Complete 1 system furnace repair and preventative maintenance check. Replaced filter. Recommend better filtration, the customer states she doesn't go in the attic. Better filtration will get a filter that can last up to 6 months at a time. With this, the customer would not have to worry about changing the filter. Recommend an inline water safety switch. System only has a pan switch. Inline will eliminate possible water overflowing onto the ceiling. Recommend gas valve Calibration. Co levels are higher than expected. The customer agreed to all recommendations.