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Why and When to Replace an HVAC Evaporator Coil

Why and When to Replace a Coil

Evaporator coils are a hot topic when it comes to HVAC repairs. Sometimes a bad HVAC evaporator coil leads to people replacing their whole HVAC unit. So, today we will be touching on what the evaporator coil is, why it goes bad, and when you need to replace it to keep your HVAC unit running smoothly.

What is an Evaporator Coil?

The evaporator coil also called an indoor coil or just a coil, is one of the most important parts of your HVAC. It is what keeps the temperature in your house regulated all year long. Whether you have just a cooling system, or a heating and cooling system, and whether you have a gas or oil furnace or a heat pump, the evaporator coil is what keeps you comfortable.

The evaporator coil is filled with refrigerant. As the HVAC blower draws air from inside your home across the coil, the refrigerant evaporates as it absorbs the heat from the air. The coil is a heat exchanger, giving you hot or cold air depending on the time of year. In the summer, the coil performs double duty, giving you cool air and serving as a dehumidifier, drawing out unnecessary moisture from the air. If you have a variable speed system, you can get an even higher degree of dehumidification for your home. Basically, your indoor coil has the most important job of the entire HVAC unit — keeping your home comfortable.

Why Does the Evaporator Coil Fail with Age?

While the coiled tubes of the evaporator coil are its primary surface, 70% of the coil’s work is performed by the finned area on the coil. This is known as the secondary surface. The bond between the finned area and the tubes is the most important manufacturing feature in any coil. Without this bond, the coil can’t properly function. Like many things, over time, the fin/tube bond becomes less and less efficient, due to the constant expansion and contraction of the coil as it does its job. Even though the construction of the coil and fin does not allow for movement, the fin/tube bond naturally weakens over the life of the coil. Because of this, the coil can become approximately 30% less efficient over the course of 20 years.

Sometimes, people try to clean the coil, however, this often pushes dirt to the center of the coil, causing more issues. This occurs even more frequently when trying to clean wet cooling coils. The BTU output of any coil is directly proportional to the amount of air going through the coil, and air flow is decreased significantly when dirt is pushed to the center of the coil. For example, if you decrease the air flow by 20%, you also decrease the BTU by 20%. Additionally, cleaning agents often corrode aluminum fins. Since every square inch of fin surface matters in performance, corrosion of the fin surface is always detrimental to the coil’s performance.

Many times, there are coil leaks simply because of old age. No coils are immune to erosion. You might find the joint in the tubes and in the header/tube connections failing over time. These joints have been brazed (melted) together during manufacturing to form a cohesive unit, but steam can be both erosive and corrosive under higher pressures. Water travels through the coil at 2-5 feet/second, so erosion is an enormous part of coil failure, regardless of how well-maintained. Erosion is always there, whether you realize it or not. Water/steam treatment and the corrosive effects of bad steam/water can all be causes of coil failure.

What Makes an Evaporator Coil Go Bad?

There are multiple reasons why an evaporator coil can fail prematurely, which is why proper maintenance is so important. A homeowner can help ensure the longevity of the coil by getting it regularly serviced. With proper maintenance, an evaporator coil can live up to 20 years. It’s important that homeowners follow the precautions and recommendations given by their service technician. Here are some important things to keep in mind, to help save you from expensive repairs down the line:

  1. Many issues can be prevented just by changing the filter in the HVAC unit every 30 days. When the filter is not changed as needed, it can draw dirt through and into the system, affecting the coil, as well as other components of your HVAC unit. The coil needs to breathe and allow the air from your home to move across it, which it can’t do efficiently with a dirty filter. If left too long, a coil clogged with dirt will freeze, causing water to pour everywhere. This is the result of the refrigerant building up too much discharge pressure because the volume of air that was needed to maintain the pressure in the system wasn’t there. In turn, this stresses the joints on the condenser coil, which can lead to a line popping and a refrigerant leak that can cost a homeowner thousands in repairs. Changing the filter regularly would have prevented this issue from the start.
  2. The blower has slotted vents for the HVAC motor to breathe. If the motor can’t breathe as needed, it will eventually go bad. It will start by drawing high amps and eventually just overheat itself until the silicone bindings inside fail completely, but that’s a subject we’ll address in another article.
  3. The condenser outside gets a huge buildup of dirt, leaves, and grass from lawn mowing, falling debris from trees, etc. The condenser coil on the outside unit is just as important as the evaporator coil on the indoor unit. In fact, the evaporator coil often fails due to issues with the condenser coil. As the condenser fan spins, it attempts to draw air across the condenser coil and through to the evaporator coil. If you let that condenser coil get too dirty it will build up the pressure, causing efficiency loss that could risk both evaporator and compressor failure, leading to expensive repairs.

How to Avoid Evaporator Damage:

Homeowners will often get recommendations from technicians for evaporator coil cleaning, condenser coil cleaning, and motor cleanings. Sometimes, homeowners are skeptical and think the technician is trying to get them to spend more money on something they don’t need. This is not the case at all. As trained service technicians, we have seen systems fail left and right from lack of proper user maintenance. An HVAC is a finely tuned machine and like a vehicle, it needs to have parts fixed and replaced regularly. So when your technician makes a recommendation don’t push it aside. Remember: a $200 cleaning now can save you from a $2,000 repair later.

When to Change an Evaporator Coil:

This is one of the easiest questions to answer. If the coil is leaking, it needs to be replaced. Of, if the cost of cleaning is more expensive than the cost of replacement, and only guarantees getting the efficiency of the coil back to 50-75%, it should be replaced. Right now, we are in a transition period where the old R-22 refrigerant is being phased out for R-410. Because of this, what would be a $1,500 fix for an R-410 system is easily a $3,000 fix for an R-22 system. If you find yourself in that scenario, it’s time to replace the coil.