House hits temperature but feels clammy
Classic oversizing. The unit satisfies the thermostat before it has run long enough to condense moisture out of the air, so the sensible load is met and the latent load is not.
AC installation in Peralta, NM sized from the building, not from the old nameplate
A load calculation before the equipment is chosen, a duct system measured rather than assumed, and a commissioning sheet you keep at the end.
The quickest way to fit an air conditioner is to read the tonnage off the unit coming out, order the same thing, bolt it in and charge it until the gauges look familiar. It takes a day, it looks identical to a proper job once the panels are on, and it reproduces every mistake the previous installer made while adding a few fresh ones.
Very few buildings look, thermally, the way they did when their first cooling system was picked out. Glazing gets replaced, attics get insulated, gaps get sealed, roofs get lighter. Each of those trims the gain the equipment has to absorb, and the original figure was generous before any of it happened. Run the numbers honestly and the answer is frequently a smaller machine, one that stays on for longer stretches, wrings more water out of the air and costs less to purchase.
What happens before a price is given
Four determinations made on site, because they decide the scope far more than the badge on the equipment does.
The first visit is an assessment, not a sales call. The load calculation is worked room by room from the actual construction: wall and ceiling assemblies, glazing area and orientation, shading, infiltration and internal gains. That produces a sensible heat and latent heat figure for the building rather than a number derived from floor area, and it is the only defensible basis for choosing capacity.
Second comes the question of whether the ducting already in the building is capable of moving the volume of air this equipment is rated around. Rated capacity assumes roughly four hundred cubic feet per minute per ton across the coil. A system delivering three hundred loses a real share of its output before anything has gone wrong, and no efficiency rating survives that. Total external static pressure is measured with a manometer and compared with the blower table for the proposed equipment.
Third is the supporting infrastructure. Electrical service capacity and breaker sizing, the condition and routing of the line set, whether the existing plenum transitions will match the new cabinet, and where condensate is going including a secondary path and a float switch. Each of these is a place where an unpriced surprise appears on the day if nobody looked first.
Fourth, and it is a genuine question rather than a formality, is whether replacing anything this year is the right call at all. A system that is out of warranty but sound, with parts still available and a coil and cabinet in good condition, is frequently worth repairing again. That assessment gets shown with the condition of the components rather than a rule about age.
Written into every installation quote
Five items that are included in the fixed price rather than discovered as extras once the old unit is out.
- The load calculation with its assumptions listed, so you can check them
- Any duct remedial work the measurements show is needed
- Permit and inspection handling where the jurisdiction requires it
- New line set or a documented reason the existing one is being reused
- Commissioning measurements recorded and handed over at completion
Signs the system you have was never sized properly
Six complaints that are usually a sizing or distribution problem rather than an equipment fault.
Households tend to live with these for years and describe them as quirks of the building. They are not. Each one has a mechanical cause and most of them trace back to decisions made on the day the last system went in.
If several of these apply, a replacement that simply matches the outgoing capacity will reproduce every one of them, which is the argument for measuring before ordering.
Short cycles, five or ten minutes at a time
Capacity well above the load. Every start is the hardest moment in a compressor's life, and a system cycling constantly accumulates them faster than it should.
Upstairs never cools while downstairs is cold
A distribution problem. Either the branch runs to the upper floor are undersized, the returns are inadequate, or a duct is disconnected somewhere out of sight.
Noisy registers and a whistling return
Air moving too fast through ducts that are too small for it. High velocity means high static pressure, which means the blower is not delivering rated airflow.
Energy bills that never matched the efficiency rating
Rated efficiency assumes correct airflow, correct charge and sealed ducts. Miss any of those and the published figure becomes theoretical.
Coil freezes in mild weather
Low airflow or low charge. In a newly installed system this usually means the duct evaluation never happened and the equipment is strangled.
How an installation runs
Six stages from assessment to handover, with the finished numbers as the closing step.
The sequence below is the same on a straightforward changeout and on a more involved job. What changes is how long each stage takes, and the schedule you are given at quote stage is a range rather than an optimistic single date, because access surprises in older buildings are common.
Assessment and load calculation
Measurements taken on site, calculation worked afterwards, and the result shared with you before equipment options are discussed.
Fixed price scope agreed
Equipment, duct remedial work, permits, line set and condensate handling all itemised. Nothing proceeds until it is approved in writing.
Recovery and removal
Existing refrigerant recovered to a machine rather than vented, and the old equipment removed and disposed of properly.
Fitting and fabrication
Cabinet set level, plenum transitions fabricated to fit the new equipment, line set routed and brazed under a nitrogen purge to keep scale out.
Evacuation and charging
Held under a micron gauge until the target is reached and stable, then charged by weight to factory charge plus line length allowance.
Commissioning and handover
Airflow, static pressure, temperature split, superheat and subcooling and amperages recorded, warranty registered, paperwork handed over.
Why brazing under nitrogen matters
One of several steps that nobody can see afterwards and that determine whether the system reaches its expected life.
When copper is heated to brazing temperature in the presence of air, the inside surface oxidises and forms a black scale. That scale does not stay where it forms. It breaks loose in service and travels with the refrigerant and oil until it reaches the narrowest passage in the circuit, which is the metering device. A partially blocked expansion valve produces a system that underperforms in a way that is genuinely difficult to diagnose two years later.
Flowing a low pressure nitrogen purge through the pipe while brazing displaces the oxygen and prevents the scale forming at all. It costs a bottle of nitrogen and a few extra minutes per joint. It is standard practice in the manufacturer installation instructions and it is skipped constantly, because the consequence arrives long after the installer has left.
Evacuation is the same kind of step. Pulling a system down to a deep vacuum and holding it there boils off moisture and removes non condensable gases. Moisture left in a refrigerant circuit combines with the oil to form acid, which is a slow and certain way to kill a compressor. Doing this properly means a micron gauge on the system and a decay test, not a vacuum pump running for a set number of minutes while somebody tidies up.
Invisible steps that decide system life
- Nitrogen purge while brazing, so no oxide scale forms inside the pipework
- Evacuation to a micron target with a decay test, not a timed pump run
- Charge weighed in and then verified by superheat and subcooling
- Condensate trap primed, secondary path provided and float switch fitted
- Temperature split and static pressure recorded before the crew leaves
Installation questions
Sizing, timing, equipment supplied by you, and what happens if the numbers miss at handover.
Because the calculation reflects the building as it is today rather than as it was when the original was specified, and because rule of thumb sizing tends to round up at every stage. Replacement glazing, added insulation or air sealing work all reduce the cooling load meaningfully.
A correctly sized unit runs longer cycles at lower capacity. That is how it pulls humidity out of the air and how it avoids the clammy at seventy two degrees complaint that oversized systems produce.
Shoulder season, so spring or autumn. The crew is not working in a hundred and thirty degree attic, the schedule is not being interrupted by emergency calls, and there is time to commission the system properly rather than rushing the last hour.
It also means the system is proven before the weather that will actually test it, which is a better position than discovering a commissioning problem during the first heat event.
This one comes up often and the answer is usually no. Coverage on equipment sourced outside the authorized distribution chain is typically refused by the maker, so you would be buying a machine with no parts protection at all, and no contractor can sensibly stand behind the labor on hardware whose shipping and storage history is a blank.
Tell us at the first phone call if a purchase has already happened. There are occasionally workable routes, and they are much easier to find in advance than on the morning the crew arrives.
Then the crew is not finished. Airflow below target sends us back to whatever is restricting the distribution. A subcooling figure outside the band means the charge gets corrected on the scale. Static pressure reading high means something in the ducting has to be opened up before the system is signed off.
This is the whole point of taking the readings. Handing over equipment that sits outside its own nameplate bands simply schedules a warranty visit for later, and that visit would be ours to absorb.
Not always, but it has to be assessed rather than assumed. A line set of the correct diameter for the new equipment, in good condition, correctly insulated and not previously exposed to a compressor burnout can often be reused after a proper flush.
Where the diameter is wrong for the new capacity, where the insulation has degraded, or where there has been a burnout, replacement is the honest answer and it goes in the quote rather than being skipped quietly.
A straightforward changeout on an accessible system is usually a single day. Where duct remedial work, a new line set or a plenum rebuild is involved it extends, and multi day jobs are scheduled as such from the start.
You are given a range at quote stage rather than an optimistic single figure. A date that was honest is worth more than a date that slips twice.
Start with an assessment rather than a catalog
In Peralta, NM the first visit measures the building and the duct system, and sometimes concludes that you should repair instead.
Call with the age and type of the current system, any recent repair history and which rooms never perform properly. That is enough to book the right length of appointment and to say whether this is likely a repair conversation.
If you are holding quotes from other contractors, bring them along. Comparing scope line by line is far more revealing than comparing headline prices, because the differences almost always sit in the items nobody itemises.
Load calculatedRoom by room, assumptions written down
Micron targetEvacuation proven with a decay test
WeighedCharge by scale, verified by subcooling
RegisteredManufacturer warranty filed for you