What crossover temperature actually controls
In a dual-fuel setup, the thermostat (and sometimes the outdoor sensor on more sophisticated controls) continuously knows the outdoor temperature. Above the crossover temperature, calls for heat go to the heat pump. Below it, calls for heat go to the gas furnace.
The crossover isn't about whether the heat pump can heat below that temperature — a modern heat pump can heat well below the typical setpoint. It's about which fuel is cheaper per BTU at a given outdoor temperature, and about reserving the furnace for genuinely cold conditions where its sustained output and faster recovery matter.
The economics behind the crossover
A heat pump's efficiency (COP — coefficient of performance) drops as outdoor temperature falls. At 50°F outdoor, a modern heat pump might deliver 3.5 units of heat per unit of electricity (COP 3.5). At 25°F, that might drop to 2.0. A gas furnace's efficiency is roughly constant with outdoor temperature (90–95% AFUE).
The crossover point is where the operating cost per BTU of heat for the heat pump equals the operating cost per BTU of heat for the furnace — a function of your actual electricity rate ($/kWh) and gas rate ($/therm), the heat pump's COP curve, and the furnace's AFUE. Below the crossover, gas is cheaper per BTU; above it, the heat pump is.
What the right number is for Sacramento
On typical residential rates in the Sacramento region, the crossover temperature for a mid-tier heat pump against a 95% AFUE furnace falls in the 30–40°F outdoor-temperature band. Specific households shift that number up or down based on their actual tariff — tiered PG&E residential rates, SMUD rates, or electric time-of-use plans produce different crossover numbers.
The right practice at install is to compute the crossover from the homeowner's actual utility rates (not a generic 'set it to 35°F' assumption) and revisit it on utility-rate changes. We help homeowners redo the calculation when their electric or gas rates shift materially.
The common install mistake
Many dual-fuel systems get installed with a 'default' crossover somewhere in the 30s and left alone. If the default was picked for a different climate (colder) or a different rate structure, the system runs the gas furnace more than it should, erasing much of the hybrid-system savings.
The telltale is a hybrid system that burns noticeably more gas than you'd expect for a mild winter. Checking the crossover setting is the first diagnostic step — and often the only fix.
Common questions
Can the thermostat set its own crossover automatically?
Some smart thermostats and all dedicated dual-fuel controllers can — the better ones take utility rates and heat-pump performance data as inputs and recalculate. The simpler ones use a fixed temperature set during install. Either way, 'automatic' doesn't guarantee 'optimal'; the inputs still need to be right.
If the heat pump can heat below the crossover, why switch at all?
Fuel cost. Below the crossover, the gas furnace produces each BTU more cheaply than the heat pump does — even though both are capable of producing it. Switching to gas at that point is the whole point of a hybrid system.
Does changing the crossover affect comfort?
Not noticeably in most Sacramento-region homes. Both the heat pump and the furnace produce warm supply air; the furnace's air is warmer per cubic foot but either delivers the same room temperature at setpoint. The decision is almost pure economics.
Can I run just the heat pump and leave the furnace permanently off?
Yes — the furnace stays available for anything below the heat pump's capacity, but if the heat pump handles the full winter at acceptable cost, running pure heat pump is fine. We set the crossover to a very low temperature (effectively 'never') and the furnace becomes a backup for exceptional cold or heat-pump failure.