Diagnostic work found an older mercury style thermostat not functioning properly and causing intermittent heating issues. Installed a truck stock HW Focus Pro thermostat. Advised homeowner that existing 20x25x1 filter was blocked. Installed a temporary 20x20x1 and homeowner said he will buy one from his local home supply store. Left an optional quote to replace an existing bypass humidifier that is not working. Homeowner advised that he will get back to us if he wants it replaced. Heater is currently operating properly.