You probably picture backup heat as a safety net that only wakes up on the coldest nights of the year, quietly doing its job when the heat pump finally gives out. However, on most cold-climate installations the backup heater is not a last resort at all — it engages far earlier and far more often than the homeowner ever realizes.
The reason is almost always a control setting, not a hardware limit. Where the backup turns on, and where the compressor turns off, are decided by two numbers — the balance point and the lockout temperature — and both are frequently left at defaults that strand the cold-weather capacity you paid a premium to install.
This post explains what those numbers mean, how they relate, and how they are set so a properly sized cold-climate system keeps heating on its compressor instead of defaulting to expensive electric resistance. For the sizing work that comes first, see our guide to cold-climate heat pump sizing.
What Is A Heat Pump Balance Point?
The balance point is the outdoor temperature at which your heat pump's heating capacity exactly equals your home's heat loss. It is the crossing point of two curves that move in opposite directions as the weather changes.
As the outdoor temperature falls, your home loses heat faster, so the heating load rises. At the same time, a heat pump pulls less usable heat from colder air, so its capacity drops — and where the falling capacity line meets the rising load line is the balance point.
A heat pump's balance point is the outdoor temperature where its heating capacity exactly equals the home's heat loss. Above it, the compressor carries the load alone; below it, supplemental heat is needed.
Above the balance point, the compressor alone can hold your setpoint indefinitely. Below it, the compressor can no longer keep up on its own, and either indoor temperature drifts down or supplemental heat has to make up the difference.
Keep in mind that the balance point is a property of the pairing between a specific home and a specific unit, not a spec printed on the box. A right-sized cold-climate system in a tight, well-insulated house can have a balance point well below 20°F, while an undersized unit on a leaky house can sit above freezing.
Thermal Balance Point Versus Economic Balance Point
There are actually two balance points, and conflating them is one of the most common control mistakes. The thermal balance point is about capacity, while the economic balance point is about cost.
The thermal balance point is the temperature where capacity equals load, as described above. Below it, the heat pump physically cannot deliver enough heat without help, regardless of price.
The economic balance point is the outdoor temperature where backup fuel becomes cheaper per BTU than the compressor. On dual-fuel systems the switchover should be set here — not at the thermal balance point.
The economic balance point is different — it is the temperature at which a backup fuel becomes cheaper per delivered BTU than running the compressor. This matters only on dual-fuel systems that pair a heat pump with a gas or propane furnace, where you have a genuine choice of fuel.
For an all-electric system, the economic comparison is rarely close. A heat pump running at a COP of 2 to 3 delivers two to three times the heat per kWh of electric resistance strip heat, which sits at a COP of roughly 1.0 — so on electricity, the compressor almost always wins until it physically cannot keep up.
How The Balance Point Is Calculated
Finding the balance point means comparing a load curve against a capacity curve at several outdoor temperatures. Both halves come from documents you can request rather than estimate.
The load side comes from an ACCA Manual J calculation, which models your home's heat loss at your local design temperature. The capacity side comes from the manufacturer's expanded performance data, often summarized in the AHRI and NEEP cold-climate listings at rating points such as 47°F, 17°F, and 5°F.
Plotting load against capacity at those points shows where the lines intersect. Many installers will do this as part of the design, and you can sanity-check it yourself with a heat-pump load calculator before committing to equipment.
Consider a home with an approximately 36,000 BTU/hr heat loss at a 5°F design temperature, paired with a cold-climate unit that delivers roughly 32,000 BTU/hr at that same 5°F. The capacity line falls just short at design conditions, so the balance point lands only a few degrees above 5°F — and below that, just a modest amount of backup is ever needed.
Where the ratings come from. The NEEP Cold Climate Air-Source Heat Pump list and the ENERGY STAR Cold Climate specification publish tested capacity at low temperatures, including the 5°F rating that drives most balance-point math.
Programs such as Mass Save and NYSERDA tie their cold-climate incentives to this listing, so the same data that earns a rebate also tells you how low the balance point can realistically go.
The figure worth identifying is the unit's maximum capacity at low temperature, not just its nominal rating. Cold-climate models certified to the ENERGY STAR Cold Climate spec and listed by NEEP are tested to maintain a defined share of capacity at 5°F and to keep operating well below zero, which is exactly what pushes the balance point down.
What Lockout Temperatures Actually Control
A lockout is simply a temperature threshold, read from an outdoor sensor, that blocks one heat source from running. Two different lockouts do two very different jobs, and they are easy to confuse.
The table below separates the two settings before we tune each one in turn.
| Setting | What it blocks | Where it typically sits | Cost of setting it wrong |
|---|---|---|---|
| Backup-heat (aux) lockout | Electric resistance strips, above the threshold | At or just below the thermal balance point | Strips run on mild days at 2–3× the energy cost |
| Compressor (low-temp) lockout | The heat pump compressor, below the threshold | Near the unit's rated minimum operating temperature | Cold-weather capacity is stranded and resistance takes over early |
A backup-heat lockout is an outdoor temperature above which electric resistance heat is blocked. Setting it near the balance point keeps costly strips off during mild weather the compressor can handle alone.
The backup-heat lockout keeps cheap compressor heat running as long as possible, while the compressor lockout decides how soon you surrender to expensive resistance heat. Setting them as a matched pair around the balance point is the whole game.
Setting The Backup-Heat Lockout So Resistance Stays Off
The backup-heat lockout exists to stop resistance strips from running on days when the compressor can clearly carry the load. Without it, many thermostats will fire the strips whenever the indoor temperature drops a degree or two below setpoint, even at 45°F outside.
A common approach is to set the resistance lockout at or slightly below the thermal balance point. That way the strips are physically prevented from engaging until the outdoor temperature actually approaches the point where the compressor needs help.
The penalty for getting this wrong is steep and invisible. Every hour of unnecessary strip heat replaces compressor heat at two to three times the energy cost, and because resistance heat is silent and effective, the homeowner rarely notices anything but the bill.
For the full picture on how strips are sized and staged in the first place, see our breakdown of electric resistance backup heat. The lockout is only as good as the staging logic underneath it.
Setting The Compressor Lockout So The Compressor Keeps Running
The compressor lockout is the mirror image, and it is the one most often set far too conservatively. Older defaults — and habits carried over from marginal first-generation heat pumps — often lock the compressor out around 25°F or 30°F, which made sense for equipment that lost most of its capacity by then.
A compressor lockout is the low temperature below which the heat pump stops and backup takes over. Cold-climate units run efficiently well below 0°F, so an aggressive lockout strands capacity you paid for.
Modern cold-climate equipment is a different machine entirely. Units built around inverter scroll compressors and certified on the NEEP cold-climate list are designed to deliver useful, efficient heat at 5°F and to keep running down to -13°F or lower.
Locking such a unit out at 25°F throws away the exact capability you paid extra for. The compressor lockout, if used at all, generally belongs near the manufacturer's rated minimum operating temperature — not at a round number inherited from the last system.
Dual-Fuel Switchover Points
Dual-fuel (sometimes called hybrid) systems pair a heat pump with a fossil-fuel furnace and need a switchover temperature rather than a resistance lockout. Here the control question is economic, not physical.
The switchover should be set at the economic balance point — the temperature where a therm of gas delivers heat more cheaply than a kWh through the compressor. That figure moves with local gas and electricity rates, so a switchover that made sense one winter can be wrong the next.
The frequent failure mode is a switchover set high — say 40°F — out of caution, so the furnace runs through most of the heating season and the heat pump barely contributes. Setting it at the true economic crossover keeps the compressor working through the mild majority of winter hours, where it is cheapest.
Why Setbacks And Defrost Quietly Call For Backup
Even with good lockouts, two everyday events can still summon resistance heat: deep thermostat setbacks and defrost cycles. Both are worth understanding because both are controllable.
Deep thermostat setbacks can trigger resistance heat on recovery, because the thermostat sees a wide gap and stages up backup. Shallow setbacks or adaptive recovery keep heating on the compressor.
When a thermostat recovers from a large overnight setback, it sees a wide gap between actual and target temperature and stages up to backup to close it fast. On a heat pump, that recovery strip heat can erase a full night's compressor savings, which is why shallow setbacks or adaptive-recovery thermostats suit heat pumps better than aggressive ones.
During defrost, the unit briefly reverses to melt coil frost and usually energizes backup heat to temper the air. This is normal and brief — frequent, lengthy defrost is the real efficiency problem.
Defrost is the other culprit. During a defrost cycle the unit briefly runs in reverse to melt frost off the outdoor coil, and most systems energize backup heat to temper the cold air during those few minutes — a normal, designed behavior covered in our guide to defrost cycle sizing.
Bringing The Settings Together
The balance point is the anchor, and the two lockouts are set in reference to it. Identify where capacity meets load, hold the resistance lockout near that point, and push the compressor lockout down to the unit's rated minimum.
None of these are set-and-forget for life — rate changes, weatherization, and equipment age all move the numbers, particularly the economic balance point on dual-fuel systems. A seasonal review of the outdoor-sensor lockouts is a reasonable way to confirm the system is still favoring the compressor.
If you are still selecting equipment, the controls conversation belongs in the same breath as the capacity conversation — start from our heat pump guide and carry the balance-point question into every quote. The right hardware with the wrong lockouts still heats your home with resistance.
This article is for informational purposes and is not financial, tax, legal, or medical advice. Consult a licensed professional — such as an HVAC contractor, CPA, or your state energy office — before acting.