Irrigation Blowout Calculator

What size compressor do you need to blow out a sprinkler system?

Size it on air volume first. Colorado State University Extension’s Fact Sheet 4.719 gives the rule in one line — “Divide GPM by 7.5 to determine the cubic feet per minute (CFM) needed” — using your largest zone, not the whole system. A 20 GPM zone needs 2.66 CFM. Pressure is the second question and has a hard ceiling: 80 psi maximum on rigid PVC, 50 psi on polyethylene. The calculator returns both numbers and checks them against the machine you own.

Volume is the number that fails, not pressure

Most people size a blowout compressor on the pressure gauge, because pressure is the number printed on the tank. It is the wrong end of the problem. CSU Extension states the failure mode directly: “Air volume is as critical as air pressure. If an insufficient volume of air is used, after forcing some water out, the air will ride over the top of the water.” That describes a system you believe you blew out — the heads misted, the gauge held, the truck moved on, and the water left behind ran downhill into the low spots and froze there. The callback arrives in April, and it is a repair, not a service call.

The 7.5 divisor is not arbitrary. A US gallon is exactly 231 cubic inches and a cubic foot is 1,728, so a cubic foot holds 7.48 gallons of water. CSU’s divisor is that ratio rounded: it converts a zone’s water flow rate into the volumetric rate of air needed to keep displacing it. That is also why the answer scales with the largest zone — the compressor serves one zone at a time, but it has to serve the biggest one.

One caveat separates that column from the machine you buy: CSU’s figure is the requirement at the point of connection, at working pressure, while a compressor’s advertised CFM is usually free air and delivered volume falls as the regulator setting rises. K-Rain closes the gap from the other direction with a whole-system spec — a compressor “capable of delivering 20 to 25 Cubic Feet Per Minute of air volume.” One is a floor, the other a working machine. The tool reports the floor, compares it to whatever you type, and publishes no correction factor between the two, because no source we can resolve publishes one.

Blowout pressure limits: PVC versus poly

Two published ceilings apply, they disagree, and the disagreement is worth showing rather than averaging away.

CSU splits the ceiling by pipe material; K-Rain, a manufacturer writing for its own installed base, caps everything at 50. The calculator shows both, checks against the material ceiling, and flags the stricter figure separately — so 70 psi on rigid PVC reads as “inside CSU’s limit, above K-Rain’s,” which is true and defensible to work from.

A third ceiling belongs to the heads rather than the pipe. CSU: “if the pressure is in excess of what the nozzles are rated, the excessive pressure will blow the sprinkler nozzles off and could cause other damage.” Enter a nozzle rating and it pulls the governing ceiling down; it can never push it up. Whatever the number, regulate at the outlet — CSU notes that “larger compressors stand the risk of pressurizing too quickly and causing damage to the system,” and that a regulator “can overcome this problem.”

One device is exempt from the whole discussion. Pressure Vacuum Breaker (PVB) and Reduced Pressure (RP) backflow prevention assemblies must never be blown out with compressed air — CSU’s wording, and it carries no exception. Isolate the assembly and follow its own manufacturer’s procedure. This tool does not model one; the minutes you spend on it go in the setup field.

How long to run each zone, and why not longer

The two published instructions here pull in the same direction: short passes, more than one, then stop.

• K-Rain: “Do not blow any circuit more than 1 minute at a time,” repeated “until nothing more than a fine mist appears from the heads.”
• CSU Extension: “Each zone/station should be blown out twice to make sure all the water is purged from the system. It is better to use two short cycles per station/zone than to have one long cycle.”

And the reason for the ceiling on the other side, from CSU: “Once a zone is ‘blown out’, avoid blowing that zone out again. Compressed air moving through dry pipes causes friction and heat which could cause damage.” Air moving through a wet pipe is doing work; air moving through a dry one is heating the pipe.

So the timing panel defaults to two cycles of one minute — published figures, not house numbers — and flags you past either. The rest of that panel is yours: the recharge allowance depends on your tank and duty cycle, and the setup block absorbs travel, hookup, backflow isolation and pack-up. Neither has a published figure behind it. Both ship prefilled anyway — half a minute of recharge, twenty minutes of setup — so the panel computes something on first load, but those two numbers are placeholders to overwrite, not sourced figures. Time one round with a stopwatch and they stop being guesses.

The pipe volume line makes the volume argument concrete. Given a nominal size, the tool reads Rain Bird’s published average inside diameter and computes gallons per foot as π ÷ 4 × ID² × 12 ÷ 231 — geometry and the exact definition of the US gallon, nothing else:

A 900-foot run of 1” Schedule 40 holds roughly 39 gallons — the water the air has to push uphill and out of the heads before the first freeze, and the fastest way to see why a shop compressor sized for an impact wrench is the wrong machine. Class 200 holds more than Schedule 40 at every size (thinner wall, wider bore), so a system plumbed in Class 200 laterals is a bigger purge than its nominal sizes suggest. If you are still working out zone flow in the first place, the irrigation run time calculator derives precipitation rate from GPM and head spacing, and the sprinkler head spacing calculator covers the grid those numbers come from.

Pricing a winterization stop without inventing a market rate

No primary source publishes a defensible market rate for a sprinkler blowout, so this page publishes none — there is no “typical blowout costs $X” figure anywhere on it, and the calculator has no market band baked in to check your answer against. It builds the number from your own cost stack instead: the only version of the answer that survives a bad season.

That does mean the price panel is prefilled rather than blank. The four money fields — burdened labor rate, compressor cost, target margin and minimum charge — ship with round illustrative values so the tool computes on first load and you can see the mechanism before you have typed anything. They are placeholders, not figures we are asserting, and the number the page paints before you touch it is arithmetic on those placeholders rather than a claim about your market. Replace all four with your own; a price built on someone else’s cost base tells you nothing.

On-site minutes come out of the timing panel: zones × (cycles × cycle length + recharge), plus setup. Multiply by crew size for person-hours, apply your burdened labor rate, add the compressor’s cost for the job, and you have job cost. The price then comes from margin by division — price = cost ÷ (1 − margin) — not from multiplying cost by the margin percentage. On the same $130 of cost at a 45% target, division gives $236 and multiplication gives $189. The $189 stop is making 31%, not 45%, so a route of them lands nearly a third short of the margin it was priced for, with no line item anywhere looking wrong.

Two inputs deserve attention. Your burdened labor rate is not the wage you pay — payroll taxes, insurance and unbilled time sit on top of it, and the labor burden rate calculator resolves it properly. The compressor line is a rental day rate split across the stops you make that day, or an ownership share from the equipment cost per hour calculator. Leave it at zero and you get a labor-only price — fine, as long as you know that is what it is.

The output that tells you whether the route is worth running is revenue per crew-hour, not the per-stop price: a dense street of eight-zone systems and a scattered rural round at the same ticket price are not the same business. Once the number holds up, put it in writing — the lawn care service agreement template is where a seasonal shutdown visit gets scoped, and the fall and winter service calendar sequences it against the rest of the shoulder season.

When the blowout window opens

Winterization is a race against the first hard freeze, not a date, and which months that means depends entirely on where you are. In the Northern Hemisphere the work clusters in September through November, moving earlier with latitude and elevation — a Colorado front-range route runs weeks ahead of a mid-Atlantic one. In the Southern Hemisphere the equivalent window falls across March to May. Neither is a rule this page can state for your territory: the forcing function is your own first-freeze date, and the safe posture is to be finished before it rather than booked against it.

The calculator covers the part that does not move — how much air the system needs, how much water is in it, how long the stop takes, and what it has to earn. Everything else in the seasonal line sits on the lawn and landscape hub.