Before you add square footage
Adding a room changes the load, not just the thermostat
In Jacksonville a large part of the cooling job is pulling water out of the air. That is why the wrong equipment can hold the set point and still leave the house feeling damp.
Before you add square footage
Adding a room changes the load, not just the thermostat
In a hot-humid climate a large part of the cooling job is removing moisture. This works out both halves for the area you are adding, so the conversation with your mechanical contractor starts from a number.
This is an orientation figure, not a Manual J. A real calculation is done room by room on the finished plan and accounts for infiltration, duct location, internal gains and shading.
Estimated cooling load
1.5 tons
- Sensible
- 0 BTU/h
- Latent, moisture removal
- 0 BTU/h
- Total
- 0 BTU/h
- Design airflow
- 0 CFM
- Works out at
- 0 sq ft per ton
Square feet per ton is a myth worth retiring
The old rule of 500 or 600 square feet per ton was a guess that happened to work on a narrow band of houses. A tight new addition in Jacksonville can run past 800 square feet per ton, and a leaky 1960s room with a west glass wall can need under 400. The only defensible answer is a room-by-room Manual J on the finished plan.
Latent load is the part people miss
In a hot-humid climate a meaningful share of the cooling job is removing moisture rather than lowering temperature. Oversized equipment satisfies the thermostat quickly, short cycles, and never runs long enough to dehumidify. The house feels cold and clammy. That is the classic symptom of a system sized by square footage.
Bigger is worse, not safer
Oversizing costs more up front, short cycles, wears the compressor, leaves humidity high and makes rooms uneven. Where the calculated load lands between two nominal sizes, a variable-capacity system is usually the better answer than rounding up.
The duct is a separate question from the box
Extending an existing trunk into an addition only works if the trunk has static pressure headroom and the air handler has the capacity. Most 1990s systems do not. We measure total external static pressure before we recommend anything, and a reading well above the equipment's rated value means the duct is the problem, not the box.
Return air decides comfort
Adding supply without a return path is the most common defect we are called to fix in converted garages and bonus rooms. Air that cannot get back to the air handler does not move. A transfer grille, a jumper duct or a dedicated return is not optional.
When a separate zone beats extending the system
If the addition is far from the air handler, used on a different schedule, or the existing equipment is within a ton of its limit, a dedicated mini-split usually costs less than re-ducting the whole house and it gives the new space its own control.
Numbers we measure before recommending anything
| What | Typical value | Why it matters |
|---|---|---|
| Airflow per ton | Around 400 CFM | Typical design airflow for a cooling system in this climate; some equipment is designed lower for humidity control. |
| Total external static pressure | 0.5 in. w.c. rated | Most residential air handlers are rated at 0.5 inches of water column. Measured readings of 0.9 or higher are common and mean the duct is strangling the equipment. |
| Supply trunk velocity | 700 to 900 FPM | Push much past that and you get noise at the registers and pressure loss along the run. |
| Return grille face velocity | Under 500 FPM | Undersized returns are the most common cause of a noisy system. |
Diagnosis
What the symptom usually means
Cold but clammy in August
Almost always oversized equipment short cycling. It hits the set point before the coil has run long enough to condense meaningful moisture.
One room never cools
Usually a return air problem, an undersized branch, or a duct run that is too long and too restrictive for the pressure available at that point in the trunk.
Noisy registers or a roaring return
Face velocity too high. The grille is undersized for the airflow, which is often a symptom of a system fighting high static pressure.
Condensation on registers or ceilings
Humid air reaching cold surfaces, which points at duct leakage in an unconditioned attic, poor insulation on the duct, or an envelope leak.
Runs constantly and never catches up
The opposite problem: undersized for the real load, or a duct system that cannot deliver the airflow the equipment is rated for.
HVAC questions
Questions
What clients ask us about equipment
Only if the trunk has static pressure headroom and the equipment has spare capacity. We measure total external static pressure before recommending anything. Readings well above the air handler's rated value mean the duct is already strangling the system, and adding a branch makes the whole house worse rather than just the new room.
Because oversizing is the most common comfort failure in this climate. Oversized equipment satisfies the thermostat fast, short cycles, and never runs long enough to remove moisture. The house ends up cold and clammy, the compressor wears out sooner, and rooms are uneven. Where the load lands between two nominal sizes, variable capacity is usually the right answer rather than rounding up.
It is a room-by-room heat gain and loss calculation on the finished floor plan, accounting for orientation, glazing, insulation, infiltration, duct location and internal gains. Yes, on any project that adds conditioned area or changes the envelope. Ask to see it. Any contractor who sizes from square footage alone is estimating your comfort rather than calculating it.
Air pushed into a closed room has to get back to the air handler. Without a transfer grille, a jumper duct or a dedicated return, the room pressurises, airflow stalls and it runs hot and stale. It is the single most common defect we are called to fix in converted garages and bonus rooms, and it is cheap to design in and expensive to retrofit.
Yes, and often the best one. For a room that is far from the air handler, used on a different schedule, or being added to a system already near its limit, a dedicated ductless system usually costs less than re-ducting the house and gives that space its own control. We have installed them on garage conversions and sunrooms where the alternative was opening finished ceilings.
Bring us the plan before the equipment
We run the load on the finished drawing and measure the existing system before anyone quotes a box.