The first sign is rarely dramatic. A load that used to dry in one cycle now comes out damp at the seams, and the towels feel heavier than they should. You run it again. Then you run it again the next day. Somewhere in the back of your mind you remember that she never had this problem, and you wonder whether the machine is simply getting old.
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Often the machine is fine. The problem is the path the air takes. A clothes dryer pulls air through the drum, pushes it through a duct, and blows it outside through a hood on the wall. Lint collects along that route, and when enough of it builds up, the air slows down. The dryer then runs longer, and the back of the machine can get warm in a way it should not.
What the clog looks like
Lint does not always show up where you expect. The screen inside the door can look clean while the duct behind the dryer is thick with fuzz. Watch for these changes: clothes need an extra cycle, the drum feels warm at the end of a load even when the clothes are still damp, lint collects on the floor behind the machine, or the outside flap barely lifts when the dryer runs. A faint burnt or scorched smell is a reason to stop and look right away.
Notice whether the change came gradually. A slow drift over many months usually means the duct is slowly filling. A sudden change often means something has shifted, a duct has come loose, or the flap is stuck.
What to check tonight
Unplug the dryer before you move it. If it runs on gas, do not pull it away from the wall until someone who knows gas lines has told you how to do it safely. Pull the machine out a few inches, look at the duct connection at the back, and check whether the flexible duct is kinked, crushed, or pulling apart at the clamp.
Then go outside. Plug the dryer back in, run it on the air fluff setting for a few minutes, and walk to the outside hood. Put your hand near the flap. You should feel a steady stream of warm air pushing it open. If the flap barely moves, or if air only leaks at the edges, the vent is restricted somewhere between the machine and the wall.
Wipe the lint screen after every load, and vacuum the lint from around the back of the machine when you are there. Your father may find this a small, reasonable habit to keep. It is easier to ask him to clean one screen than to explain a bigger repair later.
What to ask the person who cleans the vent
Ask whether they clean the whole run, from the back of the dryer to the outside hood, rather than only the part they can reach from the machine. Ask whether they check the outside flap and the wall connection. Ask them to show you the flap opening before and after the work. Ask whether they will give you a short written note of what they found, what they cleaned, and what they recommend next, and keep that note with the household papers.
When you compare two people, ask each the same questions and see which answers are specific. A good answer names the parts they checked. A vague answer is a reason to keep looking.
When the vent is not the problem
If the vent is clear and the flap opens well, the cause may be the heating element, the moisture sensor, or a load that is simply too large for the drum. Those are repair questions, and the person who services the machine is the right one to answer them.