Reznor heater’s two not working turned on from thermostat unit not responding. tested high-voltage power found high-voltage power at unit. Performed further diagnostic found system tripped on high limit. could be tripped due to multiple reasons system getting too hot due to too much gas, fan shutting off prematurely, or crack in heat exchange chamber. performed a visual inspection found cracks in heat exchanger chambers the part that is cracked separates the combustion gases (carbon monoxide) separate from airflow in the building. due to the cracks recommend having them fixed and or replaced. recommend not using heaters at this time due to the possible carbon monoxide leakage.