When a gas or electric bill jumps and nothing about the way you live has changed, the heating and cooling system is one of the first places to look, because it is one of the largest energy users in most homes. The usual causes are a clogged filter or dirty coil, leaky ducts, a thermostat that lost its schedule, a fan left running all day, short cycling, a refrigerant leak, or equipment that has simply aged. Before chasing any of them, make sure the bill is telling you what you think it is.
Step one: read the usage, not the total
- Compare the same month last year, not last month. Heating use climbs as the weather cools.
- Look at the units. Gas is billed in therms, electricity in kilowatt-hours. If the dollars rose but the units didn’t, the change is in the rates, not in your equipment.
- Check the weather. A colder month than last year explains part of a higher heating bill.
- Note which bill rose. Gas up in winter points to the furnace (or the water heater). Electricity up points to an air conditioner, a heat pump, or a blower that never stops.
What the clues point to
| Clue | Likely cause | What it takes |
|---|---|---|
| Gas use up, furnace runs long, rooms slow to warm | Clogged filter, dirty burners, an aging furnace | Filter, tune-up, efficiency check |
| Electric use up in winter with a gas furnace | Thermostat fan set to On, blower running around the clock | Fan back to Auto |
| Heat pump home, “aux” or “emergency heat” showing often | Electric backup heat doing work the heat pump should do | Thermostat setup; heat pump diagnosis |
| System starts and stops every few minutes | Short cycling | Find the cause: filter, sensor, sizing |
| Some rooms never comfortable, dust near registers | Duct leakage into the garage, attic or crawlspace | Duct testing and sealing |
| AC runs all afternoon and cools weakly | Dirty coils or a refrigerant leak | Coil cleaning; leak repair |
| Bills well above similar homes nearby | Poor insulation, air leaks, old equipment | An assessment before any upgrade |
| Temperature never changes on schedule | Program lost after an outage, or stuck on Hold | Reprogram, or a new thermostat |
What you can check this week
- Replace the air filter.
- Check the thermostat program: the right times, not on Hold or Vacation, fan on Auto.
- Make sure registers are open and not covered by furniture or rugs.
- Pull out last year’s bills and compare usage month by month.
- Look at the ducts you can see in the garage, attic or crawlspace for a run that has come apart, is torn, or is blowing air into the space.
When a technician is worth it
If the increase is large and you can’t explain it, a performance check answers what the bill can’t. A technician can test the ducts for leakage by pressurizing them and measuring how much air escapes, measure the furnace’s temperature rise and the system’s static pressure, and on an air conditioner or heat pump check the refrigerant charge and the condition of the coils. That evaluation belongs before any talk of new equipment.
Our advice is to fix leaks and settings first: a new furnace connected to leaky ducts still heats the garage. If replacement does make sense, we explain the numbers and let you decide; see heat pump installation, the repair-or-replace guide and our HVAC cost guide. A yearly check under a maintenance plan keeps filters, coils and burners from drifting back.
Local note
Daly City sits in the fog belt: heating is needed in most months and air conditioning on only a few days, so an HVAC problem usually shows up on the gas bill, or on a heat pump’s winter electric bill. South San Francisco, on the bay side, is a little warmer and less foggy, so cooling plays a slightly bigger part there. Near the coast, salt air corrodes the outdoor coils of air conditioners and heat pumps; a corroded, clogged coil moves heat poorly and runs longer, and regular coil cleaning keeps salt deposits from building up.