If you own or rent in a triple-decker, a two-family, or anything where the laundry is not on an outside wall, read this before you buy a part. A dryer that tumbles normally but dries nothing is a heating fault, and in this city the heating fault is usually somewhere else entirely.
Why the vent is the first suspect here and not elsewhere
A dryer is a fan with a heater in front of it. It works by moving a specific volume of air per minute through hot, wet clothes and out of the building. Every foot of duct and every elbow costs some of that airflow, and manufacturers publish the budget: typically 35 equivalent feet of 4-inch rigid duct, with each 90-degree elbow counted as roughly 5 feet and each 45 as 2.5.
Now picture the standard Boston arrangement. Laundry in a second-floor closet or a basement corner. The duct leaves the machine, turns, climbs a chase between floors, turns again into a joist bay, runs the depth of the building, turns a third time, and exits a gable or a foundation wall. Four elbows and thirty feet of straight run is 50 equivalent feet before you have accounted for the flexible transition behind the machine, which people crush flat pushing the dryer back.
That installation was over budget the day it was built. Add ten years of lint and it is a different appliance.
What restricted airflow actually does
This is the part worth understanding, because it explains why replacing parts does not fix it.
Restricted exhaust means hot air lingers in the drum. The high-limit thermostat sees temperature it should not and opens, cutting the element. The dryer keeps tumbling — the motor is on a separate circuit — so the machine looks like it is working and the clothes come out damp. That is the mild version, and it is reversible.
Keep running it and the thermal fuse blows. A thermal fuse is a one-shot device that exists specifically to kill the heat before a lint fire starts. It is doing its job. Replacing it without clearing the duct means it blows again, usually within weeks.
Keep going and you crack the heating element, which is the expensive failure and the one that gets fitted first by whoever came out with twenty minutes to fill.
So the classic sequence is: company one fits an element, company two fits a thermal fuse, company three tells you the machine is old. All three parts were fine. All three visits were paid for. The duct was full.
The four-minute test you can do yourself
Test one — disconnect and run. Pull the machine out, take the flexible transition off the back, and run the dryer on high for three minutes with the exhaust going straight into the room. It will be humid and lint will escape, so open a window and do not do it for long. If it gets properly hot with the duct off, your heating circuit is fine and your duct is the fault. This single test settles most of these calls.
Test two — feel the outside vent. With the dryer running, go outside and put your hand at the exterior hood. You should feel a strong, warm push of air — enough to be obvious, not a gentle waft. If the flap is barely lifting, you have your answer. If you cannot find the exterior vent at all, that is worth knowing too; we have found more than a few dryers in this city exhausting into a wall cavity, an attic, or a disconnected duct in a basement ceiling.
Test three — look at the lint screen housing. Pull the screen and shine a light down the slot. Packed lint below the screen means the whole run behind it is worse.
Test four — time a normal load. A well-vented dryer does a normal load in 40 to 50 minutes. Ninety minutes is a restriction, even if the clothes eventually dry.
Gas versus electric
Electric dryers use two heating elements at 240V. If your dryer tumbles but never heats and the duct is genuinely clear, check that both legs of the 240V supply are live — a half-blown double-pole breaker or one open fuse leaves the motor running on 120V and the heat dead. This is common in older Boston panels, particularly in buildings still running fuse boxes, and it looks exactly like a dead element.
Gas dryers use an igniter, a flame sensor and two gas valve coils. The usual failure is the coils, which weaken with heat cycling and stop holding the valve open. The signature is distinctive: the igniter glows, the burner lights, it runs for a few seconds, and it goes out — then the whole cycle repeats. That is coils, not the igniter, and it is a modest part.
The maintenance interval nobody is given
Clean the full duct run annually. In a triple-decker with a long run, or a household doing five-plus loads a week, twice a year. This is not upselling — it is the difference between a dryer lasting six years and lasting fifteen, and it is the leading appliance-related fire cause in residential buildings.
Two things worth doing while you are back there: replace any flexible foil or plastic transition with rigid or semi-rigid aluminium, and check the exterior hood flap actually opens. Louvred plastic hoods on this housing stock get painted shut, packed with lint, or nested in by birds, and any of those undoes the whole run.
When it really is the dryer
It does happen. Elements crack from age, gas valve coils weaken, thermostats drift, control boards fail. Those are honest repairs and worth doing on a machine under about twelve years old.
We just will not fit one of them until the duct has been ruled out, because you should not pay twice for the same complaint.