Replacing a furnace in Tiburon, Belvedere or Mill Valley comes down to three questions: how efficient it should be, how big it needs to be, and how it will vent and fit in the space you have. Heating runs far more hours here than cooling, especially in shaded Mill Valley homes, so the furnace you choose either earns its keep or doesn’t in most months of the year.
80% or 95%+ AFUE?
AFUE (annual fuel utilization efficiency) is the share of the gas that ends up as heat in your home. An 80% furnace sends the rest up a metal flue as hot exhaust. A condensing furnace rated 95% or higher pulls extra heat out of the exhaust with a second heat exchanger, so the exhaust is cool enough to vent through plastic pipe, and it produces water that needs a drain.
| 80% AFUE | 95%+ AFUE (condensing) | |
|---|---|---|
| Venting | Metal flue or lined chimney, usually through the roof | Plastic pipes, often out a side wall or the roof |
| Condensate | None | Needs a drain or a small pump |
| Fits well where | The vent can’t be rerouted; the furnace runs few hours | Long heating season and a practical vent route |
| Running cost | More gas for the same heat | Less gas for the same heat |
For most homes here we recommend 95%+, because the furnace runs long enough for the efficiency to count. The exception is a furnace in a location where a new vent route or a drain would be very difficult; there an 80% model can be the sensible choice.
Sizing and staging
The size comes from a heat-loss calculation for your house: walls, windows, insulation, air leakage and the rooms you heat. Older furnaces were often oversized, and simply matching the old capacity carries that mistake forward. A single-stage furnace is either fully on or off. A two-stage or modulating furnace runs at low fire most of the time, which means quieter operation, more even rooms and fewer cold spots at the far end of long duct runs. That matters in larger hillside and waterfront homes with several zones. A variable-speed blower adds quieter airflow and works well with zoning and whole-house air cleaners.
Where the furnace goes
Furnaces here sit in garages, closets, attics and crawlspaces, in upflow, downflow or horizontal configurations. Access and clearance change the job: an attic or crawlspace install takes more time than a garage swap, and older homes sometimes need the gas line, return air or wiring brought up to current requirements. We note all of this on the first visit so it appears in the upfront price rather than as a surprise on installation day.
Installation day
- The old furnace is disconnected, removed and hauled away.
- The new furnace is set, leveled and connected to the ducts with sealed transitions.
- Gas piping, venting, the condensate drain and electrical are connected.
- The thermostat is installed or rewired.
- Start-up: gas pressure and temperature rise are set to the manufacturer’s specifications, safety controls are tested, and combustion and venting are checked for carbon monoxide.
- Walkthrough: filter location and size, thermostat settings, and what normal operation sounds like.
Most replacements are finished in one day.
What affects the cost
The efficiency level, single-stage versus two-stage or modulating, the blower type, the venting and drain work, the location and access, and any duct or gas-line changes. You get a written price before any work starts, and financing is available with approved credit. If you also want cooling, adding air conditioning or replacing the pair with a heat pump is easier to plan at the same time.
Local pages
See furnace installation in Tiburon, Belvedere and Mill Valley. If your current furnace may still be repairable, start with furnace repair, and for boilers and other heating systems see heating repair.