Furnace Replacement in San Diego, Sized and Vented Right

A gas furnace in a San Diego home runs a few hundred hours a year, far less than one in Denver, so age alone is not always the deciding factor. The furnaces we replace most often are 15–25 years old, sitting in a hall closet or garage of a 1970s–1990s tract home in Mira Mesa, Poway or Chula Vista, and have reached one of five points:

• A cracked or rusted heat exchanger. This is the one failure that ends the conversation. Combustion gases can enter the supply air, and the part is not economical to replace on an older furnace.
• Repeated ignition or limit failures. A hot-surface igniter or a flame sensor is a cheap fix once. When the furnace keeps locking out after a control board, an inducer motor and a limit switch have all been replaced, the next repair is money that should go toward a new unit.
• Carbon monoxide concerns. A CO alarm that trips when the furnace runs, soot around the burner compartment, or a flame that rolls out of the burner box all call for the furnace to be shut down and evaluated, not patched.
• Age 15–20+ with rising bills or uneven heat. Older single-stage furnaces with belt-drive or PSC blowers heat in short blasts and cost more to run than a modern two-stage or variable-speed unit.
• A new AC coil that no longer matches. When the air conditioner is replaced, the new evaporator coil sits on top of the furnace. If the old blower cannot move enough air across it, cooling performance drops and the coil can ice.

80% vs. high-efficiency condensing furnaces

A standard 80% AFUE furnace sends about a fifth of the fuel's heat up a metal flue, and a condensing furnace (90–98% AFUE) captures most of that heat with a second heat exchanger. The difference in San Diego is less about the gas bill, which is modest for most homes here, and more about what the installation involves:

• An 80% furnace usually reuses the existing metal (B-vent) flue and needs no drain.
• A condensing furnace vents through PVC pipe (often out a side wall or the roof) and produces acidic condensate that needs a drain, a neutralizer in some jurisdictions, and freeze-free routing. In an attic or an interior closet without a nearby drain, this adds cost.
• If the furnace shares a flue with a gas water heater, removing the furnace from that flue can leave the water heater “orphaned” on an oversized vent, which has to be corrected as part of the job.

For a coastal Zone 7 home with a small heating load, an 80% two-stage furnace with a good blower is often the sensible choice. For an inland Zone 10 home in Escondido or El Cajon that heats more mornings a year, or where the venting route is easy, a condensing furnace makes more sense. We price both when the venting allows it.

Furnace and AC coil matching

The furnace blower is also the air conditioner's blower. A new furnace has to move the airflow the AC coil, the refrigerant charge and the ducts were designed for, roughly 350–450 CFM per ton of cooling. We check the coil model against the furnace's blower table, confirm the coil cabinet fits the new furnace footprint, and, if the AC is also at end of life, quote the matched pair so the system is rated as a whole.

The heat pump option

If both the furnace and AC are due, a heat pump replaces both with one outdoor unit and an indoor air handler, and drops the gas connection entirely. San Diego winters are mild enough that a properly sized heat pump heats without backup strips in most homes. It is not automatically the right answer: an electrical panel review, the age of the existing AC and how you heat the house all factor in. See our heat pump vs. AC + furnace comparison.

Sizing

Furnaces in older San Diego homes are frequently oversized, sometimes by double, because a 1970s builder installed one size for the whole tract. An oversized furnace short-cycles, overheats the rooms nearest the supply and leaves far bedrooms cool. We size from a load calculation, not the label on the old furnace. Read how HVAC sizing works.