In short
The dual-fuel changeover temperature is the outdoor temperature at which a hybrid system switches from the heat pump to the gas furnace. Set it at the warmer of two points: where gas becomes cheaper per unit of heat on your rates, and where the heat pump can no longer hold the house. Set too high, the furnace does all the work.
On a hybrid system, the dual-fuel changeover temperature is one number in the installer menu, and it decides which appliance heats the house on any given winter day. Warmer than the setting, the heat pump runs; colder, the gas furnace takes over.
What the changeover setting does in a hybrid system
The setting locks the heat pump out below a chosen outdoor temperature and brings on the furnace instead. It is an either-or choice. The heat pump's indoor coil sits in the furnace's airstream, so the two do not heat together the way a heat pump and electric auxiliary heat do.
The control has to know the outdoor temperature. It gets it from a wired outdoor sensor or from an internet weather feed in a smart thermostat. A wired sensor is the more dependable source, because a weather feed stops updating when the Wi-Fi drops. A sensor mounted in direct sun or beside a dryer vent reads warm and delays the switch.
Economic changeover versus capacity changeover
Two different temperatures compete for the setting, and the rule is to use whichever is warmer.
- Economic changeover: the outdoor temperature below which a unit of heat costs less from gas than from the heat pump. It depends on your Enbridge gas rate, your hydro price plan (time-of-use, tiered or ultra-low overnight) and how the heat pump's efficiency falls as it gets colder.
- Capacity changeover: the outdoor temperature below which the heat pump can no longer replace the heat the house is losing. This is the heat pump balance point, and a cold-climate model pushes it lower.
There is no single correct number for Ontario. Two neighbours with the same equipment can land on different settings because one is on time-of-use pricing and the other chose the ultra-low overnight plan. A changeover left at a factory default has not been matched to anything.
Signs the changeover is set wrong
A wrong setting shows up in which appliance you hear running, and when.
- The furnace runs on mild days and the heat pump only works in October and April: set too high, and the heat pump you paid for is wasted.
- The heat pump runs without stopping on the coldest nights and the house drifts below the setpoint: set below the capacity point.
- The system flips between fuels on days that hover around the setting: check where the outdoor sensor is mounted.
- Your gas bill barely moved after the hybrid install.
- You changed hydro price plans, or gas rates moved: the economic point moved with them.
One caution before changing anything. Some incentive programs have required specific control settings as a condition of the rebate, and the rules change, so read the current terms on the dual-fuel heat pump rebate page first.
Who sets the changeover and what it costs to adjust
The installer sets it at commissioning, in the thermostat or in the equipment's own controls. Adjusting it later is a settings change, not a part, so there is no price band for it; if it needs a visit, the service call is $90–$150. Have a recent gas bill and hydro bill on hand, since the calculation starts from your rates.
We install and set up hybrid heat pump systems, including for homeowners asking whether they can keep their furnace and add a heat pump. Contact ZK Mechanical to have the setting checked against your equipment and your bills.
Questions people ask about Dual-fuel changeover temperature
Is the changeover temperature the same as the balance point?
No. The balance point is a property of the house and the heat pump: the outdoor temperature where the unit's output equals the home's heat loss. The changeover temperature is a setting someone chooses. It can sit at the balance point, or warmer than it when gas becomes the cheaper fuel well before the heat pump runs out of capacity.
Can I change the changeover temperature myself?
Often, yes. On many thermostats it is in the installer or equipment menu. Write down the original value, move it a couple of degrees at a time and watch comfort and bills before moving it again. On communicating systems the setting lives in the equipment controls, and a rebated system may have conditions attached, so ask before changing it.
Why does my furnace come on when the heat pump is defrosting?
That is normal on many hybrid systems and is separate from changeover. During a defrost cycle the heat pump briefly runs in reverse to melt frost off the outdoor coil, which would send cool air through the vents. The control fires the furnace for those few minutes to temper the air, then hands back to the heat pump.