Most furnace installations in Menlo Park, West Menlo Park, Atherton and Portola Valley are replacements: an aging gas furnace coming out of the attic, a hall closet or the garage of an older ranch home, with ducts and a flue that were built around a different furnace. The furnace itself is the easy part. Sizing, venting, the gas supply, the ducts and access decide whether the new one works well and what the job costs.
Sizing: start with the house, not the old label
Older furnaces were often sized generously, and many houses have since gained new windows, insulation or an addition. With winter lows in the 40s, an oversized furnace heats in short bursts and shuts off: hot and cold swings, more wear, louder starts. A room-by-room load calculation (insulation, windows, air leakage, which way the rooms face) sets the size before anyone compares prices. Two-stage and modulating furnaces suit a mild climate because they spend most of the winter at low fire, with longer and quieter cycles.
80% or 90%+: what each means for your house
| Standard efficiency (80%) | Condensing (90%+) | |
|---|---|---|
| Venting | Metal flue, often the existing one | Plastic pipes through a wall or the roof |
| Condensate | None | Drain line, sometimes a small pump |
| Flue shared with a gas water heater | Usually stays as is | Water heater’s vent must be rechecked |
| Gas use | Higher | Lower |
| Easiest in | Closets and garages with a good flue | Spots with a short path outdoors |
The third row is easy to miss. When a furnace and a gas water heater share one flue and the furnace switches to plastic venting, the water heater is left alone in a flue sized for two appliances. It may need a liner or a smaller vent. That gets checked before the quote, not after.
Attic, closet or garage
Attic. Furnaces lying on their side above the ceiling are common in ranch homes. The work includes getting the old unit out through the hatch, a solid platform, a drain pan and condensate routing for condensing models, and a lit, safe path for future service. A small hatch can limit which furnaces fit.
Hall closet. Space is tight. The furnace needs service clearance, combustion-air openings have to stay open, and the return air often comes through an undersized grille. A replacement is the right time to fix that.
Garage. Easy access, but garage installs follow their own rules for gas appliances, including protection from cars, and the return air must not come from the garage itself.
What the installation includes
- A home heating assessment and load calculation before equipment is chosen.
- A ductwork evaluation: return size, leaks at the furnace connections and static pressure. Attic ducts in older houses are often leaky or crushed; see duct repair.
- A gas line inspection: pipe size for the new furnace, shutoff valve, connector and a leak test.
- Venting: reuse, reline or new plastic pipes, as the table shows.
- Electrical: the furnace circuit and its service switch.
- Safety testing and startup: gas pressure, temperature rise, draft, carbon monoxide in the flue, and a full cycle through every safety control.
- Thermostat setup and programming. If the old one cannot run a two-stage furnace, a new thermostat is part of the quote.
What moves the price
Most installations fall between $2,500 and $8,000. The low end is a standard-efficiency furnace in the same closet on the same flue. The cost rises with a condensing furnace that needs new venting, a variable-speed blower, attic access and platform work, a larger gas line, duct changes and permit fees. If the air conditioner is also near the end of its life, replacing both at once means the coil and refrigerant lines are handled in one job; see AC installation. Estimates for new and replacement systems are free, and financing explains what to compare in any payment offer.
Repair or replace?
Replace when the furnace is near the end of its life, needs a major part such as a heat exchanger or control board on an aging unit, or can no longer keep the house warm. Repeated repairs on old equipment usually cost more than a right-sized replacement. If the furnace is otherwise sound, a furnace repair is the cheaper answer.
Furnace, heat pump or both
If you are replacing an old furnace and an old air conditioner together, price a heat pump next to them. It heats and cools with one outdoor unit. A dual-fuel system, a heat pump backed by a gas furnace, is another path where the electrical panel is not ready for all-electric heat. An electric furnace is rarely the right pick where a heat pump would work, because it uses more electricity for the same heat.
Get these in writing before you approve
- Model numbers for the furnace, coil and thermostat, with efficiency ratings.
- The load calculation result.
- What happens to the venting and to a shared water heater flue.
- Gas, electrical and duct work, each described.
- Permit and inspection, and who arranges them.
- Warranty terms and how the equipment gets registered; our warranty page covers what to keep.
- Anything excluded.