Room turns and changeovers

What a changeover is

A changeover is the time from the last good piece of one run to the first good piece of the next. Everything in between counts: stopping, cleaning, setting up, restarting, and the slow or off-spec output right after a restart, which some people call run-up.

For a room, the same idea reads as last plant out to first plant in.

Two reasons it matters:

• It's time with no output. A changeover is scheduled time, so it counts against availability, the share of scheduled time a machine or room is actually producing.
• It sets batch size. When changeovers are long, planners make big lots to spread the cost, and big lots bring inventory and waiting, as in Lesson 4. Short changeovers make small lots affordable.

Some of what happens in a changeover is required, such as a surface that has to be clean or a waste step that has to be witnessed. This lesson doesn't touch those. It goes after waiting, handoffs, searching and work that could have been done earlier.

Internal and external work

The most useful split in a changeover is between two kinds of task:

• Internal work can only be done while the equipment is stopped or the room is empty.
• External work can be done while the equipment is still running, or before the changeover starts, or after it ends.

In a flower room turn, disinfecting the walls is internal: the plants have to be out first. Staging disinfectant, PPE and carts is external. So is booking a contractor's badge and escort, which can be done days ahead. Cleaning a spare set of irrigation emitters in the wash area is external too, as long as the room is still growing while you do it.

The change that pays most is usually moving work from internal to external. Every hour of external work you finish before the room is empty is an hour the room doesn't sit idle.

SMED, and a story to be careful with

SMED stands for Single-Minute Exchange of Die. "Single minute" doesn't mean one minute. It means a changeover that takes less than ten minutes. The method has three stages:

1. Separate the internal work from the external work.
2. Convert internal work to external work wherever you can.
3. Streamline everything that's left, internal and external.

The Lean Enterprise Institute lists a six-step version of the same idea: measure the changeover, list the internal and external elements with their times, convert, shorten what's left inside, shorten the external work, and standardize.

Shigeo Shingo gave the method its name. His own account says a Toyota press changeover fell from four hours to under three minutes in 1969. Other writers dispute details of that story. So write "Shingo reported", and don't treat it as a promise. In one published kaizen case a changeover went from 90 to 47 minutes, a cut of about 48%. Real results vary a lot, and I found no reliable published statistics for cannabis changeovers, so I won't quote one. Time your own.

Critics also point out that SMED's neat three stages can hide something: the slow output after a restart. That's why the first good unit, and not the restart, ends the changeover.

What a rule fixes, and what it doesn't

A rule can fix three different things: an outcome (a surface must be clean), a step (waste destruction must be witnessed), or a duration (a sanitizer must stay on for the contact time on its label). Most of the cleaning rules I read fix outcomes. That leaves the method, the sequence and the timing as the places to look.

Here is how that works out, with the sections that matter:

• Cleaning and sanitizing. Processing has to be safe and sanitary, and contact surfaces cleaned and sanitized as often as needed with an EPA-registered sanitizer used as its label says (935 CMR 500.105(3)). The outcome and the label's contact time are fixed. How you get the room ready for the cleaning is not.
• Your own procedures. You must have written operating procedures and follow them (500.105(1)). If a cleaning checklist is in your procedures, you follow it, and changing the checklist means changing your procedures.
• Waste. Plant waste has to be rendered unusable, witnessed by an agent and recorded, and the record kept for three years (500.105(12)). The step is required. It can overlap other work, for example by booking the witness ahead.
• Visitors and contractors. In limited-access areas, they are badged, escorted and logged (500.110(4)(e)). Book them ahead.
• Edibles. An edible's use-by date can't be altered or relabeled later (500.150(3)(b)), so there is no relabel shortcut.

One more thing. The empty room isn't pure waste. Cleaning between crops reduces the disease that carries over from one crop to the next, according to plant pathology work. So cut the waiting and the rework in a room turn, and leave the disinfection alone.

A room turn, step by step

Every number below is made up to show the steps. I couldn't find a reliable published number for how long a room turn takes, so yours will be different. The window is last plant out to first plant in. The "before" column is one crew doing everything in sequence.

Follow the three stages. Separate: staging, booking and prep that can happen outside the window comes to 1.5 + 2.0 + 1.5 + 2.0 = 7.0 hours, which takes 36.5 down to 29.5. Convert: cleaning the spare irrigation set earlier moves 2.0 hours outside, down to 27.5. Streamline: the two-person strip crew saves 3.0 hours, the booked witness 0.5, and the HVAC preconditioning 3.0, which is 6.5 hours, down to 21.0. That saves 15.5 hours, or 42.5%.

Look at what didn't change. Washing, disinfecting, the label contact time and the sign-off are all untouched. Only 1.0 hour of the 21.0 left is pinned to a rule or a label: the 0.5-hour witnessed step and the 0.5-hour contact time.

The arithmetic: shorter changeovers, smaller batches

Here is how a changeover links to batch size. Every number is made up. A line runs 40 units a minute for a 480-minute shift, and does one changeover for each product it runs.

That is the link. A shorter changeover doesn't only save the time. It lets you make more products in smaller batches in the same shift, which is what flow and pull from Lesson 4 need.

Knowledge check

Your task: watch one changeover

This is a walk, not a project. Fix nothing.

1. Pick one changeover to watch: a room turn, an extraction clean-down or a label change.
2. Start your timer at the last good unit, or the last plant out. Stop it at the first good unit, or the first plant in.
3. Write each activity, who does it, and when it starts and stops.
4. Tag each one I (the equipment had to be stopped), E (it could have happened earlier or later) or R (a rule, a label or your own procedure requires it).
5. Time the waiting separately: how long nothing was happening, and what it was waiting for.

Use generic labels, such as "clean step 3". Leave out strains, customers and counts. If the changeover runs long, watch one stretch and ask for the rest.

Open the printable changeover sheet

Sources

Where the facts in this lesson come from. Facts last checked October 1, 2026. If you find one that is out of date or wrong, tell me.

• Lean Enterprise Institute lexicon: changeover, with setup reduction, single-minute exchange of die, overall equipment effectiveness and batch-and-queue
• Pellegrini and others, a SMED case study (IEOM, 2012), for the three stages and the 90-to-47-minute example
• Smalley, Origins and facts regarding TPS (2006 draft), for the dispute over Shingo's account
• McIntosh and others, Changeover improvement: reinterpreting Shingo's SMED methodology, IEEE Transactions on Engineering Management 54(1) (2007), for the run-up point
• 935 CMR 500, adult-use marijuana regulations (effective September 11, 2026): 500.105(1), (3) and (12), 500.110(4)(e) and 500.150(3)(b)
• Buirs and Punja (2024), Plants 13(6): pathogens and sanitation in cannabis production