Think of dirt on a coil as a blanket. That is the last thing you want on the part of your system whose entire job is moving heat from one place to another.

Dirty coils affect cooling by blocking heat transfer, and the effect is different depending on which of your two coils is dirty. One sits inside your home. The other sits outside. They get dirty from completely different sources, and they fail in different ways.

We pull indoor coils in Houston that look like felt. It is dust that landed on a wet surface and stayed there. Below we cover what each coil does, why the indoor one fouls faster here, and what you can safely clean yourself.

Dirt on a coil works like a blanket. It blocks the heat transfer the coil exists to perform, so your system moves less heat with the same effort.

The indoor coil, called the evaporator, absorbs heat from the air in your home. When it is dirty, less heat gets pulled out, so the air from your vents is not as cold. Restricted airflow can also drop the coil below freezing and ice it over. Coil cleaning is on the ENERGY STAR maintenance checklist for that reason.

The outdoor coil, called the condenser, releases that heat outside. When it is dirty, the heat has nowhere to go. Pressures rise inside the system and the compressor works harder to do the same job.

Either way, run times lengthen, bills rise, and the compressor takes the strain.

Most advice about dirty coils treats them as one thing. They are not. They sit in different places, face different conditions, and need different attention.

The sources matter. Your indoor coil gets dirty from inside your house, so your filter is the only thing standing between your dust and that coil. Your outdoor coil gets dirty from the yard around it.

That means they foul on different schedules. It also means cleaning one does nothing at all for the other.

A coil is thin metal with a lot of surface area. Air passes over one side, refrigerant runs through the other, and heat crosses between them. Thin metal and wide surface are what make that work.

Dirt undoes both. A layer of grime adds insulation in the exact spot where you need the opposite. Heat that should pass through the metal gets held back instead.

So the refrigerant picks up or releases less heat on every pass through the coil. The system is running. It is just accomplishing less each cycle.

Dirt also fills the narrow gaps between the fins. Less air makes it through, which means less air is available to exchange heat in the first place.

Put those together and your system runs longer to reach the same temperature. Nothing has broken. The equipment is simply doing a harder version of the same job.

Your evaporator coil does two jobs at once. It pulls heat out of the air, and it pulls moisture out with it. That moisture condenses on the coil surface every hour the system runs.

In a dry climate, the coil dries out between cycles. Here it does not. Across a seven-month cooling season, that surface stays wet almost continuously.

That changes what happens to dust. On a dry coil, some of it blows through and moves on. On a wet coil, it lands and stays. What builds up over a few seasons is closer to mud than dust.

A surface that stays damp and dark also gives biological growth a place to establish. Whatever grows there sits directly in the path of the air moving into your home.

When we open an air handler in August, the coil is wet. It has been wet since April.

Signs your indoor coil needs attention:

• Cooling has weakened even though the filter is clean
• A musty smell when the system first starts
• More dust settling in the house than usual
• Water showing up around the indoor unit

That last one connects to something else. The same moisture running off the coil drains to the pan and out through the condensate line. A fouled coil often arrives with drain problems attached.

Ice on an air conditioner in August looks impossible. It is one of the more common calls we get.

The refrigerant inside your evaporator coil is genuinely cold. What keeps the coil surface above freezing is the warm air your blower pushes across it. That constant flow of warm air is doing more work than people realize.

Restrict the airflow and the balance tips. The coil temperature drops below freezing, and moisture in the air freezes onto the surface instead of draining away.

Then it compounds. The ice blocks more airflow, which drops the temperature further, which freezes more moisture. A thin layer of frost becomes a solid block over a day or two.

When that ice melts, it does not always find the drain. Water overflows the pan and goes wherever the pan sits, which in most Houston homes is above your ceiling.

One thing to know before you assume the coil is dirty. A low refrigerant charge causes the exact same symptom for a completely different reason. A frozen coil needs a diagnosis, not a guess.