Troubleshooting guide

Why Does My Air Compressor Run Continuously or Build Pressure Slowly?

Demand, leaks, inlet restrictions, controls and wear keep a compressor running. Includes a supply-versus-demand balance and a leak test.

Mechanical, HVAC & Fluids · Last reviewed October 17, 2026

A compressor has to supply the air the plant uses plus the air that leaks away. If demand and leaks together approach or exceed its output, it never unloads, the pressure sags and the machine runs all the time. The fix depends on which side of the balance is out.

Compressed air stores a lot of energy, and compressors have hot surfaces, moving belts and high voltage. Depressurize and lock out before opening any part of the system. Work on pressure vessels and safety valves belongs with qualified personnel.

Symptom	Likely cause	What to check
Runs loaded all the time, pressure falls at peak use	Demand above supply, a new load or a leak	Flow demand against the compressor rating. Leak test at no production
Runs often, short loaded periods with nothing in use	Leakage in the piping, fittings, hoses or tools	Load/unload times with no demand (see below). Listen and use leak detection
Slow to reach pressure, even with no use	Inlet filter clogged or inlet restricted, worn valves or rings, or slipping drive belt	Inlet filter, belt tension, and compressor output test by a technician
Pressure does not reach the cut-out setting	Capacity below demand, or a stuck inlet or unloader valve	Control components and the settings of the pressure switch or controller
Runs hot and loaded longer on hot days	High ambient temperature, dirty cooler or poor ventilation	Cooler cleanliness, room ventilation and the hot-air discharge
Water in the lines, pressure drops at tools	Pressure loss in filters, dryers and long lines, not a compressor problem	Pressure drop across filters and dryer. Pipe size and length

A supply-versus-demand balance

A compressor delivers 25 cfm of free air at its working pressure. During a no-production test the compressor is loaded for 20 s and unloaded for 60 s in each cycle, with nothing using air. The compressor is only making up leakage:

Leakage = 25 cfm × 20 ÷ (20 + 60) = 6.25 cfm, which is 25% of capacity

With 15 cfm of production demand, the compressor must supply 15 + 6.25 = 21.25 cfm, or 85% of its capacity. It runs loaded 85% of the time. If the inlet filter clogs and capacity falls by 15% to 21.25 cfm, supply just equals demand: the compressor runs continuously, and any extra demand makes the pressure fall.

Item	cfm	Share of 25 cfm capacity
Production demand	15.0	60%
Leakage (from the test)	6.25	25%
Total demand	21.25	85%
Capacity with a clogged inlet (−15%)	21.25	85%, no margin left

Fixing the leaks is often the cheapest way to add capacity. A leak test costs little and shows whether the compressor is undersized or the system is simply wasteful.

Write a useful problem statement

Compressor type, rated flow and pressure, and control type (load/unload, modulating, variable speed).
Load and unload times with no production, and the loaded fraction during production.
Pressure at the compressor and at the point of use, and any recent changes in demand.
Inlet filter and belt condition, and the last service.

Related tools and labs

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Questions

How big are typical leaks?

Surveys of industrial systems often find that a significant share of the output, commonly cited as 20 to 30%, is lost to leaks. Your own test is the real answer.

Can I turn the pressure up to cover low demand?

Raising the setpoint increases the leak rate and the energy use. It treats the symptom, not the cause.

What does a compressor loaded fraction of 100% mean?

It means the compressor never unloads. It has no spare capacity, so any extra demand lowers the pressure.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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