In Daly City, replacing a furnace usually means a gas furnace in the garage of a house built in the 1940s–60s, often sharing a flue with the water heater and feeding ducts that rise into the rooms above. Other jobs add forced air to a home heated by wall heaters, electric baseboard or aging radiant hot-water heat, or heat a new build. Either way the work follows the same list: a load calculation, the right size and efficiency, venting that suits the house, gas and electrical connections, a tight tie-in to the ductwork, and a start-up that is measured rather than assumed. We install gas and electric furnaces in Daly City, South San Francisco, Colma and Brisbane.
When replacement makes sense
Age alone isn’t a reason to replace a furnace that still heats safely. These are:
- A cracked heat exchanger. The one case that isn’t a judgment call; see cracked heat exchanger.
- Repairs every season on a furnace past about 20 years, when the bills keep adding up.
- A house that never gets warm even after filter and duct fixes, sometimes because the furnace was sized before an addition or a garage conversion.
- A planned change: higher efficiency, a different fuel, or forced air where there was none.
If a repair is the better answer, we say so; furnace repair covers that side.
Sizing for a fog-belt house
Heating here is a long, gentle job rather than a short, hard one: mild winter lows, but heat needed in most months. We size from a room-by-room load calculation (walls, windows, insulation, air leakage, orientation), not from the old furnace’s label. Many original furnaces are oversized, and an oversized furnace heats in loud, short bursts, leaves the far rooms cool and wears its ignition parts faster. A two-stage or modulating furnace suits this climate, because it spends most of its time on low fire, running longer and quieter cycles.
The ducts get the same scrutiny. A new furnace on leaky or undersized ducts won’t deliver what it is rated for, so we check static pressure and the main runs before quoting. Where a house has no ducts, the garage below the living space often gives an easy route: sheet-metal supply runs through the garage ceiling to floor registers in the rooms above, with the ceiling’s fire separation kept intact.
Standard or condensing
| Standard efficiency (about 80% AFUE) | Condensing (90% AFUE and above) | |
|---|---|---|
| Venting | Metal flue, often shared with the water heater | Plastic pipe, usually out a side wall or the roof |
| Condensate | None | Needs a drain line or a small pump |
| Combustion air | Drawn from the room it sits in | Often piped from outdoors (sealed combustion) |
| Typical fit | Like-for-like swap on a sound flue | When the vent can be rerouted and a drain is close |
AFUE is the share of the fuel’s energy delivered as heat over a season. Two details matter in older garages. When a condensing furnace leaves a shared flue, the water heater is left alone on a flue sized for two appliances and may need a liner or a smaller vent to draft properly, so we check it before quoting. And sealed combustion keeps the burners from drawing garage air, where paint and solvent fumes collect.
Garage installations follow extra rules
- The burners and igniter must sit at least 18 inches above the garage floor, usually on a platform, because gasoline vapor is heavier than air and settles low.
- Where a car could reach it, the furnace needs protection from impact.
- Return air can’t come from the garage, and ducts that pass through it can’t open into it.
- Gas furnaces installed here must meet BAAQMD limits on nitrogen-oxide emissions, and the district has adopted stricter zero-NOx standards for the years ahead. We’ll tell you where the rules stand when we quote, and price a heat pump alongside if you’d like to compare.
What the price covers
A furnace installation typically runs $3,500 to $12,000 per system. The low end is a standard-efficiency furnace swapped like-for-like onto sound venting and ducts. The price climbs with a condensing furnace and new venting, two-stage or variable-speed equipment, a new location, a larger gas line, electrical changes, duct modifications and a new thermostat. The written estimate lists each item, we review it with you before work begins, there are no hidden fees, and the installation carries a warranty on parts and labor.
Installation day
- Gas and power off, floors and the garage protected, the old furnace removed.
- The new furnace set and connected to the supply plenum and return, with every joint sealed.
- Venting, gas piping (leak-tested), wiring, condensate drain and thermostat.
- Start-up: gas pressure, temperature rise across the furnace, combustion and draft, and each safety control checked.
- A walk-through of the filter, the thermostat and anything we noticed in the ducts.
Still deciding? The repair-or-replace guide and the mid-century home guide walk through the trade-offs.