Performed yearly maintenance and safety checks on 2016 rtu system. Found 422 ppm co in flu. 10.3 %o2. Adjusted gas pressure to find co getting worse. Called for pricing of heat exchanger. Hm22 customer wants quote for new unit. Office will handle quote. Also performed yearly maintenance and safety checks on 01 concord unit. Found 10% o2 4 ppm co in flu 0 ppm co in supply. 111 stack temp. 46 degree delta t. No concerns at this time.