Evaporator coils are a critical component of any HVAC (heating, ventilation and air conditioning) system. They play an essential role in regulating temperature and humidity levels inside a home or building.
Evaporator coils are made up of thin tubing that contains refrigerant, a special fluid that can easily change between a gas and a liquid state. As warm indoor air passes over the tubing, the refrigerant inside absorbs heat from the air. This causes the refrigerant to boil and turn into a vapor. The heat absorbed by the refrigerant is then released outside, which cools the air inside the building.
A-coils are the most common type of evaporator coil. They get their name from their A-shape design, consisting of two slanted sides joined together at the top.
The main components of an A-coil include:
- Refrigerant tubing – Usually made of copper or aluminum, the tubing is attached to the interior sides of the A-shape. Refrigerant flows back and forth through the tubing, absorbing heat from the air.
- Fins – Thin slices of metal attached to the tubing to increase surface area. More fins allow more air contact, improving heat transfer.
- Drain pan – Located under the coil. Condensation that forms on the coil drips into the pan and drains away. This removes humidity from the air.
Advantages of A-coils:
- Widely available from many manufacturers
- Easy to service and clean
- Highly reliable
- Fit into small spaces
A-coils are used in all types of HVAC systems, from residential air conditioners to large commercial units. Their versatile design makes them suitable for a broad range of applications.
N-Coils and Z-Coils
N-coils and Z-coils are less common than A-coils but used by some HVAC brands like Carrier, Bryant, Rheem and Ruud. The main difference from A-coils is the addition of a third row of refrigerant tubing and fins.
Key components of N/Z coils:
- Three sections of cooling coils and fins instead of two
- Supply and return piping on the same side
- Tight, compact design
Benefits of N/Z coils:
- Increased surface area improves heat transfer
- Smaller footprint than A-coils
- High efficiency ratings
The extra row of tubing makes N/Z coils more complex to manufacture and install. Their tight design also makes them harder to clean and service. However, the improved heat transfer capabilities compensate for these drawbacks in many cases.
Key Takeaway: The two main evaporator coil types are A-coils and N/Z coils. A-coils are the most common while N/Z coils offer increased efficiency but are harder to service.
How Do Evaporator Coils Work?
While A-coils and N/Z coils have some differences, they operate on the same basic refrigeration cycle:
- Liquid refrigerant flows into the indoor evaporator coil through an expansion valve. This valve causes the refrigerant pressure to drop significantly.
- The now very cold, low pressure liquid refrigerant passes through the thin tubing in the evaporator coil.
- Warm indoor air is blown over the coil fins by a blower fan. The air absorbs heat from the refrigerant, causing the refrigerant to boil and evaporate into a vapor.
- The refrigerant vapor is then sucked into the compressor, located in the outdoor condenser unit. The compressor pressurizes the vapor.
- The hot pressurized vapor flows into the outdoor condenser coil, where it condenses back into a liquid state, releasing its heat to the outside air.
- The liquid refrigerant passes through an expansion valve once more, starting the cooling cycle over again.
This evaporation process inside the indoor coil is what actually absorbs heat from the indoor air and provides the cooling effect. Proper working evaporator coils are crucial for an efficient, long-lasting AC system.
Materials Used to Make Evaporator Coils
Evaporator coils were traditionally made from copper tubing. Copper offers excellent thermal conductivity, allowing heat to transfer rapidly from the air to the refrigerant. It also resists corrosion, providing a long service life.
In more recent years, aluminum has become a popular alternative coil material:
- Aluminum is lighter in weight than copper, making coils easier to handle.
- It is also cheaper to manufacture while still offering good heat transfer capabilities.
- Aluminum alloy coils combine aluminum with other metals like copper to improve durability and thermal performance.
- Most residential HVAC systems now use aluminum alloy evaporator coils rather than pure copper.
The fin material is also important. Fins account for much of the coil’s surface area contacting the air, and are usually made of aluminum due to its high conductivity, light weight and corrosion resistance. Overall, aluminum alloy tubes combined with aluminum fins offer the best value and performance for most evaporator coils today.
Maintaining and Cleaning Evaporator Coils
To ensure peak efficiency and indoor air quality, evaporator coils should be professionally cleaned and inspected annually as part of routine HVAC maintenance. Dust and dirt buildup on coils can lead to:
- Reduced heat transfer and cooling capacity
- Higher energy bills as the system works harder
- Potential refrigerant leaks due to corrosion
- Excess humidity as condensation efficiency decreases
- Unpleasant dust or odors spreading from dirty coils
Careful chemical cleaning and rinsing performed by an HVAC technician can restore coils to like-new condition. Homeowners can also help between maintenance visits by:
- Changing filters monthly
- Keeping outdoor condenser coils clear of debris
- Ensuring unobstructed airflow around vents
- Having ductwork cleaned every 3-5 years
Signs of a Problem With Your Evaporator Coil
Watch for these warning signs that may indicate evaporator coil issues:
Higher electric bills – Clogged coils lower efficiency, increasing runtime and power consumption.
Reduced cooling capacity – Your system can’t keep up and runs constantly but the house doesn’t feel cool.
Ice buildup on coils – Indicates low refrigerant level or very dirty coils. Ice blocks airflow and must be defrosted.
Water leaks – Signify corroded coil tubing that finally sprang a leak. Refrigerant may also be leaking out.
Noisy operation – As coils become obstructed, fans and compressors strain harder causing unfamiliar noises.
If you notice any of the above, call a professional HVAC technician to inspect and service your evaporator coil. Left uncorrected, these issues can lead to system failure or compressor breakdown.
Evaporator Coil Replacement
If an HVAC technician determines your evaporator coil is damaged beyond repair, a full replacement will be necessary. Some key facts about coil replacement:
- Labor and materials average $1000-$2000 depending on location and system size.
- The refrigerant must be pumped down and lines cut to remove the old coil. This requires special equipment.
- New lineset tubing is generally installed along with the new coil.
- The system must be thoroughly evacuated of any moisture before recharging with refrigerant.
- Upgrading to a higher efficiency coil can pay back through lower energy bills over time.
- A new coil usually comes with a 5-10 year warranty.
Since the evaporator coil is a major component integrated into the rest of your system, it’s hard to predict the exact cost or work involved until an HVAC technician evaluates your particular situation. Get a professional assessment before proceeding with replacement.
Evaporator Coils in Heat Pumps
Heat pumps are a special type of HVAC system that reverses the direction of refrigerant flow, allowing the unit to provide both heating and cooling. This requires an additional component called a reversing valve.
When the heat pump runs in cooling mode, the evaporator coil inside the house acts just like a normal AC system, absorbing heat from indoor air.
In heating mode, the reversing valve flips the refrigerant flow so that the indoor coil becomes the condenser. It now absorbs heat from the outdoor air and releases it indoors to warm your home.
So in a heat pump, the indoor coil alternates between acting as the evaporator and condenser depending on whether the system is cooling or heating. The coil itself remains the same. This dual-purpose design is what gives heat pumps their versatility to handle both hot and cold weather.
Evaporator Coils in Commercial HVAC Systems
Larger commercial buildings like offices, hospitals and schools often use more powerful centralized HVAC systems instead of individual residential units. These systems employ the same evaporator coils to provide cooling, just on a bigger scale:
- Large packaged rooftop units – Contain both the evaporator and condenser coil in one outdoor package unit, providing conditioned air to the building’s ductwork.
- Split systems – Use a separate indoor air handler with evaporator coil and large outdoor condenser just like residential systems. Scaled up for bigger spaces and airflows.
- Chillers – Massive central units that cool water instead of directly cooling air. The chilled water is circulated to “fan coil” units containing evaporator coils throughout the building.
- Variable refrigerant flow (VRF) – Allows multiple indoor evaporator coils in different zones, all connected to one or more outdoor condenser units. Enables precise temperature control.
Regardless of the overall system design, the evaporator coils themselves operate on the same principles to provide cooling and dehumidification. Proper coil selection, maintenance and replacement is equally important in large commercial systems.
Key Takeaway: Heat pumps, chillers and large commercial HVAC systems rely on properly functioning evaporator coils just like residential air conditioners.
How often should evaporator coils be replaced?
With proper maintenance, evaporator coils typically last 10-15 years. Heavy use, improper cleaning, leaky ductwork or other factors can shorten lifespan. Have coils inspected annually and replace if necessary.
Do N-coils work better than A-coils?
The extra row of tubing in N-coils improves heat transfer efficiency by 5-10% in most cases. However, they cost more and don’t last as long. Efficiency ratings are more important than coil type when comparing HVAC systems.
Should I upgrade to a new efficiency coil?
If your current coil has suffered damage or major loss of efficiency, upgrading to a newer high-efficiency model can pay back over time through energy savings. Consult an HVAC pro on the best replacement.
What causes evaporator coils to freeze?
Low refrigerant charge, very dirty coils, incorrect refrigerant airflow or other issues can all cause ice to buildup on evaporator coils. This blocks airflow and requires immediate attention from an HVAC technician.
Do all HVAC systems have an evaporator coil?
All modern air conditioners, heat pumps and central HVAC systems rely on an indoor evaporator coil to provide cooling. The main exception would be a Chiller system that cools water instead of air directly.
The humble evaporator coil is one of the most critical HVAC components, even though homeowners rarely see it. Proper functioning evaporator coils allow your heating and air conditioning system to reliably produce comfortable indoor temperatures and humidity levels.
By understanding the basics of how evaporator coils operate and the signs of potential problems, you can help optimize performance. Partnering with professional HVAC contractors for regular maintenance and repairs will keep your evaporator coils running efficiently for years to come.