High limit and safety limit function - GRAA or Rooftop Sterling Heaters – Trane Commercial HVAC Help Center

On each furnace there is a High Limit and Safety Limit.
The High Limit is a 3 wire limit on the entering air side of the heat exchanger positioned on the header plate, accessible via small panel behind the gas train.
The Safety Limit is also a 3 wire limit on the leaving air side of the heat exchanger, located on the heat exchanger jacket. It is visible to you after you take off the furnace jacket panels.
Both the High Limit and Safety Limit on either of the furnaces would have to open to stop pwr from getting to the R terminal. Subsequently W1 will also lose pwr when both limits open.
Additionally: Both limits are 190° and there are instances when the heated air from the first furnace will trip the High Limit on the second furnace, however the second furnace will still be able to run since the there is the Safety Limit in the circuit. This is normal and nothing to fear as it is safe per ETL testing.

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