New AC System: The Right Starting Point

A new cooling system starts with facts about the house, not with equipment brochures. Settling load, duct condition, electrical capacity and refrigerant generation early is what makes competing proposals comparable to each other.

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Glazing Orientation Drives Afternoon Load

West-facing windows deliver their heaviest gain exactly when outdoor temperature peaks, which is why two houses of equal area can need different capacity. Shading and glass type both move the number.

Air Sealing Lowers the Calculated Result

A house tightened since the original system went in has less infiltration to condition, so the correct replacement is frequently smaller. Matching the outgoing tonnage would carry an old assumption forward.

Panel Capacity Is Checked Before Ordering

A new outdoor unit needs a correctly sized circuit, a disconnect and physical room in the load centre. Discovering a full panel on installation day turns one working day into a wait for an electrician.

Two-Stage Equipment Runs Longer at Low Output

Extended run time at reduced capacity removes more moisture and holds temperature more evenly than repeated full-output cycles. The benefit is comfort and humidity rather than any claim about the utility bill.

Scope Lines Explain What Model Numbers Hide

Line set replacement, plenum fabrication, duct sealing at the equipment and permit handling can each be present or missing. Reading those lines side by side is more informative than comparing equipment brands.

A Site Assessment Produces Written Findings

Measured static pressure, duct condition, electrical capacity, line routing and the permit position belong in the proposal. A quote without them is describing equipment rather than the project.

Square Footage Alone Never Produced a Load

Two houses of identical floor area differ by orientation, glazing, insulation and infiltration, and the cooling loads diverge substantially. A calculation that starts and ends with area is a guess wearing a number.

Humidity Complaints Often Follow Oversized Equipment

Short run cycles satisfy temperature quickly and leave latent load untouched, so the air stays damp at setpoint. The fix belongs at selection time, because no thermostat setting recovers dehumidification the run time never allowed.

Panel Space Is Found Before Equipment Is Ordered

A new condenser needs a correctly sized circuit, a disconnect and physical room in the load centre. Discovering a full panel on installation day turns a one-day project into a wait for an electrician.

Two Quotes, Same Tonnage, Different Projects

Line set replacement, plenum work, duct sealing at the equipment and permit handling can each be present or absent. Comparing scope line by line explains differences that the model numbers never will.

The first decision in a new cooling project is not which brand goes in the yard. It is what the house actually needs, expressed as a cooling load and a set of constraints, because every proposal that follows is only as good as those inputs.

Four Facts to Establish First

The cooling load for this building, calculated for the house as it stands today.
Whether the duct system can carry the airflow that load implies, measured as static pressure with the existing blower running.
What the electrical service can support, including the available panel space and the size of the circuit the new equipment will need.
Which refrigerant generation the project is buying into, since equipment now ships with R-454B and that changes accessories, leak detection and future service.

Why Sizing Goes Wrong in Both Directions

Oversizing is the more common error and the harder one to see. A unit with more capacity than the house needs satisfies the thermostat quickly and shuts off, which leaves the coil too little run time to remove moisture. The house ends up cool and clammy, and the compressor accumulates starts that shorten its life. Undersizing produces the opposite complaint: the system runs continuously through the afternoon and still loses ground on the hottest days of July. Neither is corrected by adjusting the thermostat.

What a Useful Site Assessment Produces

A written load calculation naming the assumptions it used for insulation, glazing and infiltration.
Static pressure readings taken at the existing air handler, with a note on which duct sections are the restriction.
An equipment recommendation identified by an AHRI matched combination, so the efficiency figure quoted actually applies.
A description of the line set route, whether the existing copper is being reused, and the test result that justifies that decision.
The permit position for the municipality, and what the inspection will cover.

Comparing Proposals Fairly

Two quotes describing the same tonnage can still describe very different projects. One may include a new line set, plenum modification and duct sealing at the equipment, while the other assumes everything existing carries over. Reading the scope line by line matters more than reading the model numbers, because the differences that affect performance usually hide in what is assumed rather than in what is listed.

Where the Project Should Start Instead

Sometimes a new system is the wrong conversation. Uneven temperatures between floors on a system that otherwise performs are a distribution problem, and zone control systems address it without new cooling equipment. Rooms with no ductwork at all, additions, attics and converted spaces, are usually better served by ductless mini-split installation. Where the existing system has a specific fault and is otherwise sound, a repair assessment settles the question first. When a new central system genuinely is the answer, the path runs through central AC installation.

Two Contractors, Two Different Load Figures

Calculations that differ by a large margin usually differ in their assumptions about insulation, infiltration or glazing. Asking which inputs were used turns an argument about tonnage into a comparison of evidence.

Ceiling Height Changes Volume, Not Just Area

A room with a raised or vaulted ceiling holds considerably more air than its floor plan suggests, and stratification puts the warmest layer where nobody is. Load figures account for it and rules of thumb do not.

Equipment Sound Is Measured, Not Described

Outdoor units carry a published sound rating, which matters on a narrow lot where the cabinet sits under a bedroom window. Placement and mounting affect what actually reaches the neighbouring wall.

Deciding What Happens to the Old Equipment

Removal, recycling and refrigerant recovery are part of the project and belong in the scope in writing. Equipment left on a pad or in a basement becomes the homeowner problem after the crew leaves.

Starting a New Cooling System Project in Chicago

What should be settled before asking for installation proposals?

The calculated cooling load, the measured condition of the duct system, what the electrical panel can support, and which refrigerant generation the equipment uses. Those four turn proposals into comparable documents. Without them each contractor is answering a slightly different question and the quotes cannot be read side by side.

How does oversized cooling equipment show itself in a finished house?

The thermostat is satisfied quickly and the system shuts down before the coil has removed much moisture, so rooms feel cool and damp at the same time. Compressor starts accumulate faster than they should. The symptom is often mistaken for a charge problem and diagnosed repeatedly without resolution.

Which house details change the load calculation most?

Window area and orientation, ceiling and wall insulation, air leakage, and whether attic or basement spaces are inside the conditioned envelope. Occupancy and internal gains matter in smaller houses. Those inputs move the result far more than total floor area, which is why the calculation asks for them.

Do proposals that quote the same tonnage describe the same work?

Frequently not. One may include line set replacement, plenum modification, duct sealing at the equipment and permit handling, while another assumes existing components carry over. The tonnage is the easiest number to compare and the least informative. Scope lines explain performance differences that model numbers hide.

When is new central equipment the wrong answer to a comfort complaint?

When one floor or one room is out of step while the rest of the house holds setpoint, the problem is distribution and zoning addresses it. Spaces with no ductwork are usually better served by ductless equipment. A specific component failure on sound equipment belongs with repair, not replacement.