HVAC Replacement Guide: Deciding, Sizing and Scoping a New System

Four signals that say it is time to talk

No single number retires an air conditioner. Four signals together usually do.

• Age and refrigerant. A system past its twelfth Phoenix summer is old by desert standards, whatever the brochure said. If it runs on R-22, every refrigerant repair now carries the cost of a phased-out gas.
• Repair frequency. One capacitor is a Tuesday. A capacitor, then a fan motor, then a contactor, then a leak inside two seasons is a pattern, and patterns end with a compressor.
• Efficiency and bills. If the APS or SRP bill climbs each summer while the thermostat sits at the same 78, the system is working harder for the same result. A dirty coil can do that too, which is why maintenance comes before a verdict.
• Comfort complaints. The west-facing bedroom that never cools, the unit that cycles every eight minutes, the sticky air during monsoon. Some of these are duct problems. Some are sizing problems that a new box will not fix unless someone sizes it correctly.

Add the signals up and read the repair-versus-replace math before anyone quotes you anything.

Why a nameplate is not a size

The fastest way to size a replacement is to read the tonnage off the old unit and order the same. It is also the most common way to get it wrong. The original may have been sized by a builder's rule of thumb in 1998, when the house had single-pane glass and no shade trees. Since then the windows were replaced, an addition went on, the attic gained insulation or lost it, a patio cover went over the west glass. The house is a different thermal load than it was. A load calculation takes the house as it stands, orientation, glass, insulation, infiltration, occupancy, and produces a cooling and heating load in numbers. Only then does equipment get chosen. An oversized unit in a block-and-stucco tract home short-cycles, never runs long enough to wring out monsoon humidity, and wears its contactor out early. An undersized one runs all afternoon and loses ground at 5 p.m.

Scope: the parts of the job that are not the box

The outdoor unit is the part that gets photographed. The rest determines whether it works.

• Ductwork. If the ducts are undersized, leaking into the attic or poorly returned, a new high-efficiency unit underperforms on day one. Static pressure and a leakage check belong in the scope, before the equipment is ordered.
• Electrical. A new unit may draw different amperage and need a different breaker or disconnect. Inverter equipment has requirements of its own.
• Plenum and coil cabinet. New coils are often physically larger. The transition has to be built, not forced.
• Line set. Reused copper must be the right diameter, flushed, and free of the old refrigerant's oil.
• Pad or rooftop curb. A ground unit needs a level, drained pad. A rooftop package unit needs a curb adapter that matches the new footprint and a crane on a day the wind allows it. The rooftop replacement guide describes what that day involves.
• Condensate. A new drain route, trap and overflow protection, because a clogged drain over a ceiling is a drywall repair.

A bid that lists only a model and a price has left these decisions for later, when they cost more.

Efficiency, staging and the bill you actually pay

Higher efficiency ratings pay off in Phoenix because the runtime is enormous; the same rating in a mild climate is a rounding error. But the rating alone is not the question. Under APS and SRP time-of-use plans, the expensive hours are late afternoon and early evening, exactly when a single-stage unit is running flat out. Two-stage and variable-speed equipment can run longer at lower draw, hold a steadier temperature and pull more humidity in August. Whether that premium pays back depends on your house, your rate plan and how long you intend to stay. The staging guide walks through the trade-offs without a sales pitch. Sometimes the right answer is a well-installed mid-tier system and money left over for duct sealing.

What an on-site evaluation covers

Expect measurement. Room dimensions and window areas for the load calculation. Attic insulation depth and duct condition, photographed. Static pressure on the existing system. Electrical service and panel capacity, observed by the technician, not opened by you. The location of the current equipment and whether it should stay there. Your comfort complaints, room by room, because those are data. Then a written scope that names what is included, what is optional and what is excluded, with a flat price for that scope before any work begins. The estimate tool is the first step; the evaluation is the second. A web page cannot size a system. It can tell you what the person who sizes it should be doing.

Paying for it without being sold to

Replacement is a large purchase and it usually arrives uninvited. Financing on approved credit exists so the decision can be made on the merits rather than on what is in checking this month. Utility and manufacturer incentives exist and change often; ask during the evaluation, and see the rebates page linked below for what is current. Neither should push you into a bigger system than the load calls for. If someone quotes you tonnage over the phone, they are quoting the old nameplate, and you now know what that is worth.

When to call

Call when two or more of the four signals are present, or when a single large repair is quoted on an old R-22 system. Call in spring or fall if you can; replacement in a shoulder season means a scheduled crane, a calm roof and a house that is not 90 degrees while the old unit comes out. Call in July if you must, and we will be there the same day. Either way the answer starts with an evaluation, not a brochure.