What it is

This is a diagnostic problem disguised as a billing complaint. The system cools, the system heats, no breaker's tripping, no safety's faulting — and the customer's electric bill jumped 40%. There's no single dead part to find. Instead, energy is leaking out somewhere as wasted runtime or wasted watts, and the system masks it by still keeping the house comfortable (or close to it).

The trap is dismissing it ("the system's working, must be rates or weather"). Sometimes it is rates or weather — but plenty of times there's a real, fixable fault that doesn't stop the system, it just makes it expensive. Energy = power × time, so the bill goes up if the system draws more watts than it should or runs more hours than it should (or, the worst case, both at once). That framing is your whole diagnostic map.

How it works

Break the bill into the two factors and chase each:

Excess runtime (hours):

  • Lost capacity / efficiency — a dirty condenser, low charge, low airflow, or a tired compressor means the system delivers fewer BTUs per hour, so it runs longer to do the same job. Every "it still cools, just not great" condition is also an energy leak.
  • Auxiliary / strip heat running when it shouldn't — on a heat pump, electric resistance heat is the single biggest hidden energy hog. A stuck reversing valve, a heat pump locked out into emergency heat, a defrost board cycling too often, or aux staging in too aggressively can run kilowatts of resistance heat the customer is paying for while thinking the heat pump is doing the work.
  • Duct/envelope losses — leaky ducts in a hot attic, missing insulation, a return pulling attic air. The equipment runs constantly fighting a load that's partly self-inflicted.
  • Thermostat behavior — a wildly oversized setback recovery, a "hold" left at an extreme setpoint, or a smart-thermostat schedule nobody understands.

Excess watts (power):

  • High amp draw from a fault that doesn't stop the system — high head pressure (dirty condenser, overcharge) makes the compressor pull more amps the whole time it runs. A weak capacitor or low voltage raises current. None of these necessarily trip anything; they just cost money every minute.
  • Simultaneous heating and cooling — the nightmare: a control or wiring fault, or a stuck relay, that energizes heat and cool at the same time, or a heat pump running compressor cooling while strip heat is also on. The house may even feel okay while the meter spins.
  • Parasitic / always-on loads — crankcase heaters that should cycle but run continuously, a condenser fan or blower that never shuts off, auxiliary equipment.

In the field

Treat it like any other diagnosis — measure, don't assume:

  1. Interview for the timeline. When did the bill jump — sudden or gradual? Sudden points at a discrete fault (stuck relay, reversing valve, aux-heat lockout). Gradual points at creeping efficiency loss (slowly fouling coil, slowly leaking charge) or a seasonal/rate change.
  1. Clamp everything and add it up. Compressor amps, condenser fan, indoor blower, and — critically — the electric heat / aux strips. Multiply by voltage for a rough wattage. Now you can see where the load actually is. Strips pulling kilowatts while the customer thinks the heat pump is running is a smoking gun.
  1. Verify only one mode runs at a time. Force a cooling call and confirm no heat (gas valve closed, no strip amps, reversing valve correct). Force a heat call and confirm the right heat source. Catching simultaneous heat+cool here explains a doubled bill instantly.
  1. Read the refrigerant side for efficiency loss. High head from a dirty condenser or overcharge raises amps. Low charge / low airflow drops capacity and stretches runtime. Bring the system to spec and the watts-and-hours both improve.
  1. Check the heat pump's mode logic. Is it actually running as a heat pump, or has it defaulted/locked into emergency (all-electric) heat? Is the reversing valve doing its job? Is defrost cycling far too often (running strips each time)? A heat pump that's secretly heating with resistance every cycle can triple a winter bill.
  1. Look at runtime and the thermostat. A connected stat or the equipment's own runtime history tells you if it's running 18 hours a day. Check the schedule, the setpoints, any "hold," and whether aux heat is set to engage too eagerly.
  1. Eyeball the ducts and envelope if the equipment checks out. Disconnected duct in the attic, a crushed flex run, a return-side leak pulling 130°F attic air — the equipment runs fine and runs forever.

Normal values & targets

Orientation only — you're comparing measured draw to nameplate and looking for things that shouldn't be on:

  • Electric strip heat: roughly 5 kW per 240V/~21A bank; a 10 kW package pulls ~40+ amps. If strips are energized when only the heat pump should be running, that's the bill right there.
  • Compressor amps: below nameplate RLA when healthy. High head from a dirty coil can push amps up 10–20%+ — pure waste.
  • Crankcase heater: small (tens of watts) and usually meant to cycle off during run; a stuck-on one is minor but real.
  • Runtime: a right-sized system in design weather runs long but not constantly. Near-100% duty cycle in mild weather signals lost capacity or a load problem.
  • Simultaneous heat+cool: should be zero. Any strip or gas heat during a cooling call (or vice versa) is a fault.

Common faults & what they mean

  • Bill jumped suddenly in winter, heat pump "works": suspect it's running on strip/emergency heat — stuck reversing valve, aux lockout, or a control defaulting to electric heat. Clamp the strips.
  • Bill crept up over a season, cooling a little weak: efficiency loss — fouling condenser, slowly leaking charge, dirty filter/coil. Restore capacity.
  • High bill + high head + high compressor amps: the system pays extra watts every minute it runs. Dirty condenser or overcharge. Fix heat rejection.
  • Both heat and cool draw current on a single call: stuck relay/contactor or miswired control. Simultaneous operation. Find and fix immediately.
  • System runs nearly 24/7: lost capacity, gross oversizing on a mild day (short-cycling waste), or a duct/envelope load. Measure capacity and runtime.
  • Bill high but everything measures perfect: it may genuinely be rates, weather, a new load in the house (space heaters, hot tub), or occupancy change. That's a legitimate conclusion — after you've measured.

Tech tips & gotchas

  • Energy = watts × hours. Always ask which one moved. It focuses the whole call. A discrete jump = a watts problem (something turned on that shouldn't be); a gradual climb = an hours problem (efficiency).
  • Hidden strip heat is the #1 winter mystery bill. A homeowner can't tell resistance heat from heat-pump heat — both blow warm air. Your clamp meter can. Always check what's actually carrying the heating load on a heat pump.
  • The system "working" doesn't mean it's efficient — and "comfortable" can hide simultaneous heat/cool. A dirty condenser still cools, at a 15–20% amp penalty; if heat and cool fight each other the house can sit at setpoint while the meter screams. Comfort and efficiency are different questions — always verify single-mode operation.
  • Don't forget non-HVAC loads. A new freezer in the garage, a failing fridge, an electric water heater element shorted to a constant-on state, or a pool pump can be the real culprit. Rule the HVAC in or out, but stay honest about what's actually on the panel.
  • Get a baseline. If the customer has prior bills or the equipment logs runtime, compare. "High" needs a reference point.

Safety / code notes

  • Clamping strip-heat and compressor circuits means working in live high-current panels — rated meters, PPE, one hand clear.
  • Simultaneous heat+cool from a wiring fault can overheat equipment and is a real hazard, not just a cost issue — correct it promptly.
  • If you find resistance heat or any circuit drawing more than its rating, check the conductor and breaker sizing against the applicable electrical code section before assuming it's "just" an efficiency issue.
  • Don't disable safeties or aux-heat lockouts to chase a number; diagnose the control logic instead.