Leaving the house for a while.

The instinct is to turn it off and save the money. In most of the country that is reasonable. In Phoenix in July it is not, and the saving is smaller than people expect.

Why off is the wrong setting

A closed Phoenix house with the sun on the roof and nothing running gets genuinely hot inside, well past anything the furniture was designed for. Over weeks that is hard on cabinetry and veneers, adhesives, electronics, musical instruments, candles, and plenty of things in a pantry.

It also means the condensate drain sits dry for months, the system does not run at all, and nobody is there if something does go wrong. The snowbird guide covers what that combination does over a full summer.

A sensible number

Most people landing on a figure for a Phoenix summer absence end up between 85 and 88.

That is high enough that the system cycles rarely and the bill stays small, and low enough that the house never reaches the temperatures that cause damage. There is no single correct number. There is a wrong one, and it is off.

Two adjustments worth making:

• Lower it a few degrees if you have pets staying, obviously, and then the whole calculation changes and belongs in the house sitter guide.
• Lower it if the house has a lot of west glass, because the afternoon load is different.

Before a long absence

Change the filter. A loaded filter over three months of running is how a coil gets dirty.

Clear the drain. A cup of white vinegar down the condensate access tee. A plug that forms while you are away trips the float switch, the system stops, and the house spends the rest of your trip at outdoor temperature.

Confirm the float switch exists. If not, fitting one before a long absence is the cheapest insurance on this page.

Clear around the outdoor unit so nothing blows against it in a monsoon.

Close blinds on the sun side. Free, and it meaningfully cuts the load.

Book the tune-up for before you go, not after you return. A fault found in advance is an appointment.

The thing actually worth buying

A thermostat you can see from anywhere. Not for the setback savings, which in this climate are modest, but for the alert that tells you the house is at 105 in July while you are elsewhere.

Paired with somebody local who can open the door, that turns a four month disaster into a two day repair. The C-wire guide covers whether your wiring supports one, and a thermostat install with the wiring in place is a flat published price.