Customer called for service for water leak at boiler. Master service tech found boiler and piping dry upon arrival. There are traces of previous leakage leading to the auto vent and some water at the vent screw. Shut vent screw on auto vent and customer to watch for leak. Ran boiler and found system pressure went from 12 to 20 psi. Checked expansion tanks and the lower one is bad and should be replaced. Pressure may be swinging enough to make auto vent leak, but auto vent should be replaced as well. Customer complained of low flow of hot water. Tankless water heater coil in boiler appears to be clogged. Recommend replacing the coil. Parts will be ordered and replaced. Customer very satisfied with service and will highly recommend Advanced Heating & Cooling.