Author: Kevin

Quicklinks to other articles in this series:

What Are the Indoor Coil and TXV?

The indoor coil is the portion of the air conditioner that absorbs the hot air from inside your home and transfers it to the refrigerant. It pairs with the condenser coil to create a continuous loop of heat transfer.

The TXV, which stands for thermostatic expansion valve, acts as a metering device for air conditioning systems. It regulates the rate at which liquid refrigerant flows into the indoor coil.

Where Are They Located?

An indoor coil will be located inside your home or business. The installation of the indoor coil is in line with the furnace, in a closet or in the attic.

You will likely find the TXV mounted near the indoor coil on the liquid line. Also the sensing bulb is mounted to the outside of the larger suction line.

How Do The Indoor Coil and TXV Work?

The radiator-like device mounted in the airstream is the indoor coil. As warm indoor air passes over the evaporator coil, heat from the home is absorbed into the refrigerant. Likewise moisture is naturally removed from the air . The combination of cooling and dehumidification provides the total cooling effect for the home. In contrast, a heat pump reverses this process. In the winter, heat transfers from the refrigerant into the home or business thru the indoor coil.

Through the liquid line, refrigerant enters the TXV valve at a high pressure. The TXV limits the amount of refrigerant entering the evaporator thus reducing it’s pressure. Along with this pressure drop, the saturation temperature, or “boiling point” of the refrigerant drops as well. This allows a change of state from liquid to gas which is the most efficient time to transfer large amounts of heat from the surrounding air into the refrigerant.

What Makes Them Important?

A well-maintained indoor coil is, along with the outdoor coil, essential to an efficient air conditioning system. Indoor coils that have fallen into disrepair will not be able to transfer heat effectively, causing your home or business to feel less cool. Restricted airflow to the coil can cause the coil to start icing up and even freeze over.

After regulation changes in 2006, most air conditioning systems now require a TXV. It controls the refrigerant flow to create maximum efficiency for the indoor coil. It also prevents excess refrigerant from returning to the compressor and flooding it.

Different Types of Indoor Coils and TXVs

The most common indoor coils in air conditioning systems are divided into three types:

“A” coils, which get their name from the shape they take, are common for vertically aligned air conditioners.

“N” or “W” coils, also named for their shape, work for vertical or horizontally aligned systems. They are smaller than other options.

Slab coils are less common and used for horizontally aligned air conditioning systems.

Similarly, uncased coils are uncommon as well. They easier to build as custom coils for unusual air conditioner designs. A third indoor coil design is the vertical coil, which is suitable for upward and downward air flow.

TXVs come in two major types, internally equalized and externally equalized, which refer to how the TXV senses pressure. Indoor coils with multiple refrigerant circuits typically use an externally equalized TXV.

Maintenance and Repairs

Dirty indoor coils are less efficient, sapping your air conditioner of its efficiency and leading to energy usage up to 30% above average. You can save money and keep cool by checking your air conditioner’s air filter monthly and replacing it as necessary. If you notice that the air filter looks “domed,” this is a sign that the air conditioner is not receiving enough airflow. Replace the filter immediately.

Do not attempt to clean the fins on the indoor coil as they are quite delicate and can bend easily. Instead, call a professional technician who has the right tools and expertise.

A faulty TXV forces an air conditioning system to work harder than necessary, putting stress on other components. Also improper installation of the TXV bulb, such as mounting it to the underside of the indoor coil’s output line, might provide false readings and cause malfunctions.

Quicklinks to other articles in this series:

What Are Refrigerants, Copper Pipes, and Filter Driers?

Refrigerant is the liquid chemical blend that soaks up heat and releases it as part of the air conditioning process; it is the lifeblood of the air conditioner. Copper pipes act as refrigerant lines to transport this vital fluid throughout the air conditioning system. The filter drier provides physical filtration and absorbs water and other contaminants that can be found in the refrigerant.

Where Is the Refrigerant & Filter Dryer?

Refrigerant runs throughout the air conditioner, from the evaporator coil to the condenser and back again, creating the basic flow necessary for cooling to occur. A technician can maintain refrigerant levels by access ports found on the outdoor unit.

The two main lines of copper piping run from the condenser to the evaporator coil and back. The larger is called the suction (or return, or vapor) line and the smaller one is the liquid line.

You will find the filter drier on the liquid line outside near the condenser or inside near the evaporator coil. It looks like a blue or grey soda can.

The Science Behind Refrigerant

Refrigerant is contained within copper coils inside an air conditioner. As the hot liquid refrigerant is pumped into your home to the indoor coil it is metered by the TXV which only allows a small amount to enter the coil. This creates a drop in temperature of the refrigerant allowing the heat from your home to soak up like a sponge and transfers into the refrigerant to be taken outside as a gas back to the compressor. Finally, once your thermostat senses that enough heat has been removed, it shuts off the system at just the right temperature.

Larger refrigerant lines that carry cold gas need to be insulated. For example; on a warm summer day condensation would form, like the outside of a glass of ice water. In contrast, the liquid line needs no insulation because it contains warm refrigerant.

Filter driers remove moisture from refrigerants. To acheive this, dryers use desiccants, such as activated alumina or silica. Some use molecular sieves, which trap moisture on a molecular level. Also, to keep out common contaminants, filter driers use screens and depth filters, such as bonded desiccant cores or fiberglass pad filters.

What Makes Them Important?

Refrigerants need to have low freezing and boiling points, a low condensing pressure, a high vaporization heat, high vapor density, and a high critical temperature. Moreover, they should be non-corrosive, non-flammable, and non-toxic. For those reasons, refrigerants are carefully synthesized chemicals.

The filter drier’s essential role is to remove water from the system. Water can cause freeze-ups and corrosion from organic acid compounds, so moisture must be eliminated.

As far as materials go, copper tubing is economical and eco-friendly since it is strong, light-weight, and relatively inexpensive. In addition, refrigerants are contained in copper tubing because the copper is efficient at transferring heat.

Different Types of Refrigerants, Copper Pipes, and Filter Driers

Refrigerants come in two types: the older version, R-22, commonly known as Freon; and R410A, known by its brand name, Puron. For many years the main refrigerant in air conditioning systems was R-22, but due to its effect on the ozone, it was severely restricted from production and will be banned entirely by 2020. R410A is the industry standard now. This is because it is efficient and less harmful for the environment, while providing the same cooling as R-22.

The two main types of copper pipes are hard-drawn copper and soft copper. Hard-drawn copper is very rigid and straight. Soft copper is flexible and comes in 25- or 50-foot rolls. There are markings that indicate that the pipes have been cleaned and dehydrated for use in air conditioning and refrigeration systems.

Filter driers come in various forms, including spun copper, steel liquid-line, steel suction-line, and steel bi-flow. Steel driers provide added filtration and water capacity over copper driers.

Maintenance and Repairs

Be aware that R-22 and R410A are not interchangeable. Your air conditioning system will use only one of these refrigerants. Newer air conditioners will use R410A as its refrigerant. A trained technician should handle refrigerant replacement because these coolants can be dangerous to handle and must be disposed in specific ways.

If you notice oil stains around your air conditioner, you might have a leak, potentially caused by damaged copper pipes. Harsh weather, internal contaminants, or installation in high traffic areas can damage copper pipes. Therefore never reuse old copper refrigerant lines when repairing or upgrading your air conditioner.

And finally, filter cores and drier shells may need to be replaced over time, but thankfully, the market provides a wide range of replacements. Often, these replacement cores and shells are a simple installation for a professional technician.