Installing central air in Sunnyvale, Santa Clara or Cupertino usually means one of two jobs: replacing an aging condenser and coil, or adding cooling to a 1950s–1970s house that was built with only a furnace and ducts in the attic or crawlspace. Either way, a good installation is decided before the old unit comes out — by sizing, duct condition, electrical capacity and where the new equipment will sit.
Start with the old system
If your current AC still runs, the first question is whether it needs replacing at all. A weak capacitor, a clogged filter or a dirty coil can make a sound system look finished, so those are ruled out first (see AC repair). Replacement makes sense when an older system needs a compressor or coil, depends on R-22 refrigerant, or keeps breaking down. Our article on signs you need a new air conditioner covers the usual red flags.
Sizing comes from a load calculation
Tonnage should come from a room-by-room load calculation (ACCA Manual J is the usual method), not from the label on the old unit. The calculation weighs floor area, insulation, window size and direction, ceiling height and where the ducts run.
- Too big: the system short-cycles, leaves rooms uneven and removes less moisture.
- Too small: it runs nonstop on the hottest afternoons and still falls behind.
Large west-facing windows and a thinly insulated attic matter more during a hot spell than square footage alone.
Choosing the type of system
- Split AC with your furnace. A new outdoor unit and an indoor coil designed to work together, sharing the furnace blower and ducts.
- Heat pump. It cools the same way and also heats, which fits our mild winters. Compare it on the heat pump installation page.
- Ductless. For Eichlers, additions and homes without usable ducts, ductless mini-splits avoid ductwork altogether.
- Condos and townhomes. Compact mechanical closets and rooftop or balcony condensers limit equipment size and access, so measurements come before model choice.
Efficiency is rated in SEER2: a higher number means less electricity for the same cooling. Two-stage and variable-speed models run longer at lower output, which evens out temperatures between rooms. See understanding SEER ratings and two-stage vs variable-speed systems.
Ducts, wiring and drainage
The new unit is only as good as what connects to it.
- Ducts: crushed, disconnected or undersized runs in the attic waste capacity. We check them, and the return air path, before settling on a size.
- Electrical: the outdoor unit needs a correctly sized circuit and a disconnect within sight of it. Some older panels need work before they can carry the load.
- Condensate: the indoor coil produces water that needs a clear drain route, and an attic coil needs overflow protection such as a secondary pan or a float switch.
- Outdoor placement: clearance for airflow and service, distance from bedroom windows and the property line, and a level pad. Replacement is a good moment to move a condenser that was crammed against a fence.
Refrigerant and line sets
New residential systems are moving to lower-impact refrigerants such as R-454B and R-32. Whether the existing copper lines can be reused depends on their size, their condition and the new equipment’s requirements; that should be confirmed, not assumed. Our article on refrigerant changes explains the shift.
Installation day
- Floors and walkways are protected, and the refrigerant in the old system is recovered by EPA-certified technicians rather than vented.
- The old equipment comes out; the pad, coil and condenser go in.
- The lines are brazed, pressure-tested for leaks and evacuated with a vacuum pump before the valves are opened.
- The system is started and checked under load: temperature drop across the coil, airflow, drain flow and thermostat setup.
- You get a walk-through: filter size and location, thermostat settings and what normal operation sounds like.
Permits and warranty paperwork
Replacing or adding central air generally needs a city permit and a final inspection. New equipment carries the manufacturer’s warranty, and many makers ask for registration within a set period after installation to give the full term, so keep the model and serial numbers handy. Our HVAC warranty guide explains what to look for.
For a complete heating and cooling replacement, see new installation. City pages: Sunnyvale, Santa Clara, Cupertino.