Pets, filters, and a Phoenix summer.

Two separate things here. What animals do to the system, and what a failed system does to animals. The second one matters more.

What pet hair actually does

Hair and dander are light, and plenty of it gets past a standard filter or around one that does not seal properly in its slot. Where it ends up is the evaporator coil, which is wet all summer and therefore sticky.

Hair bonds to a wet coil, the coil loses its ability to transfer heat, and airflow through the fins drops. The symptom is a system that cools less well every year for no reason anybody can point at, and eventually a coil that freezes.

The same hair loads the blower wheel, which moves less air while spinning exactly as fast as before.

The outdoor unit gets its own version: hair and fur mat into the condenser fins along with the dust, especially on a unit a dog lies next to in the shade.

The filter question

The instinct is to buy the densest filter available. Resist it.

A high MERV filter catches more and restricts more, and restricted airflow is how a coil freezes. A moderate filter changed monthly beats a dense filter left for a season, by a wide margin.

What matters as much as the rating:

• Correct size, seated properly, with no gap for air to bypass it. Air takes the easy route and carries hair with it.
• Actually changed. Look at it monthly through summer.

The outdoor unit

Keep two feet clear, and if a dog has picked the shaded side of the condenser as a favourite spot, expect the fins on that side to load faster. A gentle hose rinse with the power off at the disconnect helps. A chemical clean is what gets material out of the fin pack once it has set.

When the AC fails

This is the part that matters.

Animals cope with heat worse than people do. They cannot sweat the way you can, they are often in a closed house with no way to leave, and they cannot tell you they are in trouble. Flat faced breeds, older animals and anything thick coated are at particular risk.

If the system goes out in a Phoenix summer and there are animals in the house:

• Move them to the coolest, most interior room, lowest floor
• Water everywhere, more bowls than you think
• Wet a towel for them to lie on, and run a fan across it
• Close the blinds on the sun side
• Get them out if the house keeps climbing. A friend, a car with its own AC running, a boarding place, a vet

Warning signs worth knowing: heavy frantic panting, drooling, bright red or very pale gums, staggering, vomiting, collapse. Those mean a vet now, not later.

If you are leaving animals with a sitter, the house sitter guide is worth sending them, and so is your technician’s number.