollout trip should trigger a combustion and heat-exchanger inspection; a cracked or blocked heat exchanger is a life-safety condemn, not a repair. Match replacement switches to the equipment's listed setpoints and reset type. After any heat-side service, verify temperature rise is within the data-plate band and check for CO before leaving. Furnace overheat and combustion-safety provisions are tied to the equipment listing and the fuel-gas/mechanical codes — don't alter the safety string.