What is a furnace evaporator coil? (All you need to know)

Those who are familiar with the intricate mechanics of an HVAC system know exactly how important the furnace evaporator coil is.

Nobody would like their cooling system to break down in the middle of a hot summer or a chilling winter and more often than not the reason why the HVAC starts to dysfunctional is a compromised evaporator coil.

But what exactly is it and how does it function?

What is a furnace evaporator coil

What is a furnace evaporator coil?

The furnace evaporator coil is that part of the HVAC system that absorbs the heat from the air from inside of a room and works with a condenser coil to create a heat exchange process that produces cool air.

Simply put, the evaporator coil assists the refrigerant inside the HVAC to collect the heat from the air and exchange it for cool air from the condenser. But this is not all an evaporator coil is good for as it can work for both the furnace as well as the air-conditioner depending on which you are using at the moment.

Where is the evaporator coil located in a furnace?

In an HVAC system, you will find the evaporator coil located either inside the air handler or attached to the furnace. The air handler is box-shaped and contains a blower fan.

If you want to locate the evaporator coil then look for a box with an A-frame shape. There are panels that are lined with “fins” or thin metal strips that allow the passing air to get as close as possible to the coil tubing.

What does a furnace evaporator coil do?

The furnace evaporator coil forms an integral part of an HVAC because it performs one of the most important tasks in the whole system. Because the HVAC acts as both an air conditioner and artificial heating, the evaporator coil assists directly and partially in both processes.

Firstly, the primary job of the furnace evaporator coil is to help the refrigerant gas in absorbing all the heat from the air inside a home or where the central HVAC system has been fitted. This is done by housing the refrigerant and when air from the blower fan moves over the coil and the cold refrigerant absorbs all the heat from the room’s air.

After this, the refrigerant becomes warmer and moves towards the condenser coil which is located in the outdoor unit of the HVAC.

During winters, heating takes priority over air conditioning and this process reverses with the evaporator coil expels heat from the refrigerant directly into the house instead of absorbing it and guiding it outdoors.

In both cases the evaporator coil works together with the condenser coil, to bring about the desired temperature inside the house. This is why the evaporator coil is so integral and because it deals with heat and outside air during the entire time the HVAC is running, the chances of it getting dirty or damaged are high.

Why is the evaporator coil important?

In simple terms, the evaporator coil is important because it performs the key role of housing the refrigerant gas and allowing it to pass to the condenser coil to perform cooling and heating functions when required.

It is thus a vital part of the HVAC system. Holding the refrigerant helps in cooling the air that passes through it, and thus it is usually connected to the air conditioner and not the furnace as its primary function is to cool. The air blower blows air from the house into the evaporator coil where it is cooled by the refrigerant and the now cold air is blown back into the house with the help of channels and ducts.

Thus it is clear that if the evaporator coil stops functioning, it will be an impediment to the whole HVAC system operating properly.

But it has also been mentioned that the evaporator coil performs the role of heating and thus sometimes it is attached to the furnace. But to be fair the evaporator coil does not have an active role in heating.

That said, it must be stated that both the furnace and the air conditioner use the same access points which means that when the furnace is being used, the air being collected will be first warmed by the furnace and this air has to pass through the evaporator coil to be distributed in the house through the ducts.

Since the evaporator coil is not in operation when the furnace is on, the heat from the air will not be absorbed. But if there is a problem with this essential component of the HVAC then even the heated air will not be able to pass through efficiently. Thus, although the evaporator coil in furnace operation is not an actively important component, it is still integral.

If dirt and gunk start to accumulate inside the coil then the system will start to act up. As when the air conditioner is on, the coil won’t absorb the heat efficiently leading to the air that comes out being warm instead of cool. Similarly, a dirty coil will block the heated air and the room temperature will be cold and uncomfortable.

What causes an evaporator coil to fail?

What HVAC owners dread the most is probably faulty or failed evaporator coils because then they need to be replaced and replacing these things cost a hell lot of money. But what exactly causes this part to act up? Let us find out.

Dirt and gunk

Yes, dirt and many types of debris can accumulate on the evaporator coil and disrupt its proper functioning.

Specifically, the layer of dirt that accumulates on the surface of the coil acts as unnecessary insulation between the air and the refrigerant it stores. Since the refrigerant cannot cool the air properly, the air that is blown is thus warm.


This happens mainly because the coil is so often cold that it is easy for water droplets to materialize on its surface and accumulate here.

This can lead to the harmful growth of mold and bacteria ultimately blocking the evaporator coil from performing its duties properly.

Not enough refrigerant

For whatever reason, if the amount of refrigerant is not enough to deal with the amount of airflow then the evaporator coils might freeze. This can also happen if there is not enough flow of warm air.

Dirty air filter

The job of an air filter is to trap the dust on itself before it can get to the inner parts of the system. If it accumulates too much dirt then the airflow will be restricted causing the evaporator coil to freeze and fail.

The consequences of a failed coil are many but the most noticeable is a lack of warm air when the furnace is turned on and a lack of cool air when the evaporator coil is turned on.

Moreover, if the warm air from the furnace constantly gets blocked in the coil without being able to pass through then it might confuse the furnace sensors into short cycling as it will sense the heat and automatically turn off after believing it heated the house well enough.

As a result, the furnace will not be able to warm your home properly and may even get damaged.

How to Prevent Dirty Evaporator Coils?

No mechanical part will last forever but if the parts in question are as expensive as evaporator coils are then you might want to look into preserving their longevity for as long as you can.

The best way to make sure that those coils are in tip-top shape is to keep them clean. But how exactly should you do that? Here is how-

Change the air filters periodically

This is probably the easiest and also the most effective way to make sure that the AC parts do not get dirty.

Changing the air filters every 3 months or 90 days is the standard norm but if you have pets at home who shed and people who have allergies and respiratory issues then 60 days is recommended to keep one filter on.

Use a better quality air filter

After consulting with the HVAC manual. see how high and upgraded an air filter can be used with your unit. Then install it. This will ensure better performance and trap fine particles of dust from going in.

Call professionals for maintenance at least twice a year

Periodical maintenance is the key to an optimally performing AC and extension of the evaporator coil. This is essential because while you can change filters yourself and vacuum the outer side of the ducts, there is no way that you can reach the interior.

Qualified technicians will assess the performance and also check for damages, leaks, and so on.

Don’t miss:

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Wrapping Up

Thus the long story short is that the evaporator coil is a very important part of the HVAC and keeping it clean with regular maintenance is the way to keep it in peak performance.

You might think that maintaining an HVAC is eating away a lot of your money but if you think about it, the expense that you will have to incur in one go will easily make the costs of regular maintenance seem like spare change.

So do not second guess it and do not let mere dirt eat away at the HVAC.

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