Came out to inspect 2 furnaces, gas company has them both shut off at valves and has them both red tagged. Both tags say "bypassing gas at burners". Upon looking at downstairs furnace door it says "bad exchanger" written by someone. This prompted me to do a visual inspection on both furnace heat exchangers. Both appear severely rusted at curves (pictures attached). Could not identify a direct hole on heat exchangers, so decided to run them and test for carbon monoxide. Opened gas valves and used gas detector as well as soap bubbles to test for leaks. Did not find any gas leaks on either furnace. Ran heat on both systems. For both furnaces flames blue at first then when blower comes on the flames do begin to roll out a bit. For main floor supply plenum read 7ppm on CO detector after about 8 minutes. For upstairs plenum read 8ppm on CO detector after only about 6 minutes. Concluded bad heat exchangers on both furnaces. Both systems original to home from 2002. Spoke to customer about opti