What the lockout setting actually controls
As covered in the Heat Pumps in Cold Climates guide, every heat pump has a "balance point" — the outdoor temperature below which its heating output can no longer fully cover a home's heat loss. Below that point, something else has to make up the difference: electric resistance strips built into the air handler, or a separate furnace or boiler in a dual-fuel configuration. The aux-heat lockout is the thermostat setting that decides the exact outdoor temperature at which that backup is allowed to engage.
The two settings are related but not identical: a system's compressor lockout (the point where the heat pump itself stops running, if it has one) must always sit below the aux-heat lockout — backup should never be locked out before the heat pump's own operating floor (The Furnace Outlet). Getting the aux-heat lockout right means setting it close to the system's real balance point, not padding it with a large "safety" margin.
Why the lockout setting matters so much
Electric resistance heat has a coefficient of performance (COP) of essentially 1.0 — one unit of heat out per unit of electricity in. A heat pump compressor, even in cold weather, typically runs at a COP of 1.5 to 4.0+, meaning it delivers several times more heat per dollar of electricity (PickHVAC). That gap is why resistance backup typically costs 2 to 5 times more per unit of heat than the heat pump running alone (Build-Construct) — every hour of backup runtime that didn't need to happen is a real, avoidable cost.
One Massachusetts-focused comparison put real numbers on it: for roughly 50 hours of backup heat runtime over a winter, natural-gas backup cost about $97 against $290 for electric-resistance backup — a $193 difference for the same amount of supplemental heat (NuWatt Energy). A lockout set too conservatively doesn't just add a few dollars — it can turn "backup heat as a rare emergency fallback" into a recurring monthly expense.
Illustrative aux-heat lockout — recommended vs. a conservative default
Illustrative example, not a universal setpoint — your own recommended lockout depends on your specific heat pump's cold-weather capacity and your local electricity and fuel rates. Government guidance points to 35°F or lower as a starting point (BPA); some smart thermostats ship with defaults in the 40°F–50°F range.
Real-world thermostat defaults back this up: Google's own configuration guide for Nest lists a default aux-heat lockout around 40°F — more conservative than the 35°F-or-lower guidance from utility and government efficiency programs (Google Nest configuration guide). Homeowners who lower a similarly conservative default toward the 32°F–35°F range have reported eliminating 50%–60% of unnecessary aux-heat activations after the change (Duct Sealing Near Me).
Dual-fuel backup: when a furnace beats electric strips
If a home already has a gas furnace and its utility charges more for electricity during winter peak demand, keeping that furnace as backup — a "dual-fuel" or hybrid setup — can beat electric-resistance strips on cost, since the switchover point is really a fuel-price comparison rather than a fixed temperature. Typical dual-fuel switchover points run 30°F–45°F depending on the relative price of gas versus electricity locally (The Furnace Outlet). Installed cost for a dual-fuel system runs roughly $12,000–$22,000, compared with $10,000–$20,000 for an all-electric cold-climate heat pump with resistance backup — a modest premium for keeping the furnace, offset over time if local rates favor gas on the coldest days (GetABetterQuote).
Whether dual-fuel or all-electric backup makes sense for a specific home comes down to local utility rates more than any universal rule — run the actual comparison with the Utility Rates tool before deciding.
How to get your own backup heat strategy right
- Find your thermostat's current aux-heat lockout setting. It's often buried in an installer or equipment menu, not the everyday thermostat screen — check the manual or ask your installer directly.
- Compare it against your system's actual balance point. If you don't know your balance point, see the Heat Pumps in Cold Climates guide for how it's calculated — and how much sizing alone can shift it.
- Move the lockout down toward that balance point, with a small margin. Government guidance suggests 35°F or lower as a starting point (BPA); adjust from there based on your comfort and your system's real cold-weather capacity.
- Never set the aux lockout below the compressor's own lockout. If your heat pump has a hard cutoff temperature, the aux-heat lockout needs to sit at or above it, not below — otherwise the home has no heat source at all in that gap.
- If you have an existing furnace, compare dual-fuel against electric-only. Run your local gas and electric rates through the Utility Rates tool to see which backup fuel actually costs less on your coldest days.
- Recheck the setting after any thermostat replacement. Smart thermostats often reset to a factory default lockout on install — don't assume your old setting carried over.