A high-limit switch is a protective part of a furnace designed to keep the mechanism from overheating itself. A furnace is at a risk of overheating any time airflow is limited from passing through the heat exchanger. Because ecobee thermostats require a C wire and run directly from the 24v supplied by the furnace, some users will notice that their thermostat screens are black after several minutes of running the heat. This is because the high limit switch is tripping and cutting all 24v to the system.
When the high limit switch trips and the ecobee powers down, users will likely notice that their fan in the furnace continues to run for a short period without the burners before power is restored to the system. This ensures continuous airflow through the HVAC system and assists with cooling things down.
Users may note that this never happened on their previous thermostats because their old thermostat was battery powered, if the furnace cuts out, there is no visual indicator to know that the high-limit switch has tripped and the furnace simply starts up again when the switch resets.
There are several things that could cause a high limit switch to trip. These include, but are not limited to:
Restriction of airflow
- Too many vents closed off in a customers home
- Damaged or dirty furnace filters
- Dirt in the blower motor housing
- Plugged secondary heat-exchanger (high-efficiency furnaces only)
- Plugged AC Evaporator Coil
Over-Amping of electrical components
- Failing fan motors can overall pull more amperage causing the switch to trip
- Any other part of the system pulling more amperage than normal
- The belt on a belt-driven fan motor may be too loose or too tight
Bad High Limit Switch
- It is entirely possible that the switch itself is bad. The switch is a very simple mechanical part and is prone to wear and tear.