Furnace replacement in Sunnyvale, Santa Clara and Cupertino usually comes up for one of three reasons: a cracked heat exchanger, a repair no longer worth making on an aging unit, or a plan to replace the furnace and AC together. Before any new equipment is quoted, we confirm the old furnace really needs replacing, because plenty of “dead” furnaces only need an igniter or a pressure switch. If replacement is the right call, the work that matters is sizing, venting and the connection to your ducts and AC coil.
Signs it is time to replace
- A crack or hole in the heat exchanger, or combustion gases where they should not be.
- Parts for the control board, inducer or gas valve are no longer made for your model.
- Repairs keep coming back on a furnace near the end of its service life.
- Repair would cost a large share of a new unit and the furnace is old.
Uneven rooms alone are not a reason to replace. A hallway that heats while bedrooms stay cold often points to duct problems, not the furnace.
Sizing the new furnace
Swapping in the same size as the old unit is common and often wrong. Many homes here have gained insulation and new windows since the original furnace went in. A heat-load calculation using the home’s insulation, windows, orientation and air leakage gives the right output. An oversized furnace short-cycles, heats unevenly and runs louder; an undersized one falls behind on the coldest mornings. The blower also has to suit your ductwork and the cooling coil, so the duct system is part of the sizing, not an afterthought.
Standard or high-efficiency?
| Standard (about 80% AFUE) | Condensing (90% AFUE and up) | |
|---|---|---|
| Venting | Metal flue through the roof | Plastic pipe through a wall or roof |
| Condensate | None | Drain line, sometimes a pump |
| Attic installs | Drain and pan for the AC coil only | Furnace and coil drains, plus a secondary drain pan |
| Worth checking | Flue condition and clearances | Vent route, drain path, water heater venting |
One detail often missed: when a standard furnace shared its metal flue with a gas water heater and is replaced with a condensing model, the water heater is left alone in a flue sized for two appliances. That flue has to be checked and sometimes resized.
Burner staging matters too. A two-stage furnace runs on low fire most of the time in a mild climate, which is quieter and more even than a single-stage unit cycling at full output. Our article on two-stage and variable-speed equipment explains the difference.
Matching the furnace to your AC and ducts
The air conditioner’s indoor coil sits on top of or beside the furnace, so its size and the blower setting must match the AC. Static pressure is measured before and after. Older homes often have one undersized return, and the time to fix that is while the furnace is out. If the AC is near the end of its life, pricing an AC replacement at the same time avoids a second installation later.
What installation day includes
- Removing the old furnace and protecting floors along the path in and out.
- Setting the new furnace and connecting gas, venting, condensate drain, power and thermostat.
- Checking the gas connection for leaks.
- Start-up: gas pressure, temperature rise against the rating plate, flame, a combustion check, safety controls and static pressure, then a full heating cycle.
- A walkthrough: filter size and schedule, thermostat settings and registering the manufacturer’s warranty.
Furnace replacement normally needs a city permit and inspection; the quote should say who handles it.
Furnace or heat pump?
If the AC is also due, compare a furnace-and-AC pair with a heat pump, which heats and cools with one outdoor unit. The heat pump vs furnace guide walks through running costs and electrical needs.
Local notes
Many 1950s–1970s homes in the area have the furnace in the attic or a tight hall closet. Attic installs need a solid platform and a clear service path; closet installs depend on cabinet size and a return air path. Eichler homes usually heat through the slab rather than with a furnace, so see radiant floor heating instead. City pages: Sunnyvale, Santa Clara and Cupertino.