AC installation, on this page, means adding an air conditioner to a home that already has a gas furnace and ductwork, or replacing a worn-out AC on that same system. Many houses in Menlo Park, Atherton and Portola Valley have a furnace in a hall closet, garage or attic that was put in for heat alone, and owners add cooling once a few hotter spells make the west-facing rooms hard to live in. If your house has no ducts at all, central air installation is the page for you.
What goes in
- An indoor coil on the furnace’s supply side, where the blower pushes air through it.
- An outdoor condensing unit on a level pad.
- A refrigerant line set connecting the two.
- A dedicated 240-volt circuit from the panel, with a disconnect within sight of the outdoor unit.
- A condensate drain for the water the coil pulls from the air, plus a secondary pan and float switch when the coil sits in an attic.
- Thermostat wiring with a cooling conductor. Heat-only thermostats often have just two wires.
- Start-up: charge set by measurement, airflow checked, thermostat set up and the controls explained to you.
Five checks before anyone names a size
- Load calculation. The size comes from the house (insulation, windows, orientation, shade, air leakage), not from square footage or from what was there before. In older ranch homes, patchy attic insulation can change the answer more than the floor area does.
- Furnace blower. Cooling needs more airflow than gas heat; a common rule is about 350–400 cubic feet per minute per ton. A blower chosen for heating may not move enough air for the AC you want.
- Ducts. Heat-only duct systems are often small for cooling, and old attic ducts lose cold air through leaks and thin insulation. A single small return can choke the whole system.
- Electrical panel. Older panels may lack the space or capacity for a new 240-volt circuit.
- Drain route and outdoor spot. Where the condensate goes, and where the outdoor unit can sit with clearance, service access and some distance from bedroom windows.
If one of these comes back weak, the estimate says so and prices the fix, rather than leaving you with a new AC that cannot deliver.
Replacing an existing AC: what stays and what goes
- The outdoor unit is replaced.
- The indoor coil is normally replaced too, so it matches the new outdoor unit and its refrigerant. A new outdoor unit on an old coil is a common cause of poor performance.
- The line set may be reused if its size suits the new unit and it is in good shape; damaged or undersized runs are replaced.
- The pad, disconnect and drain are checked and renewed where worn.
Systems running on R-22 are the most common replacements now. New equipment uses a different refrigerant, and the coil and lines must suit it.
What an estimate covers and what moves the cost
Most installations on an existing furnace and duct system cost $3,500 to $12,000 and take 4 to 8 hours. The low end is a straightforward swap of a smaller unit in an easy spot. The price rises with:
- capacity and efficiency (variable-speed units cost more upfront);
- a coil that needs a transition piece to fit the furnace cabinet;
- electrical work, from a new circuit to panel changes;
- duct work: a larger return, sealed joints, a resized trunk;
- a long line set, a new pad on a slope, or equipment carried down a hillside path;
- an attic furnace with little headroom.
Estimates for new and replacement systems are free. A useful one is in writing and lists:
- the model numbers of the outdoor unit and coil and their efficiency ratings;
- the size and how it was chosen;
- each piece of electrical, duct and drain work included;
- parts and labor warranty terms, and the manufacturer’s registration steps;
- exclusions, and whether permits are included.
We install equipment from the major makers, including Carrier, Trane, Lennox, Daikin, Mitsubishi Electric and Rheem.
AC, heat pump or ductless?
If the furnace is near the end of its life as well, a heat pump fits where an AC would, cools in summer and can carry most of the heating through winters with lows in the 40s. If only one or two rooms overheat, a ductless mini-split may cost less than whole-house cooling. When the current AC is fairly young, a repair may be the honest answer. The estimate can price more than one path.
Install day
The old outdoor unit and coil come out after the refrigerant is recovered. The new coil is fitted to the furnace, the pad is set and the outdoor unit placed. Lines are brazed, pressure-tested and evacuated; power and controls are connected. Then the system is started, the charge is set by measurement, airflow is checked, and you are shown how to run it and where the filter goes.
Local notes
Shade trees help an outdoor unit on hot afternoons, but a unit placed right under one fills its coil with leaves and seed pods; a spot with open sky above is cheaper than frequent cleaning. In large Atherton homes with two systems, owners often replace one at a time, so it is worth having the second system looked at during the estimate to keep the two plans in step. On Portola Valley hillside lots the outdoor unit’s location decides the line length, the noise near bedrooms and how the equipment gets there; see AC installation in Portola Valley.