Walk into a commercial HVAC contractor's building on a Tuesday morning and you'll find two completely different air problems under one roof. Out back in the fab shop, a Pittsburgh lock former, a coil line, a plasma table and three guys with pneumatic screw guns are all pulling at once. Out front, six service trucks are loading up, and every one of them needs air of some kind before the day is over. The owner bought one compressor for the whole building, usually about ten years ago, usually too small.
That split is what makes compressed air for HVAC contractors different from air for a generic machine shop. You are running a light-duty manufacturing operation and a mobile service fleet out of the same yard, and the two have almost nothing in common except the invoice.
The Fab Shop Is Where Your Real Demand Lives
If you fabricate your own duct, the sheet metal side is your load. Most contractors underestimate it because no single tool looks like much. Add them up and it stops being small.
| Tool | Typical CFM at 90 PSI | Duty |
|---|---|---|
| Pneumatic screw gun / drill | 3 to 5 | Intermittent |
| 1/2 in. impact wrench | 4 to 5 | Intermittent |
| Air nibbler or shear | 4 to 8 | Intermittent |
| Die grinder (deburring seams) | 5 to 8 | Near continuous |
| Pneumatic riveter | 3 to 4 | Intermittent |
| Plasma cutter, 45 to 85 amp | 5 to 7 continuous at 90 to 120 PSI | Continuous while cutting |
| Blow-off at the bench | 4 to 15 each | Whenever someone feels like it |
Three people working, a plasma table cutting, and a couple of blow guns puts most fab shops somewhere between 25 and 45 CFM of real average demand. A 7.5 HP two stage piston delivers roughly 24 to 28 CFM at 175 PSI. A 15 HP rotary screw lands around 50 to 55 CFM. If your shop runs a plasma table daily, the screw is the honest answer, because the table wants steady flow and a piston at 100 percent duty cycle will cook itself before lunch.
Watch the plasma requirement specifically. Cut charts are written at a given pressure and flow, and the number on the chart is what the torch needs at the torch, not what the compressor is stamped for. Losing 15 PSI in an undersized drop turns a clean cut into a dross-covered mess and your operator will blame the consumables. If that sounds familiar, our piece on compressed air pipe sizing is worth ten minutes.
Plasma and Laser Air Has to Be Dry, Not Just Plentiful
Here is the mistake that costs HVAC fab shops the most money. They size the compressor correctly and skip the dryer. Water and oil in plasma air wrecks torch consumables fast, and on galvanized it leaves you chasing a cut quality problem that has nothing to do with the machine.
For a plasma table, a refrigerated dryer holding a pressure dew point around 38 to 50 degrees F, plus a particulate filter and a coalescing filter ahead of the table, is the baseline. That is ISO 8573-1 territory, and if the phrase is new to you, we broke it down in ISO 8573-1 air quality classes, decoded. If you have moved up to fiber laser, the spec tightens hard and you are into desiccant with a minus 40 dew point, which we cover in compressed air for laser cutting.
One more thing about a duct shop specifically: galvanized dust is everywhere, and it gets pulled straight into the compressor intake. Put the compressor in a separate room or at minimum away from the shear and the grinder bench, and check the intake filter on a real schedule rather than when the unit starts sounding tired.
The Service Truck Side
Service techs need air for a much shorter list, but they need it where the truck is. Realistically that is blowing out condensate drain lines, clearing a plugged evaporator drain pan, coil cleaning prep, running an occasional impact on a rooftop unit's fan assembly, and air for a small pneumatic pump or vacuum tool.
Two workable answers:
- A small portable on the truck. A one to two gallon oil-free unit will clear a drain line all day and weighs little enough that nobody resents carrying it up a ladder. It will not run a die grinder, and nobody should pretend it will.
- A gas or truck-mounted unit for crews doing rooftop work, coil replacements or anything where you need 15 to 25 CFM with no jobsite power. Our truck-mounted air compressor guide covers the trade-offs between underhood, gas deck mount and tow behind.
For coil cleaning, keep the pressure honest. Blasting a condenser coil at 150 PSI folds fins and you just turned a cleaning into a coil replacement. Most manufacturers want that work done well under 100 PSI with the flow coming from the direction opposite normal airflow.
The Nitrogen Question, Answered Straight
Every HVAC contractor asks us the same thing eventually: can I stop buying nitrogen cylinders and just make my own?
Partly. Here is the honest split.
Nitrogen purging during brazing needs a trickle, not a torrent. The accepted practice is a very low flow, on the order of 2 to 5 SCFH at roughly 1.5 to 2 PSI, just enough to displace oxygen inside the tube so you do not form copper oxide scale that later ends up jammed in a TXV or plugging a filter drier. A membrane or PSA nitrogen generator handles that flow and purity without breaking a sweat, and if you are burning through cylinders every week, the math gets interesting fast.
Pressure testing is where it falls apart. A standing pressure test on refrigerant lines commonly runs 150 to 300 PSI and higher on some systems, and an onsite generator fed by a shop compressor is not going to deliver that. That is cylinder work, or a booster, and anyone telling you otherwise is selling something. If your nitrogen spend is mostly pressure testing, keep the cylinders. If it is mostly purge gas on a fab or install crew, a generator is worth pricing.
Either way, remember the generator only produces nitrogen as clean as the air you feed it. It needs dry, oil-free air upstream, which means filtration and a dryer sized for the generator's inlet flow, not just your shop's average demand.
Sizing the Shop Compressor Without Guessing
A method that works, in order:
- List every tool that can run at the same time, with its CFM at 90 PSI from the manufacturer, not from a forum.
- Apply a realistic duty factor per tool. A screw gun runs maybe 20 percent of the shift. A plasma table runs whatever your cut time actually is.
- Add a diversity factor if you have more tools than people. Two techs cannot run five tools at once.
- Add 20 to 30 percent headroom for the leaks you already have and the work you will pick up next year.
- Size storage separately. A fab shop with a plasma table wants real receiver capacity, generally 4 to 6 gallons per CFM for a piston and at least 2 to 4 for a screw.
The number two mistake we see, after skipping the dryer, is buying on horsepower instead of CFM. Two 10 HP machines can be 10 CFM apart. Read the delivered flow at your working pressure. We wrote air compressor HP to CFM for exactly this argument.
What HVAC Shops Get Wrong Most Often
- Treating the compressor room like a closet. An air-cooled unit rejects nearly all of its input energy as heat. Seal it in a small room with no ventilation and you will run hot, degrade the oil and shorten the airend's life.
- Running PVC for shop air. Still common in contractor shops, still a bad idea. PVC gets brittle and shatters. See why OSHA says no to PVC.
- No drops, no drip legs. Sheet metal shops tend to run one long header with tools teed straight off the bottom, so condensate walks right into the plasma torch.
- Ignoring blow gun pressure. OSHA limits dead-end pressure for cleaning to 30 PSI. A shop full of open-ended blow guns is both a citation and a large chunk of your compressor's output.
- Never auditing leaks. A fab shop with ten years of quick fittings and old hose is typically leaking 20 to 30 percent of what it makes. That is the cheapest CFM you will ever buy back.
Frequently Asked Questions
What size air compressor does an HVAC fab shop need?
Most small to mid duct shops land on 7.5 to 15 HP. If you run a plasma table daily, start at 15 HP rotary screw delivering roughly 50 CFM, because the table is a continuous load and a piston will struggle with the duty cycle.
Do I need a dryer if I only cut mild steel and galvanized?
Yes. Moisture in plasma air shortens consumable life and causes rough, dross-heavy cuts regardless of material. A refrigerated dryer plus coalescing filtration is the minimum for any plasma table.
Can a nitrogen generator replace my brazing purge cylinders?
For purge gas, usually yes, since purge flow is only a few SCFH at very low pressure. For standing pressure tests at 150 PSI and up, no. Keep cylinders for testing.
What pressure should I use to clean a condenser coil?
Well under 100 PSI, blowing opposite the normal airflow direction. High pressure bends fins and makes the unit worse, not better.
Is a truck-mounted compressor worth it for a service fleet?
It depends on the work. If your techs mostly clear drains and blow out lines, a small portable is plenty. If crews do rooftop changeouts or run impacts and grinders away from power, a gas or truck-mounted unit pays for itself in trips not made back to the shop.
Where to Start
Get the fab shop right first, because that is the load that decides everything else. Measure what you actually use, size for delivered CFM at your working pressure, put a dryer in front of anything that cuts metal, and then figure out the truck side separately. Good compressed air for HVAC contractors is not exotic. It is just two honest systems instead of one compromised one.
