Compressed Air and Pneumatic Leaks in Buildings: The Costliest Utility Nobody Meters

• Leaks can waste 20 to 30% of compressor output in a poorly maintained system; an active leak program can bring leakage below 10%.
• A single 1/4 inch (6.4 mm) leak at 100 pounds per square inch gauge (psig), about 7 bar, costs about $8,382 a year. That assumes $0.05 per kWh and continuous operation.
• Every 2 psi (0.14 bar) of extra discharge pressure raises energy use by about 1% at full load. Raising pressure to cover leaks makes every leak blow harder.
• Airborne ultrasound is the standard survey method, because escaping air makes turbulence that stands out from room noise.

Compressed air in commercial buildings and data centers serves three main jobs: pneumatic HVAC controls, fire protection air supplies and general shop air. Each is often owned by a different team.

• Pneumatic heating, ventilation and air conditioning (HVAC) controls: thermostats, damper actuators and valve actuators in many older buildings.
• Fire protection: supervisory air or nitrogen for dry pipe and preaction sprinkler systems.
• Shop and maintenance air: tools, blow offs and workshop equipment.
• Process uses: labs, kitchens and some equipment with pneumatic parts.

Because these systems are spread out and rarely metered, nobody sees their total cost. That is why the title calls compressed air the costliest utility nobody meters.

A compressed air leak costs the electricity needed to make the air it wastes, every hour the system runs. DOE's figures make the scale clear.

DOE estimates leaks can waste 20 to 30% of compressor output in a poorly maintained system. An active leak program can bring leakage below 10% (Reliability Magazine summary of DOE data). The overall wire to work efficiency of a typical system can be as low as 10 to 15%, so wasted air is wasted electricity several times over.

How big is a leak?

DOE's leak table gives about 25 cubic feet per minute (cfm) through a 1/8 inch (3.2 mm) hole at 100 psig (DOE Compressed Air Tip Sheet #3). For a sharp edged hole DOE applies a factor of 0.61, giving about 15 cfm (Fabrico).

The cost formula

DOE's cost formula multiplies five numbers: leak count, leak rate in cfm, kilowatts (kW) per cfm, operating hours and the price per kilowatt hour (kWh). DOE's example assumes about 18 kW per 100 cfm and $0.05 per kWh.

As a worked example, take one 15 cfm leak running all year at those assumptions. That is 15 × 0.18 × 8,760 × $0.05, or roughly $1,180 a year. That is one leak from one small hole, at an electricity price far below most current rates.

DOE's Fact Sheet #7 puts a single 1/4 inch (6.4 mm) leak at 100 psig at about $8,382 a year, at $0.05 per kWh with continuous operation. At today's higher electricity prices the cost rises in proportion.

Compressed air leaks hide at the many small connections between the receiver and the point of use. The common hot spots are well known.

• Quick connect couplings on tools and hoses.
• Hoses and hose reels, especially worn or kinked ones.
• Filter regulator lubricator (FRL) units at points of use.
• Threaded fittings throughout the distribution piping.
• Condensate drains stuck open.
• Pneumatic actuators and positioners on dampers and valves.
• Old pneumatic control tubing, which hardens and cracks with age.

Most of these leaks are too small to hear over room noise. They run all day, every day, and add up across a building.

Pneumatic HVAC controls leak through aging tubing, fittings and actuator diaphragms, and the leak makes the controls drift. Many older buildings still run pneumatic thermostats, damper actuators and valve actuators on control air around 20 psi (about 1.4 bar).

A pneumatic actuator moves in proportion to the air pressure it receives. When tubing or the actuator leaks, the pressure it sees is lower than the thermostat intended.

The result is drift. Dampers and valves sit in the wrong position, and zones overheat or overcool, which shows up as comfort complaints rather than as a leak.

Meanwhile the control air compressor runs more and more to keep up. A control air compressor that seems to run constantly is a strong hint that the control network is leaking.

A leak shows up at the compressor as running and cycling that does not match demand. The compressor is working to replace air that nobody is using.

• Frequent cycling or long run times with no matching demand.
• Pressure that falls overnight when nothing should be using air.
• A receiver that cannot hold pressure once the compressor stops.

The overnight test is the simplest. Read the receiver pressure gauge at the end of the day and again before anyone arrives; with no demand, any fall is leakage.

Compressor health

A compressor that runs too much wears out like any rotating machine. Bearing wear, vibration and motor overheating follow Rotating Equipment Part 1, Part 2 and Part 6.

A refrigerated dryer that runs warm lets moisture into the lines. Thermal imaging or the dryer's own display shows it, and wet air corrodes piping and fouls controls downstream.

People raise system pressure because it is the quickest way to make low pressure complaints go away. It is also one of the most expensive.

For systems around 100 psig, each extra 2 psi (0.14 bar) of discharge pressure adds about 1% to full load energy use (Bonneville Power Administration tip sheet).

The setpoint creep is gradual. A few psi added after one complaint, a few more after the next, and the plant ends up running far above what any end use needs.

Raising pressure also makes every leak blow harder. More air escapes through every hole, so part of the extra energy goes straight back out through the leaks it was meant to cover.

The better fix is to find and repair leaks, then lower the setpoint to what the end uses actually need. That saves energy twice, and it reduces wear on every compressor in the plant.

Receivers, drains and dryers fail in ways that waste air or send water downstream. They are easy to overlook because they sit quietly beside the compressor.

• Timer drains stuck open waste air continuously.
• Drains stuck closed send water downstream into piping and controls.
• A receiver that cannot hold pressure points to leaks downstream or at its own fittings.
• A refrigerated dryer running warm lets moisture through.

Oily condensate puddles under compressors are a housekeeping and environmental issue. Condensate from oil lubricated compressors carries oil and should be handled, not left on the floor.

Sprinkler air and nitrogen supplies matter because dry pipe and preaction systems depend on holding pressure. They are held under supervisory air or nitrogen, and a leak makes the supply cycle frequently.

A sprinkler air compressor or nitrogen generator that runs often is a leak indicator. The system is losing pressure somewhere in its piping, valves or fittings.

Some systems use nitrogen to limit corrosion in the piping. In a small, closed room, a nitrogen leak can lower oxygen, which makes oxygen and gas sensing relevant there.

Sprinkler inspection rules belong to NFPA 25 and are covered in NFPA 25 between inspections. The point here is that the air supply's run pattern is a leak signal worth reading on every round.

Recording how often the supply starts, week by week, turns that signal into a trend. A run count that climbs steadily means the leak is growing, long before a low air alarm appears.

Airborne ultrasound is the standard way to find air leaks because escaping gas makes high frequency turbulence that stands out from low frequency room noise. It finds leaks too small to hear.

A mechanical room is loud with motors, fans and pumps, but most of that noise is at frequencies people can hear. A leak's turbulence is much higher, so ultrasound separates it cleanly and points to its source.

That makes a leak survey fast. A technician can walk a distribution system, scan fittings and tag leaks without shutting anything down.

Tagging and fixing matter as much as finding. A survey only saves energy when the tagged leaks are repaired and the list is closed.

You run a compressed air leak program by measuring the baseline, finding and tagging leaks, fixing them, and repeating on a schedule. DOE's figures show an active program can bring leakage below 10%.

1. 1. Measure the baseline with an overnight pressure decay test or a flow meter.
2. 2. Survey the system with airborne ultrasound, and tag every leak with a location and size estimate.
3. 3. Fix the largest leaks first, and close each tag when repaired.
4. 4. Check drains, FRL units and hose reels, which leak most often.
5. 5. Lower the discharge pressure to what end uses actually need.
6. 6. Repeat the survey on a schedule, because new leaks appear constantly.

Every leak fixed also reduces compressor run time, which extends compressor life and cuts maintenance. The energy saving is the headline; the wear saving is a bonus.

You measure total leakage with a pressure decay test: isolate the system, let the compressor stop, and time how fast pressure falls with no air in use. The faster it falls, the more air is leaking.

1. 1. Choose a time with no planned air use, such as overnight or a weekend.
2. 2. Close off end uses that are meant to consume air, so only leaks remain.
3. 3. Run the compressor to its normal cut out pressure, then let it stop.
4. 4. Record the starting pressure and time, then the pressure after a set interval.
5. 5. Use the pressure drop, the interval and the system volume to estimate the leak rate in cfm.

The volume includes the receiver and the distribution piping, which a compressed air specialist can estimate. Repeating the test after a repair round shows how much leakage was removed.

A simpler version works on a walk past. If the receiver gauge reads lower each morning than it did each evening, with nothing running, the system is leaking.

Replacing pneumatic controls with direct digital controls (DDC) removes the control air leaks for good, but it is a capital project. Many buildings keep pneumatics and manage the leaks instead.

Where pneumatics stay, the priorities are the control air compressor's run time, a dry and clean air supply, and replacing hardened tubing and leaking actuators. Those steps restore control accuracy and cut compressor wear.

Where a building is planning a controls upgrade, the leak survey still helps. It shows which zones have the worst drift and comfort complaints, and so where conversion pays back first.

Compressed air leaks cost energy everywhere, but the operational risk differs by building.

In an office building

Wasted energy, drifting pneumatic controls causing comfort complaints, and compressors wearing out early from overwork are the main costs. The comfort complaints often get blamed on the thermostats, not the leaks.

In a data center

Preaction sprinkler air compressors or nitrogen generators cycling constantly are the main concern. They lead to low air alarms and raise the risk of a false trip on a preaction system, which no data center wants near its racks.

Not every hiss is a wasteful leak. Several sounds and uses look like one.

• Normal, designed air use, such as planned blow offs and purge flows, which are meant to discharge air.
• Steam leaks, which also hiss, covered in Part 8.
• Refrigerant leaks at a hissing joint, covered in Part 6.

The system and the location usually settle it. A hiss at a pneumatic fitting is air; a hiss at a refrigerant joint with an oily stain is refrigerant; a plume is steam.

This is why no single sensor is enough. Sound, pressure trends, run times and a look at the fitting each rule out a different explanation.

A walk past can prove that air is leaking, that a drain is stuck and that a compressor is running without demand. It cannot measure the total leak rate.

• Directly observable: an audible hiss, a drain stuck open, a compressor cycling with no demand, pressure gauge readings below normal and oily condensate on the floor.
• Strong indicator: an airborne ultrasound hiss at a fitting, and overnight pressure decay on the receiver gauge.
• Suspected: the total leak rate in cfm. That needs a flow meter or a timed pressure decay test with the system isolated.

That is the limit of any route based check. It finds the leaks and the symptoms; a flow or decay test measures the total.

Routine visual inspection and a walkthrough record of gauges and run status turn these checks into a trend, the core of predictive maintenance.

• Listen at quick connects, hose reels, FRL units and drains.
• Receiver and distribution pressure gauges, compared with the last round.
• How often each compressor starts and how long it runs.
• Condensate drains for continuous discharge.
• Refrigerated dryer temperature or status display.
• Pneumatic control systems: listen at actuators and panels.
• Sprinkler air compressor or nitrogen generator run status.

Coming Soon

1. 1. Where is compressed air used on your site, and who owns each system?
2. 2. Does any part of your building still run pneumatic controls?
3. 3. How often does your sprinkler air compressor or nitrogen generator run?
4. 4. What does your receiver pressure do overnight?
5. 5. When was a leak survey last done, and were the leaks fixed?

How much does a compressed air leak cost?

DOE puts a single 1/4 inch leak at 100 psig at about $8,382 a year at $0.05 per kWh with continuous operation. Cost scales with electricity price, so at today's rates it is higher.

What percentage of compressed air is lost to leaks?

DOE estimates leaks can waste 20 to 30% of compressor output in a poorly maintained system. An active leak program can bring leakage below 10%.

How do you find compressed air leaks?

Airborne ultrasound is the standard method, because escaping air makes high frequency turbulence that stands out from room noise. Leaks are then tagged, repaired and the survey repeated on a schedule.

Why does my air compressor run all the time?

The most common cause is leakage: the compressor is replacing air lost through leaks. Check whether receiver pressure falls overnight when nothing should be using air.

What causes pneumatic thermostats to drift?

Leaks in control tubing, fittings or actuators lower the air pressure reaching the actuator. Dampers and valves then sit in the wrong position, and zones overheat or overcool.

Why does a preaction sprinkler air compressor keep cycling?

A preaction or dry pipe system that keeps losing supervisory air or nitrogen makes its compressor or generator cycle often. That points to a leak in the system piping, valves or fittings.

Does raising compressor pressure fix low pressure problems?

It hides them at a cost. Every 2 psi increase raises energy use by about 1% at full load, and higher pressure makes every leak blow harder. Fixing leaks and then lowering pressure is the better approach.

What is a compressed air condensate drain?

A condensate drain removes water that condenses out of compressed air in receivers, dryers and filters. Timer drains that stick open waste air, and drains that stick closed send water downstream.

Can a nitrogen leak be dangerous?

Yes, in a small, closed room. Nitrogen used in some sprinkler systems can lower oxygen levels if it leaks into an enclosed space.

• Ultrasonic Sensing in Data Centers: Hearing Faults Before They Fail
• NFPA 25 for Data Centers: Sprinkler Inspection, Testing and Maintenance, and the Closed Valve Nobody Sees
• Motor Overheating: The 10 Degree Rule, Voltage Unbalance and the Cooling Air Nobody Checks

• DOE Compressed Air Tip Sheet #3: leaks
• Reliability Magazine: DOE compressed air leak statistics
• Fabrico: DOE leak table and sharp edge factor
• Bonneville Power Administration: pressure and energy