Limit and Rollout Switches

Temperature safeties that shut down heating equipment, and what each one is actually reporting.

Component

Technician

#electrical#safety#furnace#combustion

Safety

Never bypass a limit or rollout switch. A rollout trip indicates a combustion or venting fault that must be found and corrected before the appliance is returned to service.

What it is

High limit switch

A temperature sensitive switch in the heat exchanger area that opens if supply air temperature climbs too high, usually from inadequate airflow.

Rollout switch

A switch mounted near the burners that opens if flame rolls out of the burner compartment instead of drawing into the heat exchanger. Usually manual reset.

Auxiliary limit

An additional temperature safety, often on the blower compartment or near a specific hot spot.

Why it matters

A tripped rollout switch is a serious finding

Flame rollout indicates a blocked heat exchanger, a blocked vent, a plugged burner or a combustion air problem. It can accompany carbon monoxide production. Find and correct the cause, verify the heat exchanger condition, and never jumper the switch or simply reset it and leave.

A high limit that trips repeatedly is almost always an airflow problem. Each trip cycles the burner while the blower continues, and long term operation this way stresses the heat exchanger.

How it works

Both devices are normally closed and wired in series in the control circuit. Opening either one removes power from the ignition sequence and stops the burner. Most high limits reset automatically once temperature falls. Rollout switches are typically manual reset so the condition cannot repeat unnoticed.

Step 1 of 8

Thermostat Call: The thermostat closes R to W, sending a 24V call for heat.

Gas furnace sequence of operation — each step must prove before the next one is allowed