Ontario Heat Pump AUX Heat: What It Means, What It Costs, and When to Worry
If you live in Ontario and heat your home with a heat pump, you may eventually notice AUX Heat, Auxiliary Heat, EM Heat, or Emergency Heat on your thermostat.
For many homeowners and renters, the first reaction is concern: Is the heat pump failing? Is an electric heater running? Is this why the hydro bill suddenly jumped?
Sometimes the answer is completely normal. At other times, extended backup-heat operation can point to a thermostat setting, airflow problem, equipment issue, or simply a heating system that is relying heavily on expensive electric resistance heat.
The first thing to understand is that AUX Heat and Emergency Heat are not the same thing. The second is that not every heat pump uses electric heat strips as its backup source. Some Ontario homes use a gas furnace, propane, oil, or another supplementary heating system.
This guide explains what AUX Heat means, how it differs from Emergency Heat, when auxiliary heat is normal, what a 10 kW electric heat strip costs under Ontario's Time-of-Use (TOU), Ultra-Low Overnight (ULO), and Tiered electricity rates, and when it may be time to call an HVAC technician.
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| Ontario Heat Pump |
1. What Does AUX Heat Mean on a Heat Pump?
AUX is short for auxiliary heat. It refers to a secondary heating source that helps the heat pump when the heat pump alone cannot meet the home's heating demand quickly enough.
An air-source heat pump does not create heat the same way an electric baseboard heater does. Instead, it uses electricity to move heat from the outdoor air into the home.
As the outdoor temperature falls, two things generally happen at the same time: the heat pump has less heating capacity available, while the house itself loses heat more quickly.
Natural Resources Canada describes the point where a heat pump's available heating capacity matches the building's heat loss as the thermal balance point. Below that outdoor temperature, supplementary heat may be required to meet the full heating load.
That means seeing AUX Heat during an Ontario cold snap can be part of normal operation. A brief AUX call by itself is not proof that the heat pump is malfunctioning.
Do not assume that “AUX Heat” automatically means electric heat strips. Your backup source may be electric resistance heat, a natural-gas furnace, propane, oil, or another heating system. Identify the backup heat source before estimating operating cost.
2. AUX Heat vs. Emergency Heat: What's the Difference?
The terms are often confused, but they generally describe different operating modes.
| Thermostat Display | Typical Operation | How It Starts |
|---|---|---|
| Heat | Heat pump provides normal heating | Automatic |
| AUX Heat | Supplementary heat assists when needed | Usually automatic |
| Emergency / EM Heat | Backup heat becomes the primary heating source | Usually manually selected |
On compatible systems, Auxiliary Heat can be called automatically by the thermostat or equipment controls when additional heating capacity is needed.
Emergency Heat is different. It is generally a manually selected backup mode used when normal heat-pump operation is unavailable or should not be used. On many systems, selecting Emergency Heat stops normal outdoor-compressor heating and relies on the backup source instead.
Google Nest, for example, distinguishes the two by explaining that its thermostats can call Auxiliary Heat automatically, while Emergency Heat must be manually turned on.
However, thermostat labels and control logic vary by manufacturer. A dual-fuel system can also behave differently from an all-electric heat pump. Your equipment manual and installer configuration are the final authority for your specific system.
3. When AUX Heat Can Be Completely Normal
During very cold outdoor temperatures
As the outdoor temperature drops, an air-source heat pump generally has less heating capacity available while the home's heating demand rises.
Modern cold-climate heat pumps are designed to operate at low temperatures, but that does not mean every system can meet 100% of a home's design heating load at every outdoor temperature without backup heat.
Depending on the equipment, house, sizing, and control strategy, supplementary heat may be expected below a certain temperature.
During recovery from a large thermostat setback
Suppose you lower the thermostat to 17°C overnight and then raise it to 22°C in the morning. Some thermostats interpret that large temperature gap as a need for faster recovery and bring on auxiliary heat.
For homes with electric resistance backup, deep setbacks followed by aggressive morning recovery can sometimes increase heat-strip use.
A heat pump also does not heat faster simply because you raise the thermostat several extra degrees. The control system may simply call for more heating stages.
During a defrost cycle
In heating mode, frost can build up on the outdoor coil. The heat pump periodically runs a defrost cycle to clear that frost.
During defrost, some systems energize auxiliary heat so the air coming from the supply vents does not feel excessively cold.
A short burst of backup heat during defrost is very different from AUX Heat staying on for hours at a time.
4. What Does a 10 kW Electric Heat Strip Cost in Ontario?
This is where AUX Heat becomes especially important for Ontario households with electric backup heat.
A 10 kW electric heat strip running at full output for one hour uses approximately 10 kWh of electricity.
Rated heater output (kW) × run time (hours) × electricity price ($/kWh)
Example:
10 kW × 1 hour × $0.203/kWh = $2.03
The Ontario Energy Board's current Regulated Price Plan electricity prices took effect November 1, 2025. As of September 2026, the current posted prices are:
| Ontario Price Period | Electricity Price | 10 kW Heat Strip for 1 Hour |
|---|---|---|
| TOU Off-Peak | 9.8¢/kWh | $0.98 |
| TOU Mid-Peak | 15.7¢/kWh | $1.57 |
| TOU On-Peak | 20.3¢/kWh | $2.03 |
| ULO Ultra-Low Overnight | 3.9¢/kWh | $0.39 |
| ULO Weekday On-Peak, 4–9 p.m. | 39.1¢/kWh | $3.91 |
| Tiered — Tier 1 | 12.0¢/kWh | $1.20 |
| Tiered — Tier 2 | 14.2¢/kWh | $1.42 |
Ontario's Ultra-Low Overnight plan charges only 3.9¢/kWh from 11 p.m. to 7 a.m., but the weekday on-peak price from 4 p.m. to 9 p.m. is 39.1¢/kWh.
That means the commodity cost of running the same 10 kW heat strip is about $0.39 per hour overnight versus $3.91 per hour during the weekday evening peak — exactly 10 times as much.
These examples include only the electricity commodity charge. Your actual hydro bill can also include Delivery, Regulatory Charges, taxes, rebates, loss adjustments and other utility-specific items. If the heat-pump compressor operates at the same time as electric auxiliary heat, its electricity consumption is additional.
What about 5 kW or 15 kW heat strips?
| Heat Strip Size | TOU On-Peak / Hour | ULO Weekday 4–9 p.m. / Hour |
|---|---|---|
| 5 kW | About $1.02 | About $1.96 |
| 10 kW | $2.03 | $3.91 |
| 15 kW | About $3.05 | About $5.87 |
5. Why Electric AUX Heat Can Raise Your Hydro Bill So Quickly
The reason comes down to how the two heating technologies work.
Electric resistance heat converts electricity directly into heat. A heat pump uses electricity to move heat from one place to another.
Because of that, a properly operating heat pump can deliver significantly more heat energy than the electrical energy it consumes.
For example, a heat pump operating at a coefficient of performance, or COP, of 2 is delivering roughly two units of heat for every one unit of electrical energy it uses.
Electric resistance heat, by comparison, delivers approximately one unit of heat for one unit of electricity at the point of use.
That does not mean every heat pump always operates at a COP of 2. Actual efficiency changes with outdoor temperature, equipment model, compressor speed, system condition and installation quality.
Example: 10 kW of electric backup running three hours per day
If a 10 kW heat strip ran for three hours every day for 30 days, that would equal:
At 20.3¢/kWh, 900 kWh represents approximately $182.70 in electricity commodity charges.
At ULO's weekday 4–9 p.m. rate of 39.1¢/kWh, the same 900 kWh would represent $351.90.
A real heating system will not necessarily run the entire load in one price period, and multi-stage electric heaters may not always energize at full capacity. But the example shows why extended electric AUX operation can have a noticeable impact on a winter hydro bill.
6. Electric Backup and Dual-Fuel Heat Pumps Are Not the Same
Not every Ontario heat pump uses electric resistance strips for backup heating.
Many homes use a hybrid or dual-fuel system that combines an air-source heat pump with a natural-gas furnace, oil furnace, boiler, or another supplementary heating appliance.
Natural Resources Canada notes that hybrid systems can switch between the heat pump and the supplementary heating system based on a thermal balance point or an economic balance point.
In this type of setup, the heat pump and the gas or oil furnace generally do not provide space heating simultaneously. Instead, the controls switch from one heating source to the other.
That distinction matters because an AUX or alternate-heat indication on a dual-fuel system does not mean you should automatically calculate the cost as though a 10 kW electric heat strip were running.
✔ Electric resistance backup → Check electricity use
✔ Natural-gas furnace backup → Check gas consumption as well as blower electricity
✔ Propane or oil backup → Check fuel consumption
✔ Not sure → Check the equipment model, installation paperwork, thermostat wiring information or ask your HVAC contractor
7. AUX Heat Keeps Running? Check These Things First
1. Read the thermostat display carefully
Confirm whether the thermostat actually says AUX, AUX Heat, Emergency, or EM Heat.
Also check whether Emergency Heat was accidentally selected after a thermostat change, power outage, battery replacement or system reset.
2. Identify your backup heat source
Before assuming you have electric strips, check the equipment documentation, utility bills and thermostat configuration.
Do not remove electrical service panels to identify components.
3. Think about recent thermostat changes
Did someone raise the setpoint by several degrees?
If AUX appeared immediately after a large temperature increase, the system may simply be trying to recover quickly.
4. Check basic airflow
A severely dirty filter, blocked return grille or closed supply registers can interfere with airflow and system performance.
Use a filter size and filtration level that are appropriate for your HVAC equipment. A higher MERV rating is not automatically better if the system cannot handle the additional airflow resistance.
5. Look at the outdoor unit from a safe distance
Make sure snow, leaves or debris are not blocking airflow around the unit.
Do not chip ice off the coil with tools and do not reach inside the cabinet. Some frost is normal; a defrost cycle should periodically clear it.
6. Check hourly electricity use
If your Ontario utility provides hourly usage data, compare your consumption on days with similar outdoor temperatures.
This is especially useful for ULO customers. If electricity use jumps sharply between 4 p.m. and 9 p.m. on weekdays, electric backup heat may be particularly expensive during those hours.
8. When AUX Heat May Need Professional HVAC Service
Continuous AUX Heat is a symptom, not a diagnosis. But certain patterns deserve professional attention.
- AUX Heat runs for long periods during relatively mild weather.
- The thermostat is in normal Heat mode, but the outdoor unit never appears to start.
- The house cannot reach the thermostat setpoint.
- The system repeatedly short-cycles.
- The outdoor coil remains heavily iced even after normal defrost operation.
- Your electricity use jumps dramatically without a comparable change in weather.
- Emergency Heat appears even though nobody selected it.
- The system develops a new mechanical, grinding, buzzing or vibration noise.
Smoke, visible sparking, a burning electrical smell, overheated wiring, or repeated breaker trips. Do not open high-voltage equipment panels or bypass electrical or HVAC safety controls.
9. Should You Turn On Emergency Heat When It's Extremely Cold?
Usually, not simply because it is cold outside.
Emergency Heat is not a “turbo heat” setting. It is a backup operating mode intended for situations where normal heat-pump operation is unavailable, inappropriate, or has failed.
A properly configured heat-pump system normally calls for whatever auxiliary or supplementary heating it needs without requiring the homeowner to manually switch to Emergency Heat every time the temperature falls.
If your outdoor unit has failed, is damaged, or has been taken out of service while you wait for a repair, Emergency Heat may help prevent the house from becoming dangerously cold, provided that is consistent with the manufacturer's instructions.
If you are unsure, ask the HVAC contractor:
- Should the compressor remain locked out?
- What type of backup heat is installed?
- If electric, what is the total heat-strip capacity in kW?
- What are the auxiliary-heat and compressor-lockout temperatures?
- Is the thermostat configured correctly for this equipment?
- Is it safe to operate Emergency Heat continuously until the repair?
10. How to Reduce Unnecessary Backup-Heat Use
- Use a thermostat that is properly compatible with your heat-pump system.
- Avoid very deep setbacks unless your equipment manufacturer or installer recommends them.
- Keep filters, supply registers and return-air paths clear.
- Keep snow, leaves and debris away from the outdoor unit.
- Seal major air leaks and address insulation problems that increase the home's heating load.
- Have the system maintained before the heating season.
- Ask the installer to document auxiliary-heat staging and lockout settings.
- For dual-fuel systems, ask how the heat-pump-to-furnace changeover temperature was selected.
- Compare TOU, ULO and Tiered pricing using your actual electricity-use pattern before changing plans.
Hydro One notes that a compatible smart thermostat can help monitor heat-pump performance and minimize unnecessary auxiliary or backup heating.
However, do not change installer-level balance points, compressor lockouts, or auxiliary-heat lockouts simply to stop the AUX indicator from appearing.
An overly aggressive setting can reduce comfort, prevent the house from reaching temperature during extreme cold, or conflict with the equipment's intended operating sequence.
11. Ontario Heat Pump AUX Heat FAQ
No. Brief auxiliary-heat operation can be normal during very cold weather, thermostat recovery, or a defrost cycle. Long AUX operation in mild weather, poor heating performance, or an outdoor unit that is not operating deserves further investigation.
Usually not. AUX Heat is generally called automatically when additional heating is needed. Emergency Heat is typically a manually selected backup mode that relies on the backup heating source instead of normal heat-pump operation. Exact behaviour varies by equipment and thermostat.
It can if your backup source is electric resistance heat. A 10 kW heat strip operating for one hour uses about 10 kWh. If your backup is a gas furnace or another fuel-fired appliance, the cost needs to be evaluated differently.
Using current Ontario electricity commodity prices, approximately $0.39 to $3.91 per hour depending on the price period. The lowest example is ULO overnight at 3.9¢/kWh. The highest is the ULO weekday 4–9 p.m. on-peak period at 39.1¢/kWh.
At the current 20.3¢/kWh TOU on-peak price, a 15 kW strip running for one hour costs about $3.05 in electricity commodity charges. At the 39.1¢/kWh ULO weekday evening rate, it costs about $5.87.
Some thermostats call for auxiliary heat when the difference between the current indoor temperature and the new setpoint is large. The controls may interpret the large temperature gap as a need for faster recovery.
Not simply because the outdoor temperature is low. A properly configured heat pump should manage normal auxiliary or supplementary heat automatically. Emergency Heat is generally reserved for situations where normal heat-pump operation is unavailable or should be disabled.
Not necessarily. ULO offers a very low 3.9¢/kWh overnight price, but its weekday 4–9 p.m. rate is 39.1¢/kWh. A home with heavy electric heating demand during the evening peak could potentially pay more under ULO than under TOU or Tiered pricing. Compare plans using your actual hourly consumption.
A properly compatible and correctly configured thermostat can help manage heat-pump operation and avoid some unnecessary backup-heat calls. Compatibility and installer settings matter more than simply buying a thermostat marketed as “smart.”
Bottom Line: AUX Heat Is a Clue, Not a Diagnosis
Seeing AUX Heat on an Ontario thermostat does not automatically mean something is wrong.
Short auxiliary-heat operation during a cold spell, thermostat recovery, or defrost cycle can be completely normal.
But if your home uses electric resistance backup heat and AUX stays on for long periods, the cost can add up quickly.
A 10 kW electric heat strip uses approximately 10 kWh per hour. Under Ontario's current ULO plan, the electricity commodity portion alone ranges from about $0.39 per hour overnight to $3.91 per hour during the weekday 4–9 p.m. peak.
If your hydro bill suddenly increases, do not start by changing advanced thermostat settings. Start by identifying what the system is actually doing.
1. Does the thermostat say AUX or Emergency Heat?
2. Is the backup source electric, natural gas, propane or oil?
3. Was the outdoor temperature unusually low?
4. Did someone raise the thermostat several degrees?
5. Is airflow restricted by a dirty filter or blocked grille?
6. Did hourly electricity use rise at the same time?
7. Is AUX still running for long periods during mild weather?
If the answer to the last question is yes, or the home cannot maintain temperature, a qualified HVAC technician can check equipment sizing, refrigerant-system performance, thermostat staging, electric heat-strip operation, defrost controls and lockout settings.
Ontario Energy Board — Current Electricity Rates
Ontario Energy Board — Choosing Your Electricity Price Plan
Natural Resources Canada — Heating and Cooling With a Heat Pump
Hydro One — Smart Thermostats
Google Nest — Emergency Heat for Heat Pump Systems
This article provides general Ontario home-energy and HVAC information based on publicly available sources current as of September 2, 2026. It is not an electrical diagnosis, HVAC inspection, or estimate for a specific home. Heat-pump controls, backup-heating sources, electricity price plans and equipment configurations vary. For persistent AUX operation, equipment faults, breaker trips, electrical odours, or other safety concerns, consult a properly qualified Ontario HVAC or electrical professional.
