When a heating or cooling bill climbs, the HVAC system is often part of the story but rarely the whole of it. A rate change, a cold month, more people working from home or a new appliance can all move a bill. The useful question is narrower: is the system using more energy to do the same job it did last year? That shows up as longer run times, rooms that take longer to reach the setting, or equipment that hardly ever shuts off.
Compare like with like
- Put this bill next to the same month last year, not last month.
- Look at usage (therms of gas, kilowatt-hours of electricity), not only the total. Rates change; usage is what the house actually consumed.
- List what else changed: the weather, who is home during the day, a space heater, a hot tub, an electric car.
- Watch the system for a day. Does it run longer than you remember? Does it ever rest on a mild afternoon?
If usage is up and nothing else explains it, the equipment or the ducts are the next place to look.
Where heating and cooling systems waste energy
| Cause | Why it costs more | What usually fixes it |
|---|---|---|
| Dirty filter or coils | Less airflow, longer cycles, lower efficiency | Filter change, coil cleaning |
| Leaky or disconnected ducts | You pay to heat or cool a crawlspace or attic | Sealing and reconnecting duct runs |
| Low refrigerant | The AC or heat pump runs longer for the same result | Leak repair, then the correct charge |
| Heat pump leaning on backup heat | Electric resistance heat does the job instead of the heat pump | Defrost, sensor or thermostat setup repair |
| Thermostat setup | Fan left on On, a poor schedule, a sensor in the sun | Reprogramming, moving or replacing it |
| Wrong-sized equipment | Oversized systems cycle constantly; undersized ones never stop | Duct and load review, right sizing at replacement |
| Aging equipment | Wear lowers efficiency year by year | A repair-or-replace decision |
| A leaky house | Drafts and thin insulation add load the system must make up | Weatherstripping, insulation |
The fan setting is an easy one to miss. With the thermostat fan on On, the blower runs around the clock. Unless you want constant air movement for filtering, Auto costs less. A system that turns on and off every few minutes also wastes energy on each start; our page on short cycling explains why.
Safe checks you can do
- Replace the filter and set a monthly reminder to look at it in heavy-use seasons.
- Walk the house and open supply registers; move furniture and drapes away from return grilles.
- Check the thermostat: right mode, fan on Auto, a schedule that matches when people are home.
- On a heat pump, notice how often “aux” or “emergency heat” appears on the screen.
- Look at any ducts you can see from a garage or crawlspace opening for loose joints and torn insulation. Look only; don’t crawl into tight spaces.
- On a windy day, feel for drafts at doors, windows and outlets on outside walls.
What a technician checks
A high-bill visit is a performance check rather than a hunt for one broken part. We measure the temperature change across the equipment, airflow and static pressure, the electrical draw of the motors and compressor and, on cooling and heat pump equipment, the refrigerant pressures. We look at the ducts we can reach and at how the thermostat is set up. Then you get the findings in plain language and an itemized price for whatever is worth fixing. Sometimes that is a filter and a coil cleaning, sometimes duct repair, sometimes a thermostat that suits the system.
When the equipment is old and a major part is failing, replacement can cost less over the years than repeated repairs. Our article on when to replace an HVAC system covers the signs, and furnace installation and AC installation cover the work.
Turn it off and call when
A high bill is rarely an emergency. But a system that never shuts off at all, a breaker that trips again after a reset, or a burning smell from the furnace or air handler means switch it off and call rather than wait for next month’s statement.
Heating-heavy bills in the North Bay
Winters here are cool and damp, and the heat runs for much of the year, so gas use drives many bills more than summer cooling does. Older Marin homes often have ducts in tight crawlspaces, where a joint that separated years ago can leak warm air for a long time before anyone notices. A yearly maintenance visit keeps filters, coils and settings from drifting, and a maintenance plan adds priority scheduling and discounts on repairs.