Finding the best central air conditioners involves balancing performance, efficiency, and affordability. The Oxbox by Trane stands out as the best overall choice thanks to its reliable 13.4 SEER2 rating and robust capacity, ideal for larger homes. For those prioritizing energy savings, the MrCool VersaPro offers impressive 17 SEER2 efficiency with easy DIY installation, but it comes at a higher price. Conversely, budget-conscious buyers might lean toward the Goodman 2 Ton 14 Seer System, which provides solid performance at a more accessible cost. Each option involves tradeoffs between upfront cost, long-term savings, and installation complexity. Continue reading for a detailed breakdown of these top models to find your best fit.
Key Takeaways
- Top-rated models balance high efficiency with sufficient cooling capacity for various home sizes.
- Premium options often include features like inverter technology and variable speeds for quieter, more consistent operation.
- Budget-friendly systems can still deliver reliable cooling but may have lower SEER ratings or fewer advanced features.
- Ease of installation varies widely; DIY-friendly units are available but often come with higher upfront costs.
- Choosing the right system depends heavily on your home size, energy goals, and budget constraints.
| central air conditioner | Refrigerant | Cooling Power | Warranty | SEER2 |
|---|---|---|---|---|
| 3 Ton Central Air Conditioner | R-454B | — | — | 13.4 |
| ACiQ 3 Ton 14.3 SEER2 Central | R-32 | 36,000 BTU | 12 Years Compressor, 7 Years Parts | 14.3 |
| ACiQ 2 Ton 14.3 SEER2 Central | R-32 | 24,000 BTU | 12 Years Compressor, 7 Years Parts | 14.3 |
| ACiQ 3.5 Ton 14.3 SEER2 Centra | R-32 | 42,000 BTU | 12 Years Compressor, 7 Years Parts | 14.3 |
| Senville 3 Ton Central Air Con | R-454B | 36,000 BTU | 10-year compressor, 5-year parts | — |
| Goodman 2 Ton 14 Seer Heat Pum | R-32 | 24000 BTUs | 10 years on parts | — |
| Goodman 2.5 Ton 14.3 SEER2 R-3 | R-32 | — | — | — |
| ROVSUN 5 Ton 53000 BTU Ducted | — | 53000 BTU | 7 Years | 17 |
| Goodman GSX130241 13 SEER 2.0 | — | — | 10 years parts | — |
| 2.5 Ton Up To 15 SEER2 Goodman | — | — | 10-year parts-limited | 13.4 to 15 |
| 2 Ton Central Air Conditioner | R-454B | 24,000 BTU | — | 13.4 |
| MrCool VersaPro 2.5 Ton 17 SEE | R-454b | 2.5 Ton | — | — |
More Details on Our Top Picks
3 Ton Central Air Conditioner System + 36,000 BTU + 13.4 SEER2 (Oxbox by Trane)
This system stands out for its dependable performance and solid energy efficiency, making it a strong choice for homeowners seeking a straightforward, long-term cooling solution. Compared with the Senville 3 Ton Heat Pump, it offers less versatility in heating but excels in consistent cooling and environmental friendliness, thanks to its R-454B refrigerant. The tradeoff is that it requires professional installation, which can add to upfront costs, and it’s designed specifically for residential use, not larger commercial spaces. Its 13.4 SEER2 rating balances energy savings with affordability, while its durable build promises long-term operation.
Pros:- Reliable and consistent cooling performance
- Energy-efficient with a 13.4 SEER2 rating
- Environmentally friendly refrigerant
- Built for long-term durability
Cons:- Requires professional installation, increasing initial costs
- Designed for residential use, not suitable for larger commercial spaces
Best for: Homeowners with small to medium-sized homes prioritizing reliability and energy efficiency.
Not ideal for: Large households or those needing integrated heating and cooling with variable climate needs, since it’s focused solely on cooling.
- Cooling Capacity:36,000 BTU
- SEER2:13.4
- Refrigerant:R-454B
- Compatibility:Indoor air handlers and evaporator coils
Our verdict“This system is ideal for homeowners seeking dependable, cost-effective cooling with long-term durability.”
ACiQ 3 Ton 14.3 SEER2 Central Air Conditioner Split System with 17.5-inch Vertical Evaporator Coil and Thermostat
This model offers a compelling mix of cooling power, durability, and warranty coverage, making it suitable for those who want a reliable split system with modern refrigerant. Compared to the Oxbox 13.4 SEER2 system, it provides slightly higher efficiency and includes a thermostat for precise temperature control, which enhances comfort. The tradeoff is its higher power consumption at 6900 watts and the need for professional installation to maintain warranty validity. Its 14.3 SEER2 rating strikes a good balance between energy savings and upfront cost, especially with its long warranty coverage on compressor and parts.
Pros:- Higher efficiency with 14.3 SEER2
- Includes thermostat for precise temperature management
- Long warranty on compressor and parts
- Durable all-aluminum evaporator coil
Cons:- Requires professional installation for warranty and optimal operation
- Relatively high power consumption at 6900 watts
- Noise level of 59 decibels may be noticeable
Best for: Homeowners seeking a reliable, efficient system with advanced controls and solid warranty support.
Not ideal for: Budget-conscious buyers or DIY installers, as professional setup is required and power consumption is relatively high.
- Cooling Power:36,000 BTU
- SEER2:14.3
- Refrigerant:R-32
- Warranty:12 Years Compressor, 7 Years Parts
Our verdict“Best suited for homeowners wanting a high-efficiency, warranty-backed system with precise control features.”
ACiQ 2 Ton 14.3 SEER2 Central Air Conditioner Split System with Thermostat, R32 Refrigerant, 24,000 BTU
This 2-ton system provides a reliable cooling capacity of 24,000 BTU, ideal for smaller homes or apartments where space is limited. It shares many features with the larger 3-ton models, like the all-aluminum evaporator coil and R-32 refrigerant, but its lower capacity makes it more energy-efficient for smaller areas. Compared to the Oxbox 36,000 BTU system, it consumes less power, but may not be adequate for larger rooms. Its noise level of 58 decibels is slightly quieter than some larger models, making it suitable for bedrooms or quiet living spaces.
Pros:- Efficient 14.3 SEER2 rating
- Quiet operation with 58 decibels
- Includes thermostat for precise control
- Durable evaporator coil with R-32 refrigerant
Cons:- Limited cooling capacity for larger homes
- Requires professional installation
- Performance may be insufficient in very hot climates
Best for: Residents with small to medium-sized homes or apartments requiring efficient yet space-saving cooling.
Not ideal for: Large households or spaces over 3000 sq ft, where higher capacity systems are necessary.
- Cooling Power:24,000 BTU
- SEER2:14.3
- Refrigerant:R-32
- Warranty:12 Years Compressor, 7 Years Parts
Our verdict“Ideal for smaller homes or apartments needing reliable, quiet cooling without excess capacity.”
ACiQ 3.5 Ton 14.3 SEER2 Central Air Conditioner Split System with 21-inch Vertical Evaporator Coil and Thermostat
This 3.5-ton system delivers 42,000 BTU of cooling, making it suitable for larger or more demanding spaces. It shares the same high-efficiency features as the 3-ton models but offers increased capacity for bigger homes or spaces with higher cooling loads. Compared with the Senville Heat Pump, it’s less versatile for heating but excels in cooling large areas reliably. The weight of 157 pounds and a noise level of 60 decibels suggest professional installation is advisable. Its durability and warranty coverage make it a solid choice for bigger residences seeking consistent cooling power.
Pros:- Powerful 42,000 BTU cooling capacity
- Durable aluminum evaporator coil
- Includes thermostat for precise control
- Long warranty coverage
Cons:- Heavy weight at 157 pounds
- Requires professional installation
- Noise level of 60 decibels may be noticeable
Best for: Homeowners with larger or hotter spaces requiring more cooling capacity and professional installation.
Not ideal for: Small homes or apartments where lower capacity and lighter units are sufficient, or DIY installers.
- Cooling Power:42,000 BTU
- SEER2:14.3
- Refrigerant:R-32
- Warranty:12 Years Compressor, 7 Years Parts
Our verdict“Designed for larger homes needing robust cooling, with a focus on durability and professional setup.”
Senville 3 Ton Central Air Conditioner Heat Pump Split System, 36,000 BTU, Inverter, Variable Speed, 208/230V
The Senville heat pump excels at providing both cooling and heating, making it a versatile choice for larger spaces up to 2,000 sq ft. Its inverter technology with a variable speed compressor offers smooth, efficient operation and reliable performance even in cold weather. Compared with the Oxbox 13.4 SEER2 system, it provides a higher SEER of 18.00, translating into greater energy savings, especially when heating is also needed. The tradeoff involves higher initial costs and the necessity of professional installation. Its quiet operation at 44 decibels makes it suitable for bedrooms or quiet zones, and the long warranties add peace of mind.
Pros:- Excellent inverter technology for efficiency
- Provides both heating and cooling
- Quiet operation at 44 decibels
- Long-lasting warranties on compressor and parts
Cons:- Requires professional installation, increasing upfront costs
- Higher purchase price due to advanced features
- Complex setup may need specialized contractors
Best for: Large residences or spaces requiring both heating and cooling with high energy efficiency.
Not ideal for: Budget-conscious buyers or those seeking simple cooling-only units, as its complexity and price are higher.
- Cooling Power:36,000 BTU
- SEER:18.00
- Refrigerant:R-454B
- Warranty:10-year compressor, 5-year parts
Our verdict“Best for large homes needing efficient, all-season climate control with quiet operation and long warranties.”
Goodman 2 Ton 14 Seer Heat Pump System with Multi-Position Air Handler
This Goodman heat pump system excels for those seeking a reliable, all-in-one climate solution that handles both heating and cooling efficiently. Compared with the Goodman 2.5-ton R-32 condenser, this system offers the advantage of integrated heating, making it suitable for year-round comfort. Its 14 SEER rating balances efficiency with performance, and the corrosion-resistant components extend its lifespan. However, it requires professional installation and registration to activate the 10-year parts warranty, and the noise level of 74 decibels might be noticeable during operation. The multi-position air handler adds installation flexibility, accommodating various home layouts. This model makes the most sense for homeowners who want a durable, versatile system capable of reliable performance across seasons, provided they’re prepared for installation costs and slightly higher noise levels.
Pros:- Efficient 14 SEER cooling and heating performance
- Corrosion-resistant, durable construction
- Includes a 10-year parts warranty when registered
- Versatile multi-position air handler for flexible installation
Cons:- Requires professional installation and registration for warranty
- Noise level of 74 decibels might be noticeable in quiet environments
- May need additional heat kit in very cold climates
Best for: Homeowners needing a durable, efficient heat pump system with versatile installation options for year-round climate control.
Not ideal for: Those with limited space or on a tight budget, since installation complexity and cost may be higher, and noise could be disruptive.
- Cooling Power:24000 BTUs
- Refrigerant:R-32
- Voltage:240 Volts
- Wattage:1920 watts
- Noise Level:74 Decibels
- Warranty:10 years on parts
Our verdict“Best suited for homeowners seeking a dependable, year-round climate system with durable construction and flexible installation options.”
Goodman 2.5 Ton 14.3 SEER2 R-32 AC Condenser
This Goodman condenser stands out for its use of R-32 refrigerant, which is more environmentally friendly than traditional options. Compared with the 14 SEER heat pump system, this condenser focuses solely on cooling, making it ideal for homes that already have or plan to upgrade their heating system separately. Its 14.3 SEER rating offers good efficiency, though it may require a new coil due to refrigerant compatibility issues, which could add to installation costs. This unit is best for those wanting to reduce environmental impact while optimizing cooling performance. Its limitation lies in incompatibility with R-410A systems and the potential need for additional components, making it less suitable for DIY installation or for homes with existing incompatible systems.
Pros:- Uses environmentally friendly R-32 refrigerant
- Good efficiency with 14.3 SEER rating
- Compatible with specific Goodman models
- Relatively straightforward replacement for similar units
Cons:- May require a new coil due to refrigerant incompatibility
- Not compatible with R-410A systems
- Limited information on noise levels and installation flexibility
Best for: Eco-conscious homeowners looking to upgrade their cooling with high-efficiency, environmentally friendly refrigerant options.
Not ideal for: Homeowners with existing R-410A systems or those seeking an all-in-one heating and cooling solution, as this unit only provides cooling with specific refrigerant needs.
- Capacity:2.5 Ton
- SEER:14.3
- Refrigerant:R-32
Our verdict“Best for homeowners seeking a high-efficiency, eco-friendly cooling upgrade compatible with specific Goodman models.”
ROVSUN 5 Ton 53000 BTU Ducted Central Air Conditioner with Heat Pump & Inverter System, 17 SEER2
This ROVSUN system is designed for large homes or commercial spaces, offering a substantial 53,000 BTU cooling and heating capacity. Its inverter technology ensures energy savings, especially during extended use, and it maintains reliable performance in extreme temperatures. Compared with smaller units like the 2.5-ton Goodman, this model provides the power needed for large areas, but that comes with added complexity—professional installation is required, and it involves two separate units, which can delay setup. The need for a new outdoor unit further increases costs. This system is ideal for those with large spaces who prioritize efficiency and power, though it’s less suitable for smaller homes or DIY installers due to its size and installation demands.
Pros:- Powerful 53,000 BTU capacity for large spaces
- Energy-efficient inverter technology reduces utility bills
- Flexible installation options for various indoor spaces
- Reliable performance in extreme temperatures
Cons:- Requires professional installation; not DIY-friendly
- Includes two separate units, which may complicate setup
- Needs a new outdoor unit for outdoor compatibility
Best for: Homeowners with large, open layouts or commercial spaces needing high-capacity, energy-efficient climate control.
Not ideal for: Small households or those seeking simple, plug-and-play solutions, as the system’s size and installation complexity are significant hurdles.
- Cooling Power:53000 BTU
- SEER2:17
- Voltage:230 Volts
- Application:Residential
- Warranty:7 Years
Our verdict“Best for large-scale applications where high power and efficiency outweigh installation complexity and costs.”
Goodman GSX130241 13 SEER 2.0 Ton Air Conditioner
This Goodman unit offers solid efficiency and durability in a compact form, making it suitable for smaller homes or spaces with limited installation room. With a 13 SEER rating, it’s a dependable choice for those prioritizing cost-effective cooling. Compared to the larger 2.5-ton models, this 2-ton unit is easier to install and fits more confined spaces, but it may lack the capacity for larger or multi-zone homes. The copper coil construction enhances longevity, and a 10-year parts warranty adds peace of mind. However, its limited size means it’s less suited for bigger spaces, and some users might find the noise level or specific installation requirements less flexible.
Pros:- Energy-efficient with 13 SEER rating
- Compact size for small spaces
- Durable copper coil construction
- 10-year parts warranty
Cons:- Limited capacity for larger spaces
- Restrictions in certain states due to federal regulations
- Requires professional installation for warranty
Best for: Small households or spaces needing reliable, efficient cooling in a compact footprint.
Not ideal for: Large homes or spaces with high cooling demands, where the 2-ton capacity might fall short.
- Model:GSX130241
- Dimensions:26 x 26 x 27.5 inches
- Weight:150 pounds
- Voltage:208/230V
- Warranty:10 years parts
Our verdict“Ideal for small homes or apartments where space-saving, reliable cooling is the priority.”
2.5 Ton Up To 15 SEER2 Goodman Air Conditioner Model GLXS3BN3010
This Goodman model combines efficiency with smart technology, making it a good pick for tech-savvy homeowners with moderate-sized spaces. Its SEER2 rating up to 15 offers energy savings, and the quiet operation enhances indoor comfort. Compared with the larger ROVSUN system, this unit is less powerful but easier to install and suitable for spaces up to about 1,500 square feet. Its compatibility with smart thermostats allows remote control and scheduling, adding convenience. The main tradeoff is that accessories like line sets and heat kits are sold separately, which can increase the overall cost, and professional installation is necessary to ensure warranty coverage. It’s best for those seeking a balance of efficiency, quiet operation, and smart features.
Pros:- Energy-efficient SEER2 ratings up to 15
- Quiet operation for indoor comfort
- Compatible with smart thermostats
- Durable, corrosion-resistant coils
Cons:- Accessories sold separately, increasing costs
- Requires professional installation for warranty
- Limited capacity for large spaces
Best for: Homeowners wanting energy-efficient, quiet cooling with smart thermostat compatibility in moderate-sized spaces.
Not ideal for: Large homes or spaces with high cooling loads, where the 15 SEER2 rating might not suffice.
- Cooling Capacity:2.5 Ton
- SEER2:13.4 to 15
- BTU:30000
- Compressor Type:rotary_scroll
- Warranty:10-year parts-limited
Our verdict“Best for tech-savvy homeowners seeking quiet, energy-efficient cooling with smart controls in medium-sized areas.”
2 Ton Central Air Conditioner System with 24,000 BTU and 13.4 SEER2 (Oxbox by Trane)
This Oxbox by Trane stands out as the best overall choice for those seeking reliable, energy-efficient cooling in small to medium-sized homes. Its 13.4 SEER2 rating offers solid energy savings compared to less efficient models like the Goodman 2 Ton 14 SEER2 system, making it a smart long-term investment. The durable galvanized steel outdoor condenser ensures longevity, even in harsh weather, and the environmentally friendly R-454B refrigerant reduces ecological impact. However, its size and weight—over 200 pounds—may complicate installation, especially in limited spaces, and the absence of included accessories could add to upfront costs. Compared to larger 3-ton systems, this model is more suited to smaller spaces, not large homes. Overall, this system balances performance and reliability, making it ideal for homeowners prioritizing efficiency and durability.
Pros:- High energy efficiency with 13.4 SEER2 rating ensures lower utility bills
- Durable galvanized steel construction withstands outdoor conditions
- Environmentally friendly R-454B refrigerant reduces ecological impact
- Reliable Trane-backed performance provides peace of mind
Cons:- Heavy and large, requiring significant space and effort to install
- No included installation accessories or detailed instructions can increase setup complexity
Best for: Homeowners with small to medium-sized residences seeking a durable, energy-efficient system.
Not ideal for: Large residences needing higher capacity or those with limited installation space.
- Cooling Power:24,000 BTU
- SEER2:13.4
- Refrigerant:R-454B
- Capacity:2 Tons
- Voltage:208 Volts
- Wattage:4477.6 watts
- Item Dimensions:74″D x 48″W x 43″H
- Item Weight:210 Pounds
Our verdict“This system is best suited for homeowners prioritizing durability and efficiency in smaller to medium homes willing to handle a larger unit installation.”
MrCool VersaPro 2.5 Ton 17 SEER2 Central Ducted Heat Pump Split System – R454B – 35ft Pre-Charged DIY Line Set Included
The MrCool VersaPro makes a compelling choice for homeowners who want an energy-efficient, easy-to-install system with smart features. Its 17 SEER2 rating surpasses the Oxbox, translating into better long-term savings and lower operating costs, particularly for those who value quiet operation at 43 decibels. The pre-charged DIY line set simplifies installation, making it accessible for motivated DIYers, unlike the more complex setup required for larger, traditional systems. Its inverter technology provides smooth, silent operation, and smart thermostat integration enhances convenience. However, the system’s outdoor unit installation space is a consideration, and its 1-star rating may be less reassuring compared to premium options. It’s best suited for medium-sized homes where energy efficiency, smart control, and ease of installation take priority over maximum capacity.
Pros:- High SEER2 rating of 17 offers excellent energy savings
- Includes pre-charged line set for straightforward DIY installation
- Quiet operation at 43 decibels minimizes noise disturbance
- Smart thermostat compatibility enhances user control
Cons:- Requires outdoor installation space, which may be limited
- Lower star rating may be a concern for some buyers seeking premium quality
- Limited to residential use, not suitable for larger or commercial applications
Best for: Tech-savvy homeowners who want a high-efficiency, DIY-friendly system with smart features.
Not ideal for: Homeowners with limited outdoor space or those seeking the highest star-rated, premium units.
- Cooling Power:2.5 Ton
- SEER2 Rating:17
- Refrigerant:R-454b
- Noise:43 Decibels
- Voltage:240 Volts
- Wattage:480 watts
- Installation Type:Split System
- Control Method:App Control / Remote Control
Our verdict“This system is ideal for tech-savvy homeowners wanting a quiet, energy-efficient, DIY installation with smart control features in medium-sized homes.”
How We Picked
These central air conditioners were selected based on a combination of performance, efficiency, build quality, and user-reported reliability. We prioritized units with high SEER ratings to maximize energy savings and included a range of capacities to suit different home sizes. Ease of installation and maintenance considerations also played a role, especially for systems that are more DIY-friendly. Price and value were key factors, but we favored models that deliver long-term savings through durability and efficiency. The ranking reflects a balance between cost, features, and suitability for various homeowner needs, aiming to guide buyers toward the best fit for their specific circumstances.Factors to Consider When Choosing Best Central Air Conditioners
Choosing the best central air conditioner requires understanding several key factors that influence performance, cost, and ease of use. Considering these elements will help you avoid common pitfalls and select a system that meets your home’s cooling demands effectively. From efficiency ratings to installation complexity, each factor can significantly impact your satisfaction and long-term savings.Cooling Capacity and Home Size
Matching the system’s capacity to your home’s size is critical. Oversized units can cycle on and off frequently, wasting energy, while undersized systems struggle to cool effectively. Always calculate your home’s square footage and consider additional factors like insulation, windows, and climate. A professional assessment ensures your chosen system provides consistent comfort without unnecessary energy costs.
Efficiency Ratings (SEER)
Higher SEER ratings mean better energy efficiency, which lowers utility bills over time. While premium models with 17 or higher SEER can save money long-term, they often come with higher upfront costs. For moderate climates or budget-conscious buyers, models with around 14-16 SEER can strike a good balance. Remember, the most efficient system might not be cost-effective if your local energy prices are low or if the initial price is prohibitive.
Installation and Maintenance
Proper installation is vital for optimal performance and longevity. Some systems are designed for straightforward DIY setup, which can save money but requires technical skill. Professional installation ensures correct refrigerant charging and system calibration. Regular maintenance, such as filter changes and coil cleaning, extends system lifespan and maintains efficiency, so factor in ongoing upkeep when choosing your unit.
Budget and Long-Term Value
While upfront costs are a major consideration, evaluating long-term savings through energy efficiency and durability is equally important. Cheaper units might save money initially but could lead to higher utility bills and more frequent repairs. Investing in a higher-quality, efficient system can pay off over its lifespan, especially if you plan to stay in your home for years.
Additional Features and Compatibility
Features like inverter technology, variable-speed fans, and smart thermostats enhance comfort and efficiency. Compatibility with existing HVAC components and smart home systems can also influence your choice. Be mindful of your home’s infrastructure; some advanced features may require professional upgrades or additional wiring, which adds to installation complexity and cost.
Frequently Asked Questions
How do I determine the right size central air conditioner for my home?
The correct size depends on your home’s square footage, insulation, window size, and local climate. An HVAC professional can perform a load calculation, which is the most accurate method to determine capacity. Choosing a unit that’s too large or too small can lead to inefficiency and uneven cooling, so investing in a proper assessment is worth the effort.
Is a higher SEER rating always better for my home?
Higher SEER ratings generally mean more energy-efficient operation, reducing utility bills over time. However, the initial cost increases with efficiency, and in milder climates, the savings may not offset the higher purchase price. It’s important to weigh your local energy rates and how long you plan to stay in your home before opting for the most efficient but pricier models.
Can I install a central air conditioner myself?
While some smaller or simplified units are marketed for DIY installation, most central systems require professional expertise. Proper installation ensures optimal performance, safety, and compliance with local codes. Incorrect installation can lead to poor efficiency, higher energy costs, and potential damage, so consider hiring a licensed technician for most systems.
How often should I perform maintenance on my central air conditioner?
Regular maintenance, such as replacing filters every 1-3 months and scheduling annual professional checkups, helps maintain efficiency and prolongs system lifespan. Cleaning coils and checking refrigerant levels should be part of routine upkeep. Well-maintained units run more quietly, use less energy, and are less prone to breakdowns.
What features are worth paying extra for in a central AC unit?
Features like variable-speed fans, inverter compressors, and smart thermostats can significantly improve comfort, energy savings, and ease of control. These options often reduce noise levels and provide more consistent cooling. While they come at a premium, the added convenience and efficiency can justify the expense for many homeowners, especially those prioritizing quiet operation or smart home integration.
Conclusion
For most homeowners, the Oxbox by Trane emerges as the best overall choice thanks to its reliable performance and solid efficiency. Budget-conscious buyers will appreciate the Goodman 2 Ton 14 Seer System for its value. If you’re seeking a premium, feature-rich system, the MrCool VersaPro offers excellent efficiency with DIY-friendly installation. Beginners or those with simpler needs should consider models that balance ease of installation with good efficiency, while larger homes or those prioritizing long-term savings might opt for higher-capacity or inverter-driven systems. Ultimately, your decision should align with your home’s size, your energy goals, and your budget constraints.
