The summer HVAC habits that save the most energy are simple: keep the thermostat around 78°F when you’re home and higher when you’re out, keep the filter and outdoor unit clean, make sure the ducts deliver the air they should, and shade sunny windows. In Tiburon, Belvedere and Mill Valley, where fog and evening breezes keep many summer days mild, you can add one more: let cool night air do part of the work and save the air conditioner for warm afternoons and the occasional hot spell.
Thermostat settings that lower the bill
Your system runs less when the indoor setting is closer to the outdoor temperature. The U.S. Department of Energy suggests about 78°F when you’re home and awake, and higher when you’re asleep or away.
- Is 72 too low? It’s comfortable, but the system runs longer. Raise the setting a degree at a time and stop where you’re still comfortable.
- Use ceiling fans. A fan lets most people feel fine a few degrees warmer. Fans cool people, not rooms, so switch them off when you leave.
- Turn it up when you leave. Catching up later doesn’t erase the savings: a warmer house gains heat more slowly, so there is less heat to remove over the day.
- Fan on AUTO, not ON. On ON, the blower runs nonstop, uses electricity and can push moisture from the wet indoor coil back into your rooms.
- Let a schedule do it. A programmable or smart thermostat makes these changes for you.
Many California homeowners are on time-of-use electricity rates, where power costs more in the late afternoon and evening. If you are, cool the house a little before the peak window starts, then let the setting drift up during it. Your plan lists its exact peak hours.
Let the cool nights help
Summers here are mild on most days, and the warmest stretches often come in late summer and early fall. When it’s cooler outside than inside, which is most evenings, open windows on opposite sides of the house or run a whole-house fan if you have one. Close windows and shades by mid-morning to hold the cool air in. On many days that alone keeps the AC off until afternoon.
Many hillside homes here have large windows for the view, and afternoon sun through west- and south-facing glass is often the biggest heat load in the house. Exterior shades, awnings or solar screens stop heat before it enters; interior blinds help less, but still help.
Maintenance that keeps the system efficient
- Filter. Check it monthly in summer and replace it when it looks gray or clogged. A dirty filter chokes airflow, stretches run times and can freeze the indoor coil.
- Outdoor unit. Keep a couple of feet clear of plants, leaves and stored items. With the power off, rinse the coil fins gently with a garden hose. Never use a pressure washer.
- Vents and returns. Keep them open and free of furniture and rugs. Closing vents in unused rooms rarely saves energy with a central system; it raises duct pressure and can push more air out of leaks.
- Yearly tune-up. A technician checks the refrigerant charge, cleans the coils, measures airflow and tests electrical parts. A system low on refrigerant or with dirty coils runs longer for the same cooling. Book AC maintenance in spring, before the first warm spell.
Ducts: where cooled air goes missing
In many older homes the ducts run through an attic or a crawlspace. A loose joint or torn flex duct in a hot attic dumps cooled air where nobody feels it. Warning signs are rooms that never cool down, big differences from room to room and ducts you can see are crushed or disconnected.
Sealed joints (mastic or foil tape, not cloth duct tape) and insulated ducts hold cooled air far better. Ask for the accessible ductwork to be looked at during HVAC maintenance so you know whether sealing is worth it.
What SEER2, HSPF2 and AFUE mean for your summer bill
| Rating | What it measures | Does it affect your summer bill? |
|---|---|---|
| SEER2 | Seasonal cooling per unit of electricity, tested under conditions closer to real ductwork | Yes: higher means less electricity for the same cooling |
| EER2 | Cooling efficiency at a fixed hot outdoor temperature | Yes, mostly on hot days |
| HSPF2 | A heat pump’s heating efficiency over a heating season | No, it matters in winter |
| AFUE | The share of a furnace’s fuel that becomes heat | No, a furnace does not cool |
A rating only pays off when the system is sized correctly, the ducts are tight and the refrigerant charge is right.
In a mild climate the cooling load is small most of the time, which suits variable-speed equipment. Inverter-driven heat pumps and ductless mini-splits from makers like Mitsubishi Electric, Daikin and Fujitsu slow down instead of switching on and off at full power. A heat pump cools like an air conditioner and also heats, so it can replace an aging furnace and add cooling at once. See heat pump installation and mini-split installation if you’re weighing one.
Sizing, age and the $5,000 rule
Bigger is not better. An oversized system shuts off before it pulls out much humidity, so it cycles more, wastes energy and wears parts. The right size comes from a load calculation that looks at windows, insulation, orientation and air leaks, not from the size of the old unit.
For the $5,000 rule, multiply your system’s age in years by the repair quote; if the result is over $5,000, price a replacement before you pay for the repair. It’s a rough guide only: it ignores efficiency, refrigerant type and how well the system keeps you comfortable.
If you do replace, utility and state programs sometimes offer incentives for qualifying high-efficiency heat pumps. Programs change, so check before you sign. Tiburon HVAC also offers flexible financing with approved credit.
When a high bill means something is wrong
Settings won’t fix a failing part. Watch for a system that runs almost nonstop on a mild day, air that feels only slightly cool, ice on the refrigerant lines, an outdoor unit that hums while its fan doesn’t spin, or a system that turns on and off every few minutes.
Before you call, check that the thermostat is set to COOL and below room temperature, replace dead batteries, look at the filter and make sure the breaker hasn’t tripped. If you see ice, switch cooling off and run the fan alone until it melts. Don’t open panels: some parts hold an electrical charge even with the power off.
Cost depends on the cause, the part, any refrigerant work and access. AC repairs typically range from $150-600 depending on the issue, and the diagnostic fee of $79-$99 is waived if you go ahead with the repair. You get the price after the diagnosis and before any work begins, and repairs carry a 90-day warranty. For AC repair, call Tiburon HVAC at (650) 360-9230; same-day service is available, and urgent problems are answered around the clock.