Performed a three system main checking heat, checked all electrical and mechanical components, safeties, heat exchanger, gas pressures, 1st & 2nd stage heat, flue pipe, cleaned flame sensors, checked filters, all systems are working properly at this time. Homeowner had a concern about the drain line for upstairs and master system since they are tied together, traps were clean and clear, ran a/c and system didn’t show signs of being clogged. Explained to homeowner that we can blow out drain line if he wants us to but it doesn’t show that it is backed up.