It Gets Worse As It Runs: A Decision Tree

Why this matters

"It works fine for a while, then quits" or "it gets weaker the longer it runs" is the opposite of the morning fault: instead of needing to warm up, the system is degrading as it heats up or as something accumulates. This pattern is one of the most reliable diagnostic gifts you get, because the fault grows on a timer you can watch. The cause is almost always heat building somewhere it should not, or a flow path slowly choking. Reason about what accumulates with run-time and you will find it.

Start here: what builds up while it runs

When performance falls off with run-time, exactly one of two things is usually growing:

• Heat is accumulating in a component that cannot shed it: a motor winding, an electronic device, a bearing, a contact, a transformer. As it gets hotter, resistance rises, lubrication thins, semiconductors derate, and eventually a thermal protector trips.
• A flow path is choking as it runs: a filter loading up, ice forming on a coil, sediment stirring up, air working into a liquid line, a strainer collecting debris.

Ask the customer how long until it degrades and whether it recovers after it sits. Quick recovery after a cool-down points hard at heat. Recovery only after a cleaning or a part swap points at flow.

If it trips off after running a set time

This is a thermal-protection story. Something overheats and a safety opens. If a motor runs then stops and restarts after it cools, the motor's overload is tripping; suspect overload (too much mechanical load), undersized cooling, a failing capacitor making the motor work too hard, or a winding starting to short.

If an electronic control or power device shuts down after warming, suspect inadequate heat-sinking, a failed cooling fan on the electronics, or a device operating beyond its rating. Touch-test (carefully) or use an infrared thermometer to find the hot part: the thing that trips is usually the thing that is hottest.

If a flame-based or high-temperature appliance trips a high-limit after running, suspect restricted airflow or coolant flow letting heat stack up past the safety threshold.

If it gets weaker but does not fully quit

Now suspect a slowly choking flow path. If airflow or water flow falls off over minutes, suspect a loading filter, a clogging strainer, or ice building on a coil. Pull and inspect the obvious restriction first; a dirty filter is the single most common cause of "it fades."

If a heating or cooling system loses capacity the longer it runs, suspect a coil icing over (which gets worse the longer it runs cold), a low charge that frosts up, or a heat exchanger losing transfer as it scales or sludges.

If a pump or compressor loses output with run-time, suspect cavitation from a starving inlet, air being drawn in, or a suction strainer collecting debris and starving the inlet further as it runs.

If electrical performance sags with heat

Heat and electricity interact in a way worth knowing: resistance rises with temperature, so a marginal connection gets worse as it warms. If a connection, terminal, or contact runs hot, it adds resistance, which makes it run hotter, which adds more resistance. That runaway is why a loose lug or a corroded contact fails progressively over a long run, not instantly.

Find it with an infrared scan or by feel after a run: the bad connection is a hot spot the rest of the circuit does not have. Tighten, clean, or replace it. Never just retighten a connection that already shows heat damage; the metal is annealed and will loosen again.

Confirm by watching the clock

The whole advantage of this fault is that it is on a schedule:

1. Start cold and time it. Note exactly when degradation begins. A consistent time-to-fail is a strong fingerprint.
2. Find the hot spot. An infrared thermometer or imager turns "something overheats" into "this part is at fault."
3. Watch the restriction. Measure flow or pressure drop across filters, strainers, and coils as it runs; a rising pressure drop is the choke happening in real time.
4. Prove the recovery. If cooling it down or clearing the restriction restores it, you have confirmed the mechanism.

Document the time-to-degrade and the hot spot or restriction. That is the difference between a confident repair and a guess that comes back next week.

References

• Trade-standard thermal-fault and flow-restriction diagnostic practice
• OSHA general-industry guidance on overheating equipment hazards
• Manufacturer service documentation for the specific equipment class
• See related: Why Things Overheat: The Basics; The Noise That Changed: A Decision Tree