Why Your Mini Split Stops Heating Below Freezing
We get this call constantly in January and February: "My mini split is running but it's blowing lukewarm air — something's wrong." Nine times out of ten, nothing is wrong with the equipment. It's doing exactly what it was designed to do. The problem isn't the installation or the refrigerant charge — it's that standard heat pumps weren't built for New York winters.
If you're reading this because your mini split isn't heating right now: Before you call for a repair, check the outdoor temperature. If it's below 25–30°F and your system is a standard heat pump, the equipment is likely working correctly — it just doesn't have the capacity to heat in these conditions. This page explains why, and what the difference is between standard and hyper heat systems.
How a Standard Heat Pump Works (and Where It Falls Short)
Every mini split is a heat pump. In heating mode, it extracts heat energy from outdoor air and moves it inside. That works efficiently when it's 40°F or 50°F outside — there's plenty of heat energy in the air to extract. But as outdoor temperatures drop, there's progressively less heat energy available. A standard heat pump's capacity drops with the temperature.
This is physics, not a defect. Here's what happens to a standard heat pump as NYC winter temperatures set in:
| Outdoor Temperature | Standard Heat Pump Capacity | What You'll Experience |
|---|---|---|
| 47°F (fall day) | 100% — full rated capacity | Everything works perfectly. Space heats quickly. |
| 32°F (freezing) | 75–85% | System runs longer. Takes more time to reach set temperature. |
| 25°F (cold night) | 60–70% | Can't maintain set temperature. Air feels lukewarm. |
| 17°F (typical NYC January night) | 40–55% | System runs non-stop but space temperature keeps dropping. This is the call we get. |
| 5°F (cold snap) | 25–35% or shutdown | Minimal heat output. Some units shut down entirely. |
The system is running — you can hear it, the fan is spinning, the outdoor unit is operating — but it physically cannot move enough heat into your space to keep up with the heat loss through your walls, windows, and doors. The colder it gets, the worse the gap becomes between what the system can deliver and what your space needs.
What we tell callers every winter: If your mini split was installed with a standard heat pump outdoor unit and it's below 25°F outside, your system is probably working exactly as designed. The issue isn't the installation. The issue isn't the refrigerant charge. The issue is that standard heat pump technology has a physical limitation in cold weather. Hyper heat technology was specifically developed to solve this problem.
What Is Hyper Heat?
Hyper heat is a category of cold-climate heat pump technology designed to maintain full or near-full heating capacity at outdoor temperatures that would cripple a standard heat pump. The two leading platforms are Mitsubishi Hyper-Heating H2i and Daikin Aurora.
These are not minor upgrades or software changes — they use fundamentally different compressor and system engineering to solve the cold-weather heating problem.
Mitsubishi Hyper-Heating H2i
Mitsubishi's H2i (Hyper-Heating Inverter) technology is one of the most established cold-climate heat pump platforms in the industry. H2i systems maintain full rated heating capacity down to -13°F using a flash injection compressor cycle that increases refrigerant mass flow at low temperatures. H2i is available across Mitsubishi's product lines:
- M-Series — single-zone and multi-zone residential and light commercial systems
- P-Series — commercial ceiling cassettes, concealed duct units, and floor-standing models
- City Multi VRF — large-scale commercial variable refrigerant flow systems connecting up to 50 indoor units
Mitsubishi has the largest service and parts network in the NYC market for mini split and VRF systems. H2i has been deployed in cold climates across the Northeast for over a decade.
Daikin Aurora
Daikin's Aurora series is the newest entry in cold-climate VRF technology, and it pushes the performance envelope further. Aurora is the first air-cooled VRF system capable of delivering heating down to -22°F as standard — nearly 10 degrees colder than Mitsubishi H2i's rated minimum. Key Aurora features:
- Heating to -22°F — the lowest rated heating temperature of any air-cooled VRF system currently available
- Advanced inverter compressor technology — variable speed operation optimized for cold-climate heat extraction
- Heat pump and heat recovery configurations — heat recovery models can simultaneously heat and cool different zones, recovering waste heat from cooling zones and redirecting it to heating zones
- R-32 refrigerant options — lower global warming potential than the R-410A used in most current systems
Daikin's VRV platform (their brand name for VRF) has deep commercial adoption globally. The Aurora series extends that platform into extreme cold climates that were previously only served by fossil fuel heating.
The Engineering Behind Hyper Heat
Hyper heat isn't a marketing label — it's a set of specific engineering differences that allow the compressor and system to operate effectively at temperatures where standard equipment fails.
Vapor Injection Compressor
Standard heat pump compressors have one compression stage. Hyper heat compressors use a flash injection cycle — refrigerant is partially expanded, subcooled, and re-injected into the compressor mid-cycle. This increases the mass flow rate of refrigerant at low ambient temperatures, which directly increases heating capacity. The compressor can move significantly more heat at the same outdoor temperature compared to a standard design.
Oversized Heat Exchangers
The outdoor coil on a hyper heat unit is physically larger than on a standard unit of equivalent BTU rating. More surface area means more contact with outdoor air, which means more heat extraction even when there's less heat energy available per cubic foot of air. More copper tubing, more aluminum fins, larger housing.
Intelligent Defrost
When outdoor temperatures drop below freezing, moisture in the air freezes on the outdoor coil. The system periodically reverses its cycle to melt this ice — the defrost cycle. During defrost, heating to your indoor space pauses.
Standard heat pumps defrost on a fixed timer — every 30, 60, or 90 minutes regardless of whether the coil actually needs it. This wastes energy and heating time. Hyper heat systems use demand defrost — sensors measure actual coil temperature and ice buildup, triggering defrost only when necessary. The system spends more time heating and less time defrosting.
Base Pan Heater
During prolonged cold weather, water from defrost cycles collects in the outdoor unit's drain pan and refreezes. Over time, this creates an ice dam that can prevent the unit from operating or cause damage. Hyper heat outdoor units include a base pan heater — a heating element in the drain pan that prevents ice accumulation. Standard units typically lack this and can ice up during extended cold snaps.
Wider Operating Range Electronics
The control boards, sensors, expansion valves, and fan motors in hyper heat systems are rated for a wider operating temperature range. Components are selected and tested for continuous operation at extreme low temperatures where standard electronics may malfunction or shut down for self-protection.
Standard vs. Hyper Heat: Capacity at Every Temperature
| Outdoor Temperature | Standard Heat Pump | Hyper Heat (H2i / Aurora) |
|---|---|---|
| 47°F | 100% capacity | 100% capacity |
| 32°F (freezing) | 75–85% | 95–100% |
| 25°F | 60–70% | 90–100% |
| 17°F (typical NYC January night) | 40–55% | 87–100% |
| 5°F (cold snap) | 25–35% or shutdown | 80–90% |
| -5°F (extreme cold) | Shutdown | 75–85% |
| -13°F (H2i rated minimum) | Shutdown | Still operating (H2i) |
| -22°F (Aurora rated minimum) | Shutdown | Still operating (Aurora) |
At 17°F — a normal January night in New York City — a standard heat pump delivers barely half its rated capacity. A hyper heat system delivers near-full capacity. That's the difference between maintaining your set temperature and watching the thermometer drop while the system runs non-stop.
When This Matters Most: Electric as Primary Heat Source
The distinction between standard and hyper heat becomes critical when the heat pump system is the primary or sole heat source.
If a building has a gas boiler, steam radiators, or another backup heating system, a standard heat pump mini split can work for supplemental cooling and shoulder-season heating. The backup system carries the heating load during the coldest weather.
But when the electric heat pump is expected to be the only heat source — when there's no boiler to fall back on, no steam risers, no radiators — the system must perform at full capacity on the coldest days of the year. That's exactly where standard heat pumps fall short and where hyper heat systems like Mitsubishi H2i and Daikin Aurora are designed to operate.
This scenario is becoming more common every year in NYC. Local Law 97, Con Edison's Clean Heat program, and the broader push away from fossil fuels are driving more buildings to convert from gas and oil to all-electric heat pump systems. When an electric heat pump replaces a gas boiler, it becomes the building's only heat source. In this scenario, the higher-end hyper heat options should be given serious consideration. A standard heat pump that worked fine as supplemental cooling may not be adequate as the building's sole heating system through a full NYC winter.
What We See in the Field Every Winter
These are patterns from our service calls during NYC's coldest months. They represent the most common winter calls we receive, and they almost always trace back to the same root cause.
Standard Heat Pump, No Backup Heat, Below-Freezing Temperatures
The call: "My mini split was working fine until this week. Now it's running but the space won't get above 62°F." We arrive. Outdoor temperature is in the teens. The system is a standard heat pump running at full capacity. All components are functioning correctly — compressor, fans, controls, refrigerant charge. The unit is doing everything it can. It just can't deliver enough heat at this temperature to overcome the space's heat loss.
The diagnosis: Equipment is working as designed. The system was sized and selected for cooling load and moderate heating conditions. It was not designed or rated for full heating output below 25°F.
Standard Heat Pump in a Mild Winter vs. a Cold Winter
Some callers tell us the system heated fine the previous year. Often, the difference is simply the weather. A mild NYC winter with lows in the high 20s to low 30s is manageable for a standard heat pump — it's running at 70–80% capacity, which may be enough. A cold winter with sustained lows in the teens exposes the gap. The system didn't change. The weather did.
The takeaway: A standard heat pump may appear adequate as a primary heating system during mild winters. A cold winter reveals the limitation.
Standard Heat Pump Supplemented by Portable Electric Heaters
We see this in offices, retail spaces, and restaurants across the city: the mini split runs all day, but staff are also running portable space heaters to make up the difference. The mini split is providing some heat, but not enough. The space heaters fill in the gap — at 1,500 watts each, using pure electric resistance, which is the least efficient form of electric heating. The building is paying for both the heat pump and the space heaters.
The takeaway: Portable heaters running alongside a mini split every winter is a sign the system's heating capacity doesn't match the space's heating load at low temperatures. Hyper heat systems are designed to deliver full rated capacity at those temperatures without supplemental resistance heat.
Can an Existing System Be Upgraded?
In some cases, it's possible to replace just the outdoor unit with a hyper heat model while keeping the existing indoor units and refrigerant lines in place. Whether this is feasible depends on several factors:
- Refrigerant line sizing — the new outdoor unit may require different line diameters. If existing lines are compatible, they can be reused.
- Indoor unit compatibility — the new outdoor unit must communicate with the existing indoor units. Same-brand upgrades have the highest compatibility.
- Electrical capacity — hyper heat outdoor units may have different electrical requirements. The existing circuit and panel capacity need to be verified.
- Outdoor unit placement — hyper heat condensers can be larger and heavier than standard models. The current mounting location needs to accommodate the new unit.
A site assessment determines whether an outdoor unit swap is feasible or whether a full system replacement is needed.
Frequently Asked Questions
My mini split is running but not heating — should I call for a repair?
Check the outdoor temperature first. If it's below 25–30°F and you have a standard heat pump, the equipment is likely working as designed — it just can't deliver enough heat in those conditions. If it's above 35°F and still not heating, or if the outdoor unit isn't running at all, that's a different situation and warrants a service call. We're happy to diagnose either way — call (718) 835-6820.
What is the difference between Mitsubishi H2i and Daikin Aurora?
Both are cold-climate heat pump technologies designed to maintain heating capacity at very low outdoor temperatures. Mitsubishi H2i is rated to -13°F and has been available for over a decade with extensive deployment across the Northeast. Daikin Aurora is a newer platform rated to -22°F — the lowest of any air-cooled VRF system. Both use vapor injection compressor technology. They're available in different product configurations and tonnage ranges.
Will a hyper heat system work as the only heat source in NYC?
Hyper heat systems are designed specifically for this use case. Mitsubishi H2i maintains full rated heating capacity down to -13°F. Daikin Aurora operates down to -22°F. NYC's record low is -15°F (set in 1934), and typical January lows are 15–25°F — well within the operating range of both platforms. Proper load calculation and system sizing are critical: the system must be matched to the space's actual heat loss, not just the cooling load.
Why didn't my original installer mention this?
Many installers size and select equipment based on cooling load, which is the dominant concern in summer. Heating performance in cold weather may not be part of the conversation — especially if the installer primarily works in residential applications or warmer climates. The heating specs on standard equipment are typically measured at 47°F, which doesn't reflect real-world performance on a 15°F night in January.
Does my building need to approve a hyper heat upgrade?
If you're replacing an existing outdoor unit, most buildings require approval from building management or the co-op/condo board. This typically includes updated equipment specs, a noise rating comparison, and confirmation that the new unit fits the existing location. New installations or changes in unit placement require a DOB permit.
Not Sure What System You Have?
Send us the model number from your outdoor unit and we'll tell you whether it's a standard heat pump or a hyper heat system — and walk you through your options.