Performed preventative maintenance for a Lennox furnace model #SL280UH090V48B-03, serial #5914H19058. I powered the system down and removed the access panels. I removed and cleaned carbon off of the flame sensor and cleaned out the burners and burner box. The homeowner recently replaced the air filter size 16x25x1. Checked the pressure switch for clogs and tested the drain line and condensate pump with several bottles of water to ensure that there are no leaks or clogs. Checked all electrical components needed for heating to ensure that they are secure and in good condition. I powered the system back up and called for heat. I took a combustion analysis test and and received the following readings, o2 is at 13.1% and the co levels are at 25ppm. The supply temp is at 126°F. The furnace is operating properly at this time. I inspected the Aprilaire 600 series. Upon inspection I found that the solenoid valve will not allow the water to pass. Based on the age of the system my recommendat