Gas and Oil Home Heating Furnaces - air conditioning

We had our house built 4 years ago,and had one of our air conditioning units was serviced with minor problems,one of which is that the unit fills up with water in the tray and comes out thru the over-flow outside ( which is directed above a walkway ) and every couple of months it will start leaking near the back door.The air conditioning people come out and drain the water but never solves the problem.What can i do to solve this annoying problem!:confused:

I've intermittently had similar problems, algae and other stuff builds up in these lines and they won't drain properly, it's just designed to drain condensation by gravity. I just replaced my entire unit and when we installed the new drain fittings, we put in a pcv ball valve, then a threaded tee right after it, now every 6 months or so I can shut the ball valve so nothing backs toward the pan and use air or water to blow out the line, also suggest you use a dilute bleach mixture in the line after wards to kill whats left in the line.