Most furnace replacements in San Jose go into the same attic, garage or hall closet as the old unit, and that part is routine. What decides how well the new furnace works is everything around it: its size, the venting it can use, the ducts it feeds and the air conditioner that may share its blower. We check all of that on a free estimate visit and give you a fixed price before any work starts.
Signs it’s time
- The furnace is past 20 years old and needs a major part.
- A combustion test fails or the heat exchanger is cracked. A furnace with a confirmed crack should not be run.
- Repairs keep adding up, season after season.
- Heating bills climb or rooms heat unevenly. Both can be duct problems rather than furnace problems, so we look at the ducts before recommending new equipment.
If yours simply stopped working, start with furnace repair: many no-heat calls end with one part, not a new furnace.
Sizing: why the new furnace is often smaller
Many older furnaces were sized by rule of thumb, and San Jose winters are mild. We run a room-by-room load calculation that accounts for insulation, windows, orientation and where the ducts run, and the answer is often a smaller furnace than the one coming out. An oversized furnace heats in short, loud bursts, leaves rooms uneven and wears its parts faster.
If you have central air, the furnace blower also moves the cooling air, so it has to suit the size of the AC. The indoor AC coil sits on or beside the furnace, and the two cabinets have to line up. If the AC is as old as the furnace, replacing both together, or replacing them with one heat pump, is often the better plan.
80% or 90%+ efficiency: the venting often decides
Standard-efficiency furnaces (80% AFUE) send hot exhaust up a metal flue, so they can usually reuse the existing vent if it is in good shape. That keeps the swap simple, especially in a tight attic.
Condensing furnaces (90% AFUE and up) pull so much heat out of the exhaust that it leaves cool enough for plastic pipe, run out through a wall or the roof. They also make water, which needs a drain and sometimes a small pump. With a two-stage burner and a variable-speed blower they are quieter and steadier, but in a mild climate the gas savings pay back more slowly than in a cold one.
One catch: if the old furnace shares its flue with a gas water heater, taking the furnace off that flue can leave the water heater on a vent too large for it alone. We check the vent and put any liner or resizing in the estimate.
What an installation includes
- Estimate visit: load calculation, duct and venting check, options with prices. Free.
- Permit: we handle it.
- Removal: the old furnace is disconnected and taken out.
- Installation: new furnace, gas connection with a leak test, venting, wiring, condensate drain and filter rack.
- Start-up: gas pressure, temperature rise, a combustion and carbon monoxide check, and blower speed set for your ducts and AC.
- Handover: filter changes explained, plus the paperwork you need to register the manufacturer’s warranty.
What affects the price
Furnace installation in San Jose typically runs $3,500–$12,000. The low end is a standard-efficiency furnace replacing a similar one in an easy closet or garage. Condensing and variable-speed models, attic installs, new venting, return-air or duct changes and a new thermostat move it up. If a utility or regional rebate applies to the furnace you choose, we tell you at the estimate what it requires, though most current California programs for home heating are aimed at heat pumps. The federal tax credit for efficient furnaces ended for equipment installed after December 31, 2025.
San Jose homes we see
- Evergreen tract homes: builder-grade furnaces, often in the garage or attic, are reaching replacement age. Neighbours may share a floor plan, but additions, new windows and insulation change the load, so each house gets its own calculation.
- Cambrian and Willow Glen, 1950s–60s homes: the furnace is often a second or third replacement feeding original or patched ducts, and it may share a flue with the water heater. Duct condition matters as much as the furnace.
- Blossom Valley, 1970s–80s equipment: original furnaces around 40 years old, frequently larger than the house needs. A smaller modern furnace usually carries the house more comfortably.
For North San Jose, see furnace installation in North San Jose.
Until installation day
If the old furnace has to limp along for a few days, run it only while it lights cleanly and keep the filter fresh. If you smell gas, leave the house and call the gas utility or 911 from outside. If a CO alarm sounds, get everyone out and call 911 from outside, and leave the furnace off until it has been checked.