I worked the tinner trade for about four years and I have not taken a side job in fifteen. I manage projects for a general contractor in Salt Lake now, which means change orders cross my desk every week. I am the guy who reads them.
The numbers here are Salt Lake in 2026 and they are examples, not a price list. Swap in your own rate and your own supply house. This assumes you are licensed and insured for the work.
How do you price a small HVAC change order?
For a small add on a job you already priced, charge your unit rate, in writing, before you start. If you are already on site and the job has cleared its floor, an added register only has to carry its own material and labor, so it is where your margin lives. It is not free. If the add needs a new trip, a supply house run or a design answer, it is a new price, not an add.
Nothing gets agreed out loud
This is the rule everything else hangs on.
Somebody asks you for something on the job. You talk about it. Before you do anything, it goes in a text or a recap email. What they asked for, what it costs, how long it takes, and that they said yes to both the dollars and the time.
On a side job that can be two lines in a text. On a commercial job it is an RFI or an email to the GC. Either way it is a document, and a document is the only thing anybody can point to later when memories disagree.
I document every extra that gets asked of me. The worst ones are not the big ones. The worst ones are where the intent never got fully communicated, and there was still a gap between what the owner thought he was getting and what got built. Writing it down closes that gap before it costs anybody anything.
Floor, margin, and the budget baseline
Three ideas get mixed up here, so let me separate them.
The floor is the lowest price where a job stops costing you money to show up. I worked it out in what an hour actually costs you. At an $85 burdened rate, a job carries about $340 of fixed cost before anything is installed. One basement register all in is about $145. That puts the floor for a one-register job around $500 (residential example).
Margin is what every unit pays you after the floor is covered. The $340 only gets paid once. So the second register carries its own material and labor and nothing else, and the rest of your unit rate is margin.
The budget baseline is different, and it is the one that shows up on the commercial side. The baseline is the first version of the job budget, the starting point every line item gets measured against. You never modify the baseline. You work from it. As the job goes, each line item lands over or under its baseline number, and that over or under is the variance. Every budget has some of both. Under is better than over. The trouble starts when too many line items run over.
The floor is about one job. The baseline is what every line item gets measured against, all the way to closeout. Small adds that nobody tracks do not hurt the floor. They show up as variance against the baseline, and by the time you see it, it is too late to fix.
Three kinds of “while you’re here”
Not every add is the same, and the price should not be either.
An add to scope you already priced. You are in the basement doing six registers and the owner wants two more. Same work, same conditions, same trip. Your fixed cost is already paid. This is the good one. Do not start it until it is in writing that they are paying for it, and they have agreed to the dollars and the extra time.
An add you never walked. “Can you run one to the room over the garage too?” You have not seen that room. You do not know what is in the way. Never offer a price for a scope you cannot be informed on. On commercial work, we price a set of drawings, we are lucky enough to win the bid, we start, and the majority of the time we find something we missed. That happens with drawings in hand. Do not do it to yourself on a guess.
The “this room is really hot” add. This one sounds small and almost never is. A room runs hot because it faces south, or the duct to it is undersized for the airflow it needs, or it is zoned together with an open space that reacts to the system differently. Fixing that is design. On a commercial job it goes to the engineer as an RFI, and I would rather wait for that answer than have my GC own a design decision. On a side job, it is the same question I raised in how to price a side job: can the equipment even move the extra air? If you do not know, do not price it yet.
And then there is the new trip. If you already packed up, or the add means coming back another day, your fixed cost resets. Maybe not all $340, but most of it. That is a new job with a new floor.
Worked example: two more registers in the basement
You bid six basement registers at $200 each. $1,200. Your floor was cleared at register one.
The owner asks for two more while you are there. Open basement, same trunk, you have the material on the truck.
Each added register costs you about $145 in material and labor. At your $200 unit rate, that is about $55 of margin on each. Two registers is $400 on the invoice.
What you do: text him before you cut. “Two more registers, same $200 each as the estimate, $400 total, adds about an hour. Good to go?” He says yes. Now it is a document. Now it goes on the invoice as its own line.
What you do not do is throw them in because you were already there. Those two were the most profitable registers on the job. Giving them away does not make you a nice guy. It makes the next guy’s price look high.
When a small add is still under the floor
Some adds look like the basement example and are not. It is back under the floor when:
- You were already packing up. Coming back is a new trip.
- It needs a supply house run you did not plan. That is an hour of your day and a trip in the truck.
- The job was basement and the add is attic. That is a different rate for a reason, and I explained why in the hour-cost post.
- It changes how much air the system has to move. That is a design question before it is a price.
On a commercial job, the paperwork has a floor too
A GC exec told me once that no change order is free. He is right.
Here is how I look at the cost of just producing one on the sub side. An hour in the field to find it and document it. An hour communicating it to the office. An hour for the office to write it up and send it. At $85 an hour that is $255. Add travel, the computer and software it runs on, and markup, and I round it to about $400.
That is my number. Yours will be different. The point is not the $400. The point is to ask the question before you write one: does the cost of generating this change outweigh what it is worth?
A $300 extra that costs $400 to process is a loss before anybody reviews it. A couple of extra hours folded into a bigger change is a reasonable way to absorb a small one. But that should not become the standard. I check hours on a change against what similar work cost on changes I have seen before. Padding gets noticed. If you are trying to absorb something small, say so before you send it.
I will do a whole post on what a change order costs once it leaves your office. It is more than you would guess.
What to say in the driveway
When somebody asks you in the field, give them a number or tell them you need to check. Do not guess.
If it is an add to work you already priced: “I can do that. Same unit rate as the estimate. I will send it to you in writing before I start.”
If you do not know what it takes yet: “I am very interested, but I need to check on the material before I guess.”
There is no pressure to answer on the spot. Guessing is what leaves you frustrated later. The material is not available, it takes longer than you thought on the fly, and the quality suffers because you are not in a good head space. Take the time to price it.
Small adds are how two days turns into three
“While you’re here, will you…” is how the day gets away from you. A two day job turns into three. A three year project turns into three and a half. Some people call it scope creep.
It is worse when none of it was written down. At the end, when there are more overs than unders, you have no recourse. Most contracts I have written have a deadline to submit a change, and owners rarely bend on it. Documenting fast and submitting on time is what protects you.
Catching things early is the cheapest version of every one of these problems. I wrote about that from the drawing side in who owns it when the duct design is wrong and shop drawings vs construction drawings.
Once you have the number, you still have to hand it to somebody. I built a free estimate and invoice tool for tradespeople, no signup, no subscription: quickdrawinvoice.com
Common questions
What is the difference between a floor and margin?
The floor is the lowest price where a job stops costing you money to show up. Margin is what each unit pays you after the floor is covered. On a one-register job the register carries the whole floor. On an eight-register job, every register after the first is mostly margin.
Should adds be free once the floor is covered?
No. The floor being covered means the add does not have to carry your fixed cost again. It still carries its own material and labor, and your unit rate still applies. Those are your most profitable units. Do not give them away.
Do I charge for another trip if I am already there?
Not for the install time if you are already set up. Yes if the add creates a new trip, an unplanned supply house run, or you had already packed up.
What if they say it is only one more?
One more still has material and labor in it. Price it at your unit rate and put it in writing before you start.
Is a small change on a commercial job worth a change order?
Sometimes it is not. Producing a change order has a real cost on the sub side, and it is worth working out your own number. Small items can sometimes be folded into a larger change, but say so up front, and do not make padding a habit.