Why is my heat pump's aux heat running so much?
If you've ever glanced at your thermostat on a cold morning and seen the words "aux heat" — and then opened a startling winter hydro bill — this article is for you.
What aux heat actually is
A heat pump moves heat from outside air into your home, and it does this remarkably efficiently: for every unit of electricity it uses, it typically delivers two to four units of heat. Auxiliary heat (also called backup, resistive, or strip heat) is different: it's essentially a giant toaster inside your air handler. It delivers exactly one unit of heat per unit of electricity — a third or a quarter of the heat pump's efficiency.
Aux heat exists for good reasons: extreme cold beyond the heat pump's capacity, defrost cycles, and rapid recovery after a big setback. The problem is when it runs outside those moments — because every unnecessary hour of aux is heat you're buying at three or four times the price you needed to pay.
Why it runs when it shouldn't
In the homes we've studied, unnecessary aux heat almost always traces back to configuration, not equipment failure:
- The lockout temperature is set wrong (or not set). Many systems can be told "don't use aux above −5 °C." If that threshold is missing or set too warm, aux runs on mild days the heat pump could handle alone.
- Big thermostat setbacks. Dropping the temperature at night and asking for a fast morning recovery is precisely the situation where many thermostats reach for aux. A gentler schedule often heats the same house for less.
- The thermostat's "comfort" logic. Some smart thermostats aggressively use aux to close a temperature gap quickly. Comfortable, yes — and expensive in a way nothing on the screen tells you.
- Plain misconfiguration after install. Settings left at defaults, outdoor sensors not connected, staging thresholds never tuned to the house. The installer's incentive ends when the system heats; nobody's job is to make it heat cheaply.
How to tell if it's happening to you
The thermostat won't tell you what aux cost you. But your own hourly usage data will — it's free from your utility (we wrote a step-by-step guide to downloading it). The signature is distinctive: on cold days, look for abrupt jumps of several kilowatts that appear on top of your normal usage and last from minutes to hours. A heat pump alone draws a relatively smooth few kilowatts; aux strips add big rectangular blocks on top.
If those blocks appear on mildly cold days — say, above −10 °C — there's a real chance your system is misconfigured, and the fix may be a settings change rather than any new equipment.
The honest caveat
Sometimes aux heat is doing exactly its job: genuinely extreme cold, an undersized heat pump, a defrost cycle. The point isn't that aux is bad — it's that you deserve to know whether yours is running for good reasons, and what it's costing you either way. That's a question your own data can answer — and answering it is exactly what a Kettle audit does.