Heat Pump Bill Doubled? Meet Your Auxiliary Heat Strips

What actually happened to the bill

Deep cold moves in for a week, the bill doubles, and people assume the heat pump broke. Usually nothing broke. What happened is that the backup heat ran, and backup heat is expensive by design.

A heat pump is cheap to run because it moves heat rather than making it. It keeps that advantage down to a certain outdoor temperature — the balance point, commonly somewhere around 25 to 35°F depending on the system and how tight the house is. Below that point, the heat pump alone can’t keep up with the house’s heat loss, so the auxiliary heat strips stage in to help.

What the strips are, and why they cost so much

The auxiliary heat is a bank of electric resistance elements in the air handler, typically 5 to 15 kW. They work exactly like the elements in a toaster or a water heater: electric current through resistance wire, heat out. Resistance heat costs roughly three times as much per unit of heat delivered as what the heat pump costs to move the same heat.

So a cold snap that keeps the strips running for days produces a bill that shocks people. That is normal physics, not necessarily a broken system. The heat pump didn’t fail — it reached the edge of its capacity and called in the expensive help.

The EM HEAT trap

Your thermostat has an emergency heat setting. EM HEAT runs the strips only and locks the compressor out entirely. It exists for one situation: the outdoor unit is dead and you need heat until somebody can get there.

It is the most expensive way to heat a house. Leaving it on for weeks by accident — a flip switched during a cold night and never switched back — is a classic bill disaster. If your bill tripled and the outdoor unit looks perfectly fine, check that setting first.

Checks in order

1. The thermostat. Is it set to EM HEAT by mistake? Second question: what is your setback? A big overnight setback makes the system call for the strips every morning to recover. On many thermostats a jump of 4 degrees or more triggers auxiliary heat regardless of outdoor temperature. In deep cold, use smaller setbacks or none at all. A modest setback is cheap; a big one that fires the strips for an hour every morning is not.

2. The outdoor unit. Is it running? Is it iced solid? Is it dead silent when the indoor blower is running? If the heat pump has failed, the strips are carrying the entire house, and every hour of that is expensive. That’s a repair call, not a thermostat setting.

3. Filter and airflow. A clogged filter restricts airflow across the indoor coil and the strip elements, which hurts efficiency and can trip the system’s high-limit safety. Change it. It’s a $10–$30 part.

Doing the math on strip heat

A strip package pulling 10 kW for one hour uses 10 kWh. At typical US residential rates of about $0.12 to $0.18 per kWh, that’s $1.20 to $1.80 per hour of strip heat. Run the strips ten hours a day through a two-week cold snap and you can see where the bill comes from without anybody doing anything wrong.

Compare that to the heat pump running alone, which does the same job on roughly a third of the energy. That gap is why the strips are meant to be a helper, not the primary heat source.

What actually helps

• Use modest winter setbacks, not aggressive ones. Deep setbacks buy back pennies and spend dollars on recovery.
• Don’t touch EM HEAT unless the outdoor unit is down.
• Keep the outdoor unit clear of snow, leaves, and drifts so it can run at full capacity instead of dropping out early.
• Replace the filter on schedule.
• If every winter produces a brutal bill regardless of what you do, the conversation to have with a pro is insulation and air sealing first, then system sizing. An oversized strip package or a house with poor envelope performance will both show up as the same symptom on your bill.

What not to touch

STOP: no DIY inside the air handler around the strip elements. Those elements and their wiring run on 240 volts and stay energized from a breaker you may not have identified. The disconnect and the breaker are the only things a homeowner should touch, and only to shut power off before a tech arrives.

STOP: breaker-panel experiments are not a fix. Do not swap in a bigger breaker to stop it tripping, do not double-tap a breaker to “share” the load, and do not leave a panel cover off. A tripping breaker on a heat strip circuit is telling you something is wrong — oversized elements, a failing sequencer, a loose connection, or undersized wire. That’s an electrician or an HVAC tech, not a bigger breaker.

Bottom line

If the outdoor unit is running and the thermostat isn’t stuck on EM HEAT, a shocking winter bill is usually physics doing its job. Confirm the thermostat setting, confirm the outdoor unit, change the filter, and ease off the setbacks. If the outdoor unit is dead or iced solid, that’s the thing to fix — the strips are just the symptom showing up on your statement.