Bellwether MechanicalBaton Rouge, Louisiana (225) 555-0148
A raised Louisiana house under live oaks at dusk, every window lit, with a condenser unit on a pad at the near corner.

Baton Rouge & Ascension Parish · LMP #2087

It is 96° and 89% out there.
We decide what it is in here.

Bellwether Mechanical designs residential air systems on paper before anybody carries equipment up a ladder. Room-by-room load calculation, duct sized to it, and a drawing set you keep.

Request a load calculation (225) 555-0148
Design standard
ACCA Manual J / S / D
Climate zone
2A — hot, humid
Drawing set
Issued with every install

Most air conditioning in this parish is sized by looking at what was there before.

Why that matters here

A unit that is too big cools the air fast, satisfies the thermostat, and shuts off before it has pulled any water out. You end up at 74° and still damp, with a system that short-cycles itself to an early grave. In a dry climate you might never notice. On the Gulf coast you notice by June.

We start with the arithmetic instead. It takes about two hours in your house and it decides everything that comes after.

SHEET A-100  —  REV 3

Five decisions, in the order we make them

Most of what goes wrong with a Louisiana air system was decided before the equipment was bought. Scroll the drawing. This is the same sequence we work through at your kitchen table, on paper, with you.

Exploded drawing of a whole-home air systemAn isometric cutaway of a single-storey Baton Rouge house with the roof lifted clear. It shows the west-facing windows and the heat they admit, the outdoor condenser and its refrigerant lines, the indoor air handler and supply plenum, the sheet-metal trunk in the attic, three branch runs down to ceiling registers, and the path conditioned air takes into each room.
FIG. 1 Whole-home air system, cutaway — roof lifted for clarity

  1. We measure the building, not the old unit.

    Square footage, orientation, glazing, insulation, and how leaky the place actually is. A 1968 slab ranch with west glass on the long wall behaves nothing like the same square footage facing north.

  2. In this climate the second number is the one that matters.

    Sensible — heat 28,400 BTU/h
    Latent — moisture 11,900 BTU/h

    Nearly a third of a Baton Rouge cooling load is pulling water out of the air. Equipment picked on square footage alone almost never accounts for it.

  3. Sized to the calculation. Usually smaller than you expect.

    40,300 BTU/h total puts this house on a three-and-a-half ton condenser matched to an indoor coil that is actually rated with it, not a mixed pair that happened to be on the truck. Right-sized equipment runs longer cycles, and long cycles are what dry a house out.

  4. Variable speed, because run time is the whole point.

    We commission the blower below the factory default airflow per ton. Colder coil, more water off it, drier air in the room. The plenum is built to the drawing, sealed with mastic, and the pan drains somewhere you can see it fail.

  5. Every branch sized for the room it feeds.

    Manual D: trunk, take-offs and registers calculated per room instead of splitting the supply evenly and hoping. This is the step most replacements skip, and it is why the back bedroom sits eight degrees off the hallway.

Sheet index

What we do

  1. A-101 Load calculation & system design Room-by-room Manual J, equipment selection, duct design. The two hours that decide the next fifteen years.
  2. A-102 System replacement & installation Matched equipment, a plenum built to the drawing, and a commissioning report at handover.
  3. A-103 Duct design, sealing & repair Static pressure tested, leakage measured, branches resized. Often the cheapest comfort you can buy.
  4. A-104 Humidity control For the house that hits 72° and still feels like a locker room. Usually an airflow problem, not an equipment problem.
  5. A-105 Repair & maintenance Diagnosis with the gauges on, a written finding, and a price before we start.
  6. A-106 The drawing set What you are handed at the end, and why the next contractor will thank us for it.
An attic air handler on a raised platform with a straight sheet-metal trunk line and evenly spaced insulated branch ducts.

Fig. 2 — supply trunk, attic install

Straight trunk, take-offs on the calculated centres, every joint mastic-sealed rather than taped. Duct tape fails in an attic within about four summers. The difference costs a few hundred dollars and shows up on the power bill for as long as you own the house.

Issued at handover

Three things you keep

Every installation leaves behind paperwork that outlives us. If you sell the house, it goes in the folder with the survey and the roof warranty.

  1. The load calculation

    Every room, its heat gain and its moisture gain, the assumptions we used, and the total the equipment was chosen against. If a future contractor disagrees with our arithmetic, they can see it.

  2. The duct drawing

    Trunk, branch sizes, register locations and the airflow each one was designed to carry. Pinned inside the air handler closet as well as handed to you.

  3. The commissioning report

    Measured static pressure, airflow, superheat and subcooling on the day we finished, with the technician's name on it. Numbers, not a checklist of ticks.

They told me I needed a smaller unit than the one I was replacing and I thought they were talking themselves out of money. The house has not been above 50% humidity since. First August in nine years my wife has not complained.

Ray D. · Zachary · three-and-a-half ton changeout, 2025

Where we work

Baton Rouge · Prairieville · Gonzales · Zachary · Central · Denham Springs

Roughly a forty-minute radius of the shop on Choctaw Drive. We keep it that tight on purpose, because a design-build job needs us back two or three times before it is finished.

Start with the arithmetic.

A load calculation takes about two hours at the house and costs $285, credited in full against the installation if you go ahead with us.

Request a load calculation (225) 555-0148
Call (225) 555-0148