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What should happen on the last afternoon
Commissioning is the part of an installation that decides how the system performs for the next fifteen years, and it is the part most often skipped — because nobody can see whether it happened.
What commissioning actually is. Once a system is physically installed it still has to be set up: the refrigerant charge adjusted to the manufacturer’s figures for that equipment in that house, the airflow measured and adjusted, the controls configured for how the building is used, and every one of those verified with an instrument rather than by feel. It takes about two hours on a house and it is the last thing on the last day.
Why it goes missing. Not incompetence, usually. A job runs late, a crew has another start in the morning, the customer is hot and wants it on, and everything measurable gets replaced by “it’s cooling nicely”. The system works. It just works at something other than what it was designed to do.
How you can tell. One question: ask for the numbers. A system that was commissioned has a sheet with figures on it. One that was not has an invoice.
Pick where you are and what kind of system it is. Everything below is written out in full.
A mini-split arrives with a factory charge for a stated line-set length. Anything longer needs refrigerant added by weight, to a figure the manufacturer publishes — not to a pressure, not to a feeling, and not to whatever is left in the cylinder. If nobody weighed it, nobody knows what is in your system.
The line set has to be evacuated properly first. A deep vacuum, held and verified with a micron gauge, so that air and moisture are out of the pipework before the charge is released. This is the step most often shortened, because it is twenty minutes of a machine running while nobody appears to be working. Moisture left in a system does not announce itself; it damages the compressor slowly and the failure arrives years later looking like bad luck.
Flare connections are the other one. They have a torque figure and a required flare shape, and a joint made by feel will hold for a while and then weep. A ductless system that is low on charge two summers after installation was almost always made, not damaged.
What to ask for: the line-set length, the weight of refrigerant added, the micron reading the vacuum was held at, and the final operating readings. Four numbers. A good installer has them written down already.
Thinking about a new system
Then the useful thing to know now, before you speak to anybody, is that commissioning is a thing you can ask for by name, and asking changes the conversation.
Most quotes describe equipment: a brand, a model, an efficiency rating, a price. Very few describe what happens after it is bolted in. When you ask a contractor how they commission a system, you learn a great deal from the shape of the answer — a specific one names measurements, a vague one names the manufacturer.
The other thing worth knowing early: commissioning cannot rescue a system that is the wrong size or connected to ductwork that cannot carry the air. Those are decided before the equipment is chosen. A correctly commissioned oversized system is still oversized. So the order is: load calculation, ductwork assessment, equipment selection, then commissioning — and a contractor who starts at the third step is starting in the middle.
Quotes in hand
This is the moment where one email separates them, and it costs you nothing.
Send all of them the same message: “Please confirm what commissioning is included, what measurements will be taken, and that I will receive them in writing.”
The replies will differ more than the prices do. What you are looking for is specifics — superheat and subcooling, static pressure, airflow, combustion figures where there is gas, and a document at the end. What you will sometimes get is a paragraph about how their technicians are factory trained, which is an answer to a different question.
And look at whether it is priced. A quote with commissioning as a visible line is easier to compare and harder to quietly drop. One where it is “included” may well be included; it may also be included in the same way the warranty is included.
If one quote is cheaper and cannot answer the question, you now know what the difference is buying.
Installation booked
Two things to settle before the day, because both become awkward once the van is outside.
When is it being commissioned? On a job that finishes at four on a Friday in August the honest answer is sometimes “we will come back” — and that is fine if it is agreed, dated and not billed as complete until it happens. It is not fine as a thing everybody hopes gets remembered.
And will conditions allow it? A cooling system cannot be charge-verified properly in cold weather, and there are manufacturer limits on outdoor temperature for checking a charge. An installation in December is a legitimate reason to defer that step to spring — which is another reason to have the date agreed rather than assumed.
One practical request while everybody is there: ask for photographs of anything about to be covered. Duct connections, the line set where it passes through a wall, the condensate route. It costs nothing on the day and it is the only record anyone will ever have.
It was installed last week
The best possible time to ask, and the shortest window. Everybody involved still remembers the job and nobody has moved on.
Ask for the commissioning sheet. Politely, in writing, by email so there is a date on it. You are not accusing anybody — you are asking for something you are entitled to and which a contractor who did the work has already written down.
What three replies mean. A sheet arrives: good, keep it, that is the baseline for the next fifteen years. Somebody comes back to take the readings: also good, that is a contractor putting it right. Nothing arrives, or you get a verbal assurance: you now know, and you can decide whether to press it or to have somebody independent measure it.
And do not judge it by how it feels. A system that is wrong in either direction — overcharged or undercharged, too much airflow or too little — usually still cools a house in its first week. The damage is slow and the bill is annual.
It has been in for years
Then commissioning as such is behind you, and what is worth having is a baseline — the same measurements taken now, written down, so that everything after this is comparable.
That is genuinely useful even on a system that is behaving. It tells you whether the charge is right today, whether the airflow is what it should be, and whether the controls are doing something sensible. And it gives every future visit something to measure against instead of an opinion.
What it often finds. A charge that was adjusted at some point by somebody who added rather than measured. Static pressure well above what the equipment is rated for, usually because of a filter or a duct restriction that arrived later. A thermostat configured for a house nobody lives in any more. None of those is dramatic and all of them cost money quietly.
What it will not do is turn a badly sized or badly ducted system into a good one. If the measurements say that is what you have, we will tell you, and it is a different conversation.
Central air with a gas furnace
On the cooling side: superheat and subcooling measured at a steady state, with the target taken from the manufacturer’s data for that equipment and that metering device. Not a pressure, not a rule of thumb, and not the temperature of the air at the register.
On the airflow side: total static pressure measured across the system, blower speed set to deliver the airflow the equipment needs, and the temperature split across the coil checked as a cross-reference.
On the gas side: manifold pressure set, combustion analysed and the figures recorded, the heat exchanger inspected and the flue checked as installed rather than as drawn. This part is a safety item as well as a performance one, and it is the one with a carbon monoxide consequence if it is got wrong.
And the controls: heat and cool stages configured, fan timings set, and the thermostat set up for how the house is actually occupied rather than left on a factory schedule.
A heat pump
Everything on the cooling side above, plus the half that almost nobody checks: heating mode.
Defrost has to be proved, not assumed. A heat pump that cannot defrost properly will ice up on the first cold damp morning of the winter, and the first anybody hears about it is in December. Forcing a defrost cycle and watching it complete takes ten minutes in October.
Auxiliary heat is the expensive one. Electric strip heat is the costliest heat you own, and if the controls are set to bring it on at the slightest demand it will run on mild mornings all winter. The changeover settings, the staging and any outdoor temperature lockout are decisions, and on a great many installations they are left at defaults chosen for a colder climate than this one.
And charge matters more on a heat pump than on a cooling-only system, because it has to work in both directions. A charge that is acceptable in cooling can be wrong in heating, which is why the manufacturer’s procedure matters rather than a general rule.
A ductless or mini-split system
See the note above, which is the substance of it: line-set length, refrigerant weighed in, a verified vacuum, and flares made to a torque figure.
Two things to add. The indoor units have settings — fan behaviour, whether the unit keeps circulating after the compressor stops, how the controller reads room temperature — and those defaults suit a showroom rather than a bedroom. Ten minutes of configuration changes how the system feels to live with.
And the condensate is the commonest failure. A wall unit drains by gravity through a small pipe with very little fall, and a blockage puts water down your wall rather than into a pan with a float switch. Where a pump is fitted, it should have a safety that cuts the unit off. Ask whether yours has one.
A packaged rooftop unit
Everything from the split system above, plus three things that only exist on commercial equipment — and all three are routinely left at factory settings.
The economiser. It has a changeover setting that determines when the unit uses outside air instead of mechanical cooling. Set correctly for this valley it saves real money for a real part of the year. Left at a default chosen for somewhere humid, it does very little. And the dampers have to be proved to actually travel, which means opening the section and watching.
Airflow and belt drive. Fan speed set to deliver the design airflow, belt tension and alignment correct, and static pressure measured rather than assumed from the fan curve.
The sequence of operation. Occupied and unoccupied schedules, staging, minimum outside air position, and any lockouts — written down and handed over. This is the document that goes missing between one facilities manager and the next, and it is the reason a building behaves oddly for years with nobody able to say why.
The next step
Ask every contractor you speak to how they commission a system. The shape of the answer will separate them faster than the price will. And get the load calculation and the ductwork looked at first — commissioning cannot rescue the wrong size.
The next step
Email all of them the same sentence asking what commissioning is included, what will be measured, and whether you get it in writing. The replies are the comparison.
The next step
Agree the commissioning date now, in writing, even if it is a return visit. And ask for photographs of anything about to be covered up.
The next step
Email and ask for the commissioning sheet this week. Everybody still remembers the job. Do not judge the system by how it feels — it will feel fine either way in the first month.
The next step
Get a baseline set of readings taken and written down. Not a repair, not a quote — a measurement. Everything you do afterwards becomes comparable, including deciding whether to do anything at all.
Where this points at us
We commission our own installations as standard and it is a line item on the quote, priced visibly, so you can compare us honestly to somebody who does not.
We will commission or baseline somebody else’s system as a separate job, and give you the sheet, whether or not anything further follows. If the answer is that it was set up correctly, we will say so and there is nothing to sell you.
What we will not do is tell you the previous contractor was incompetent. Most missing commissioning is a scheduling failure rather than a skill failure, and a measurement is a more useful thing to have than an opinion about somebody who is not in the room.
And we will not promise a percentage. A correctly commissioned system performs better than a badly commissioned one. By how much depends on how badly, which is not a number anybody can print honestly.
The sheet you should already have
A visit, the readings taken properly, and a written commissioning record for the system you already own. It is the baseline every future visit needs, and it belongs to you.