Section R403.1.2 makes backup heat control a mandatory installation item
Section R403.1.2 of the residential IECC is marked Mandatory. That means the heat pump supplementary heat control applies regardless of which compliance path the house uses. The same requirement appears in the IRC energy chapter as N1103.1.2.
This is not a comfort preference. It is an energy code requirement.
The 2015, 2018, and 2021 IECC versions require a heat pump with supplementary electric resistance heat to have controls that prevent supplemental heat from running when the compressor can meet the heating load. Defrost is the exception.
The 2024 edition states the rule as a short list. Supplemental heat may run only when the compressor cannot provide enough heat to satisfy the thermostat setting, the heat pump is in defrost, the compressor cycle malfunctions, or the thermostat malfunctions. Nothing else is on that list.
The 2024 IRC version names electric resistance, fuel gas, and liquid fuel supplementary heating. A dual-fuel heat pump backed by a gas furnace is covered too. This is not limited to strip heat.
The adopted code edition controls the requirement on your job
The newest code book is not automatically the code for your installation. The local jurisdiction decides which edition it adopted. Check that edition before you apply the wording to a job. The adopted code edition matters more than the newest book on your shelf.
The code sets the condition for supplemental heat. It does not give you an outdoor lockout number. It does not tell you to use one universal balance point. The setting comes from the equipment and the installation.
Read the manufacturer’s installation instructions for the heat pump and thermostat. Then consider the capacity at which the system was selected. Whether the compressor can meet the heating load depends on that selection. The relationship between equipment capacity and the load is covered in this explanation of Manual S equipment selection.
That is why copying a setting from another job is poor practice. Two heat pump systems can have different equipment, different loads, and different thermostat configurations.
Factory staging can bring on aux heat when the compressor can catch up
Many thermostats ship with default staging that brings on aux heat after a large enough gap develops between the room temperature and the setpoint. A timer can do the same thing. In both cases, backup heat may run even though the compressor could catch up.
Setback recovery on a cold morning is the classic trigger. The homeowner raises the setpoint. The thermostat sees a large gap. The strips or furnace start. The compressor may still be capable of meeting the heating load, but the factory staging setting has already called for supplemental heat.
If the system uses an outdoor temperature sensor, configure it. If the thermostat has an aux heat outdoor lockout or compressor balance point setting, configure that too. Do not leave those settings at their defaults without checking them against the equipment instructions and the system selection.
Check the W2, AUX, and E wiring against the thermostat configuration. Emergency heat is different from normal auxiliary operation. It is a manual mode that a homeowner can leave on by accident. Explain that mode before you leave the job.
The first cold-week calls often start with a high bill, not a no-heat failure
There is a pattern shops see in the first cold week of the season. Calls from heat pump systems installed that year are rarely, “No heat.” They sound more like, “The heat works, but the bill doubled,” or, “The thermostat says aux heat and it will not go away.”
The house is warm. Nobody thinks the call is urgent. The caller hangs up when the phone rings out and calls whoever answers.
Trace those calls back and a lot of them lead to a thermostat left at factory staging on an installation completed in spring or summer. Nobody had a reason to see backup heat run during the cooling season. The problem stayed hidden until the first real heating demand.
This is also why an unanswered call can become someone else’s service call. The cost of a missed no-heat call is not limited to emergencies. A customer with a warm house and a high electric bill still expects a response.
Dial 72 answers 24/7 and texts the shop a call summary after every call. It triages by the shop’s own rules. No-heat calls in extreme weather, calls involving vulnerable people, or calls involving medical equipment can escalate to the on-call tech. Everything else books a slot.
That means the “aux heat is on and my bill is high” call can book as a daytime visit instead of waking the on-call tech. The summary text tells the tech that the caller has a heat pump and is seeing aux heat. The tech can walk in knowing to open the thermostat installer settings first.
Dial 72 also remembers returning customers. A call from a house you installed that year is recognized as yours.
Put thermostat configuration in the checklist and job file
ACCA’s quality installation standard expects documentation of how the system was set up. The thermostat configuration belongs in that record, not only in the installer’s memory. Use the quality installation standard as the model for the install record.
Change the checklist. Make aux staging a line item. Make the outdoor lockout a line item when the system has an outdoor sensor. Make emergency heat operation and homeowner instruction a line item. Record the settings in the job file.
Before the first cold snap, pull this year’s heat pump installs. Check the thermostat settings on the next visit or maintenance call. Do it before the homeowner sees “aux heat” and starts calling about the bill.
