Why Is Your Compressor Running When Production Has Stopped?

At 02:00 the production line is silent but the compressor room is not. An air compressor running when not in use is usually responding to something: real demand, air escaping from the system or a control setting that keeps it online.

Anglian Compressors, a Branch of Atlas Copco Compressors, treats this as a system question before a compressor fault. The aim is to find where the demand or run command comes from and correct it within the plant’s defined pressure limits.

Establish What “Running” Means

On a fixed-speed rotary screw compressor, the motor may continue running after system pressure reaches the upper control point. The inlet valve closes, the package unloads and the controller waits for renewed demand or its programmed stop condition. Our Atlas Copco manufacturer guidance confirms that low demand can leave a compressor running unloaded until its stop logic acts.

Unloading and then stopping after the programmed delay may be normal. Staying unloaded all shift, repeatedly reloading or receiving a continuous run command needs investigation.

Unloaded Running Still Has a Cost

An unloaded package still powers the motor, cooling system, oil circuit and controls. Atlas Copco reports unloaded consumption of up to 25% of full-load energy. A Carbon Trust manufacturing guide gives an upper figure of up to 40% of full-load power for an idling compressor.

Consider a 90 kW installation that stays unloaded for eight hours after each of 250 production days. At full load, those hours would represent 180,000 kWh (90 kW × 8 h × 250 days), so the published upper figures give a screening consumption of up to 45,000–72,000 kWh per year.

A real figure needs measured package input power, verified unloaded hours and the site’s contracted tariff. A compressed air energy audit can replace the estimate with a recorded demand and power profile.

Is the Overnight Demand Real?

Some facilities need continuous control or instrument air even when output is zero, so confirm which users are authorised to stay live before treating every overnight flow as waste.

During a controlled idle period, check whether any of these are still connected:

• Desiccant dryer purge or regeneration air
• Timed drains, or a drain stuck open
• Pneumatic control circuits, air knives, blow-offs and cabinet cooling
• Idle machines whose solenoid valves remain supplied
• Unused branches and external buildings
• A second compressor or receiver connected through a faulty non-return valve

A desiccant dryer can legitimately use compressed air to regenerate. HSE describes twin-chamber dryers in which one chamber supplies air while the other regenerates, so count the purge as part of the idle load. A drain stuck open is a loss that runs all night.

Leaks consume air for as long as the pipework stays pressurised. Our guide to finding compressed air leaks covers common leak locations, and our leak detection service gives a structured route from locating losses to documenting repairs.

What to Capture During a Quiet Shift

Record these without opening the machine or changing settings:

• Controller state: loaded, unloaded, standby, stopped, remote or fault
• Header pressure trend: rising, steady or falling
• Total run hours against loaded hours
• Starts during a known idle period, with times
• Active alarms and recent alarm history
• Any master controller or second compressor commanding the machine
• Which zones and users should remain pressurised, and who can change them
• Recent maintenance, pipework additions or control changes

Do not alter set-points to make the symptom disappear. A lower stop pressure can leave insufficient pressure at the point of use when production resumes.

What the Out-of-Hours Load Profile Shows

A load profile logged across a genuine idle period usually points in one of three directions.

• Flat overnight load/unload cycling. Pressure slowly falls and the machine reloads. Air is leaving the system through leakage or a user that is still connected.
• Pressure falling behind a proven isolated valve. With the downstream system shut off, pressure loss on the compressor side points to the compressor package, receiver, drain, relief valve and non-return valve.
• Continuous unloaded running with no stop. Suspect a timer or control setting, a remote command or a fault. Check the programmed control and master sequencer before replacing hardware.

If pressure climbs above the normal band or a relief valve lifts, stop the machine safely and do not reset it repeatedly. Our quick guide to central air compressor controls explains how pressure bands and the master sequence interact.

Hands-on testing needs a competent person. The Health and Safety Executive’s maintenance guidance says machinery should be isolated and stored compressed air released before maintenance begins. “Unloaded” describes a control state and says nothing about electrical isolation or stored pressure.

Two Common Questions

Should an Air Compressor Run All the Time?

It should run when the site needs air. Outside those hours, unloaded running adds cost without producing air.

How Long Can You Leave an Air Compressor Running?

There is no general cut-off. The manufacturer’s manual and controller settings define the permitted duty, unloaded stop delay and starts per hour.

When to Bring in a Compressed Air Engineer

Bring us in when the true demand remains unknown, pressure indication is questionable or testing needs access inside the package. As a Branch of Atlas Copco Compressors, our engineers have direct access to factory engineering information and proprietary diagnostic platforms such as SMARTLINK, AIRScan and AIRchitect.

Book a free energy audit and we will measure the installation, show where air and electricity are being consumed and document the likely payback of any changes. Use the form below to tell us when the compressor runs, what equipment remains active and which pressure limits apply.