The lockout temperature is the outdoor reading below which your thermostat stops allowing a particular heat source to run. For most Maine homes the answer is this: lock out the electric backup (auxiliary or strip heat) at about 35°F or lower, and leave the compressor unlocked, or set its lockout at 0°F or below if your system needs one. The right numbers also depend on your home's balance point, which comes from a heat-loss calculation, not a guess.
Below we explain the two kinds of lockout, how they relate to the balance point, and how to spot a setting that is quietly raising your electric bill.
TL;DR
- There are two lockouts. The auxiliary heat lockout blocks electric strip heat above a set outdoor temperature. The compressor lockout stops the heat pump itself below a set temperature.
- Industry guidance recommends an auxiliary lockout at 35°F or lower, and a compressor lockout, if used at all, at 0°F or below. Many modern cold-climate systems do best with no compressor lockout.
- The balance point is the outdoor temperature where the heat pump's output equals your home's heat loss. It comes from a load calculation, not square footage.
- If backup heat runs in mild weather, suspect sizing, a fault, defrost behavior or big thermostat setbacks before you suspect the lockout number.
- Changing these settings is a job for the person who commissioned your system, because the right value depends on your equipment.
Table of Contents
- Two lockouts, two very different jobs
- What the balance point is, and why it matters more than the lockout
- Why Maine is a special case
- What actually drives backup heat use
- How to check what your system is doing
- Settings by system type
- Pairing a heat pump with solar
- Our perspective: set it from the house, not from the thermostat default
- When to get help, and what it may cost
- How Hita can help
- FAQ
- Sources
Two lockouts, two very different jobs
People search for "lockout temperature" and land on advice about two separate controls. Mixing them up is the most common mistake.
Auxiliary (strip) heat lockout
Many ducted heat pumps include electric resistance strips as a backup. Strip heat delivers one unit of heat for each unit of electricity, a coefficient of performance of 1.0, while a heat pump moves several units of heat per unit of electricity. Federal energy researchers describe auxiliary heat as an emergency backup, not a normal operating stage, because relying on it drives bills up sharply.
The auxiliary lockout is the outdoor temperature above which the thermostat will not call for those strips. A Bonneville Power Administration reference guide recommends setting it at 35°F or lower, and notes that incorrect lockout settings account for the majority of inspection failures in its program. Homeowner forums often suggest 30 to 35°F as a starting point, which is consistent with that.
In plain terms: when it is 38°F outside and your heat pump is struggling, the strips should not be the answer. The compressor should be doing the work.
Compressor lockout
The compressor lockout is the opposite. Below the temperature you set, the thermostat will not run the compressor at all, and something else heats the house: a furnace, a boiler, a wood stove, or electric strips.
That made sense for older single-speed heat pumps that lost most of their output in the cold. It makes much less sense for modern inverter-driven cold-climate models. The same BPA guidance says a compressor lockout, if used, belongs at 0°F or below, and disabling it is often preferred. Where a lockout is used, it usually exists to protect equipment or hand off to a second fuel in a dual-fuel setup, not to save the heat pump from the cold.
| Auxiliary heat lockout | Compressor lockout | |
|---|---|---|
| What it blocks | Electric strip heat | The heat pump compressor |
| Blocks it when | Outdoor temp is above the setting | Outdoor temp is below the setting |
| Typical guidance | 35°F or lower | 0°F or below, or disabled |
| Risk if set wrong | Strips run in mild weather and inflate bills | Heat pump shuts off while it could still deliver efficient heat |
What the balance point is, and why it matters more than the lockout
The balance point is the outdoor temperature at which your heat pump's heating capacity exactly equals your home's heat loss. As GreenBuildingAdvisor explains, capacity falls as the air gets colder, while the load on your house rises. Where the two lines cross is the balance point.
Above the balance point, the heat pump covers the whole load. Below it, you need something extra for the gap: strips, a second fuel, or a larger or better-chosen heat pump.
That is why lockout settings and the balance point belong in the same conversation:
- The auxiliary lockout should sit near or below the point where the heat pump truly can no longer keep up. Set it too high and strips fire when the compressor could have handled the load.
- The compressor lockout should sit well below the balance point, since the heat pump still contributes useful heat there even if it cannot cover everything.
A worked example
Say a Maine home has a design heat loss of 30,000 BTU/h at its design temperature. A cold-climate heat pump might deliver roughly half to two thirds of its nameplate capacity at 5°F. Our guide to heat pump capacity at 5°F covers the 48 to 68% range in detail. If the installed system delivers 20,000 BTU/h at 5°F, that covers about two thirds of the load at design conditions.
That is a good result. It means the balance point sits somewhere well below freezing but not near the coldest nights, and the backup source only fills a modest gap on the few coldest days. Now compare a system sized only from square footage, which might hit its balance point at 25°F. That house would use backup heat for much of January.
The numbers above are an illustration, not a prediction for your house. Your real figures come from your own load calculation and the manufacturer's capacity data for your exact model.
Why Maine is a special case
Maine homes face long stretches below 20°F and occasional nights well below zero. That puts the balance point conversation at the center of any heat pump decision here. Two things follow.
First, equipment choice matters. A system rated to hold meaningful capacity at low temperatures changes where your balance point falls. Efficiency Maine's rebate claim form shows the program works from heating capacity at design temperature, counting dry-bulb capacities toward a minimum share of peak load. In other words, the program itself treats design-temperature capacity, not square footage, as the measure that matters.
Second, the lockout settings that suit a mild-winter state do not transfer automatically. A default pulled from a manual written for a milder climate may leave you with strips running more than necessary, or a compressor that quits earlier than it should.
What actually drives backup heat use
When homeowners notice a high bill, the lockout number is often blamed first. Research from the national labs points to a wider set of causes. A Department of Energy lab report on auxiliary heat lists the key drivers: low outdoor and compressor lockout temperatures, undetected system faults, heat pump sizing mismatch, defrost cycles and setback recovery.
Here is how to think about each.
Sizing mismatch
An undersized system hits its limit early, and backup heat covers the shortfall. An oversized one cycles on and off and may not reach its efficient operating range. Sizing from a real heat-loss calculation avoids both. Our piece on heat-loss sizing and ducted versus ductless systems explains why square footage rules of thumb fall short.
Undetected faults
A system struggling at 30 to 40°F is often not at its technical limit at all. A clogged coil, restricted airflow or low refrigerant can make a healthy-looking unit underperform, and the thermostat then calls for strips. The same lab research lists undetected faults as a primary cause of extra auxiliary use. Refrigerant work is regulated, and we explain what owners can and cannot do in our refrigerant recharge guide.
Defrost cycles
In cold, damp weather the outdoor coil frosts over and the unit briefly reverses to melt it. During that short cycle, some ducted systems turn on strips to keep the air from feeling cold. That is normal in moderation. If it is constant, or the unit never seems to finish defrosting, have it checked. Our article on a heat pump blowing cold air walks through what a normal few minutes looks like.
Thermostat setbacks
This one is entirely in your hands. Efficiency Maine advises that heat pumps run best holding a steady temperature, and that turning them down when you are away or asleep can use more energy, because the unit must work hard to recover. Unlike an oil or gas furnace, a heat pump asked to climb 6 or 8 degrees in the morning may call for supplemental electric heat to do it. If your thermostat is programmed with deep overnight setbacks, try a steady setpoint for a week or two and watch what happens.
How to check what your system is doing
You do not need tools for a first look. Work through these in order.
- Note the outdoor temperature when backup heat runs. If strips or a second heat source engage at 38°F, the lockout is probably too high, missing or not configured.
- Look at your thermostat's schedule. Remove large setbacks and see if backup use drops.
- Check the filters and the outdoor unit. Clear snow and ice from around it, and keep it free of debris. Do not wrap it; see why in our note on winter heat pump covers.
- Watch for repeated defrost cycles or an outdoor unit that is encased in ice.
- Run through the basics in our no-heat checklist, then call a professional if it does not resolve.
Then stop. Changing lockout values inside an installer menu or on a communicating thermostat can void settings the system was commissioned with. The right values depend on your model, your controls and your fuel mix. Ask the contractor who commissioned the system, or a qualified technician, to read the current settings and explain them.
Settings by system type
There is no single number that fits every house. Use this table as a conversation starter with your installer, not as a setting to type in yourself.
| Setup | Auxiliary lockout | Compressor lockout | Notes |
|---|---|---|---|
| Ductless only, no electric strips | Not applicable | Usually disabled | Backup is a separate heat source such as an existing boiler or stove |
| Ducted with electric strips | 35°F or lower, set from the balance point | Disabled, or 0°F or below | Keep strips as emergency backup |
| Dual fuel (furnace or boiler backup) | Not applicable | Set where the second fuel becomes cheaper to run | Depends on fuel prices and equipment capacity |
In a dual-fuel house, the cutover point is a cost question as much as a capacity one. If oil or propane is very expensive in a given winter, a lower cutover keeps the heat pump in charge longer.
Pairing a heat pump with solar
If your home also has solar panels, the electricity your heat pump uses can be partly offset during sunny hours, which helps most in shoulder seasons. A heat pump with solar is a common pairing, though in Maine winter production is lower, so it does not replace getting the sizing and lockout settings right.
Our perspective: set it from the house, not from the thermostat default
We see the same pattern again and again in Maine homes. A system gets installed, the controls are left near factory defaults, and the first cold snap brings a surprise electric bill. The owner assumes the heat pump "can't handle Maine." Usually the heat pump is fine. The setup around it is not.
Our approach starts with a heat-loss calculation for the actual home, because that is what tells you where the balance point sits. We choose and size equipment so it is built to keep working when temperatures drop to −22°F or lower, and then we treat the lockout and thermostat settings as part of the design, not an afterthought. Backup heat should be rare and brief.
Maintenance is the other half. A dirty coil or a developing fault shifts your real balance point upward long before anything breaks, and you pay for it in strip heat. That is why we push proactive care rather than waiting for a no-heat call.
If you take one thing from this page: do not chase a lockout number in isolation. Ask what your balance point is, and ask who verified it.
— John
When to get help, and what it may cost
Call a professional if backup heat runs often above 35°F, if the unit ices up repeatedly, if the compressor short-cycles, or if your electric use jumped without a change in routine. Early diagnosis is cheap compared with the alternative. One national estimator puts compressor replacement in the range of $1,300 to $3,000 for Maine, and professionals commonly weigh that against full replacement when a system is more than 10 to 15 years old. A failing compressor that has been overworked by poor settings or neglected faults is the kind of outcome a check-up can head off.
If you are planning a new system or a replacement, Efficiency Maine rebate rules and eligibility affect the equipment choices, so it helps to plan both together. Our installation planning page explains how we approach it.
How Hita can help
We install and replace cold-climate heat pumps in Maine, sized from real heat-loss calculations, in ductless, multi-zone and ducted configurations. See our services for new installations, change-outs, and preventive maintenance and troubleshooting. A one-time deep cleaning starts from $249.
If you would rather not wait for symptoms, our care plans cover the upkeep that keeps backup heat out of the picture: the Annual Cleaning Plan is $219 per year and the Maine Heat Pump Care Plan is $319 per year. Terms are on the plan terms page. We confirm service by ZIP code before scheduling or payment, so we will tell you plainly whether we cover your address. You can start at hitamaine.com.
FAQ
What should I set my heat pump lockout temperature to?
For electric backup heat, 35°F or lower is the widely used guidance, and the exact figure should come from your home's balance point. For the compressor, the recommendation is to leave the lockout off or set it at 0°F or below. Ask the installer who commissioned your system to confirm the values for your model.
What is the balance point of a heat pump?
It is the outdoor temperature at which the heat pump's heating capacity equals your home's heat loss. Above it, the heat pump carries the whole load. Below it, you need backup heat or a system with more low-temperature capacity.
Does a heat pump stop working below a certain temperature?
Not at a fixed number. Output falls as it gets colder, and the unit may stop if a compressor lockout is set, but modern cold-climate models keep delivering heat well below freezing. How much they deliver at 5°F or colder depends on the specific model, so check its published capacity data.
Should I turn my heat pump down at night?
Generally no. Efficiency Maine's guidance is that heat pumps work best at a steady temperature, and large setbacks can trigger electric backup heat during recovery. A modest, consistent setpoint is usually cheaper than a deep nightly drop.
Why is my backup heat running when it is only 35 or 40°F?
Common causes are an auxiliary lockout set too high or never configured, an undersized system, a dirty coil or other fault, a defrost cycle, or a large thermostat setback. Have a technician check the settings and the equipment before assuming you need more heating capacity.
Sources
- Bonneville Power Administration: PTCS Lockouts and Balance Points reference guide
- U.S. Department of Energy: Air Source Heat Pumps in Cold Climates
- GreenBuildingAdvisor: A Simple Way to Calculate Heat Pump Balance Point
- Efficiency Maine: Rebate Claim Form
- OSTI: Key drivers of auxiliary heat usage
- Efficiency Maine: Heat Pump User Tips
- HeatingCoolingSource: HVAC Repair Cost Calculator