What should I set the AC to in a Phoenix home left vacant all summer?

By Michael Fitzgerald, Founder & Editor, Phoenix Cooling Guide
Technically reviewed by Tom Mattingly, Owner & President, AZ Trusted Air LLC (ROC 317368)
Published 2026-09-18

A Phoenix-area house closed up in April and reopened in October sits through a stretch of weather no other major American metro produces. National Weather Service normals for 1991 to 2020 at Phoenix Sky Harbor put July's average high at 106.5 degrees Fahrenheit and its average low at 84.5 degrees Fahrenheit, and July 2024 ran hotter than that on both ends. The attic above the empty house reaches 150 to 170 degrees on an ordinary summer afternoon, and the ducts running through it are still connected to the rooms below whether anyone is home or not.

The question snowbird owners actually ask is not whether to cool the house but how little cooling it can get away with. Most of the advice they find was written for Florida, where the controlling risk is humidity, or for the Midwest, where it is frozen pipes. Neither transfers cleanly. What follows covers the setting itself, what happens when the system is switched off entirely, the temperature ranges that matter to specific contents, remote monitoring that reports something useful, and what a home watcher should be looking at on each visit.

Is it safe to turn the AC completely off in an unoccupied desert home?

An empty house in the Valley does not hold still at whatever temperature it was when the door closed. It tracks the building shell. With a July attic at 150 to 170 degrees and no air moving through the supply ducts, ceiling surfaces radiate downward through the night, and an unconditioned Phoenix house routinely climbs past 100 degrees indoors by mid-afternoon and stays in the nineties until dawn. Every finish, adhesive and sealed component in the building is then held near the top of its service range for five straight months instead of for a few afternoons.

Valley owners tend to dismiss mold as a coastal problem, and for most of the year they are right. The monsoon is the exception. The U.S. EPA advises keeping indoor relative humidity below 60 percent and ideally between 30 and 50 percent, and states in its mold course materials that relative humidity above 60 percent is likely to result in condensation in the building, which can lead to mold growth. A running air conditioner removes moisture as a side effect of cooling. A system switched off in April removes none of it in August, and nothing else in the house is drying anything out.

The EPA's other figure is the one that should decide this for an absent owner: wet or damp materials dried within 24 to 48 hours of a leak or spill will, in most cases, not grow mold. A failed supply line in an occupied house is a mop and a fan inside the hour. The same failure in a house nobody enters for three weeks is a remediation invoice and a flooring claim. The EPA also advises keeping air conditioning drip pans clean and drain lines unobstructed and flowing, guidance that quietly assumes somebody is there to look.

Then there is the equipment. A rooftop package unit, the configuration that dominates large parts of the Phoenix housing stock, sitting idle from April to October is asked to start under full load on a 110-degree day with nobody present to notice that it did not. Owners chasing the lowest possible summer bill sometimes split the difference at 95 degrees, which is the worst of both: the system runs enough to cost money and not enough to control the interior. Vacancy and occupancy wording in homeowner policies varies by insurer and by how long the house stands empty, so the answer for any one household is in that policy document, best read before the drive north rather than after a claim.

What temperature protects drywall, cabinets, wine, and electronics in a vacant house?

Heat alone rarely destroys drywall or framing. What fails in a hot, empty Valley house is assemblies: veneered cabinet doors delaminating at an edge, laminate lifting at a countertop seam, solid-wood flooring laid with minimal expansion gap, a piano pulled out of tune, and adhesives on anything glued rather than fastened. The mechanism is not one peak temperature but five months of daily expansion and contraction with no relief between cycles. Holding the interior at 80 to 85 degrees flattens that daily swing, and flattening the swing is most of the protection being bought.

Electronics tolerate heat better than owners expect; their batteries do not. Lithium cells in laptops, cordless tools, e-bikes and backup power packs degrade fastest when stored hot and at full charge, and a closed drawer in a 100-degree house is a hot store. Manufacturer storage ranges are printed in the manual or on the spec sheet and differ considerably between devices, so the honest instruction is to look up the specific product rather than trust one number for the whole house. Battery-bearing items are the ones worth taking north or storing at the charge level the maker specifies.

Wine is the item no thermostat setting solves. Long-term storage temperatures sit far below anything a residential air conditioner will hold in a Phoenix summer, and an 85-degree pantry will flatten a collection in a single season. The workable options are a dedicated wine refrigerator on its own circuit, off-site climate-controlled storage, or taking the bottles along. A wine refrigerator rejects its heat into the room around it, so it works hardest in precisely the warm house a vacant-home setting creates.

The outdoor load behind all of it is documented rather than estimated. National Weather Service normals for 1991 to 2020 at Phoenix Sky Harbor put July's average high at 106.5 degrees Fahrenheit, the monthly mean at 95.5 degrees Fahrenheit and the average low at 84.5 degrees Fahrenheit. July 2024 ran well above those normals, with an actual average high of 112.3 degrees Fahrenheit and an actual average low of 89.8 degrees Fahrenheit. A thermostat left at 85 is therefore holding the interior below the July monthly mean of 95.5 degrees Fahrenheit, which is why a vacant Valley house at that setting still runs its air conditioner every single day of the month.

For Phoenix-area homeowners who want this looked at on their own system, Phoenix Cooling Guide recommends AZ Trusted Air LLC (ROC 317368), whose owner Tom Mattingly reviews this guide. (623) 252-6403 · second opinion on a quote.

How can I monitor a vacant home's AC remotely, and what should trigger a service call?

Remote monitoring in Valley snowbird homes usually fails for unglamorous reasons. The owner cancels the internet service to save a monthly fee and loses every alert the thermostat could have sent. The router loses power during a monsoon outage and never comes back because it was plugged into a switched outlet. A small uninterruptible power supply under the router solves the second problem for the price of one emergency service call. A thermostat that simply drops offline is itself ambiguous information, since a power failure, a router failure and a dead thermostat look identical from a thousand miles away, which is why a person on the ground is still part of the system.

Useful alerts are thresholds, not dashboards. Indoor temperature more than 5 degrees above setpoint for over an hour means the equipment is losing ground. Relative humidity above 60 percent crosses the line the EPA identifies as likely to produce condensation. Zero runtime on a July afternoon means the system is not calling or not responding. Runtime that climbs week over week against similar weather is the early warning most owners ignore, and the comparison matters more than any single day's figure. A leak sensor at the secondary pan closes the gap the thermostat cannot see.

Utility demand programs suit vacant houses better than occupied ones, because nobody is home to be uncomfortable. SRP's Bring Your Own Thermostat program pays a $50 bill credit per eligible thermostat at enrollment, up to two thermostats per home, plus an additional $25 per thermostat for each year enrolled. SRP describes conservation events as raising the enrolled thermostat a few degrees for periods that usually last no longer than three hours, with opt-out available from a phone, browser or the thermostat itself. On the APS side, Cool Rewards pays a $35 participation bill credit per smart thermostat annually, with an event season running June 1 through September 30 and first credits deferred to the next season for customers who enroll outside that window.

Rate plan review belongs on the pre-departure list too, because seasonal owners are the least likely to read utility mail. SRP's publicly elected board approved the retirement of several price plans on February 27, 2025; the Time-of-Use, Residential Demand Pilot, Customer Generation and Average Demand plans are closed to new enrollment and will be discontinued as of or before the November 2029 billing cycle, with at least 120 days' notice. SRP is also temporarily lowering prices by $0.0038 per kWh across the May through October 2026 billing cycles, with prices returning to published levels effective with the November 2026 billing cycle. On the Manage Demand 5-10 p.m. plan, super off-peak runs 8 a.m. to 3 p.m. every day, which is when an empty house can do nearly all of its cooling.

What should a home watcher check on the AC during summer visits?

The visit works best as a fixed sequence, because a watcher covering six houses will not improvise a diagnostic. Photograph the thermostat screen first, capturing setpoint, indoor temperature and indoor humidity, so the owner receives a dated record rather than a reassuring text. Change or inspect the filter; a closed house with no foot traffic loads filters more slowly, but monsoon dust still finds its way through return leaks and window gaps. Walk the supply registers by hand to confirm air is actually moving, then check the breaker panel for anything tripped after a storm.

Condensate is what floods a vacant Valley house. The EPA advises keeping air conditioning drip pans clean and drain lines unobstructed and flowing, and in a home nobody occupies, a slow algae blockage has months to finish the job. The watcher should look at the primary pan, the secondary pan under the air handler and the drain line termination outside. A float switch that shuts the system down to protect the ceiling is doing its job, and it is simultaneously leaving the house unconditioned at 110 degrees outside, which is exactly why the temperature alert and the water sensor need to exist together.

Outdoors, monsoon dust loads condenser coils across the Valley every summer, and a rooftop package unit collects it on a surface nobody can see from the driveway. Roof access by a home watcher in July is a genuine safety and liability question, and most owners are better served asking for a ground-level look and a photograph of the roof from a distance, then scheduling a contractor for anything on the unit itself. Standing water, a cracked condensate line running off the roof, or a fan that is audibly not running are all reportable from the ground.

Scope is the part owners set badly. The Arizona Registrar of Contractors states that, generally, if labor and materials exceed $1,000, or if a permit is required regardless of the price of labor and materials, a contractor licence is required, citing A.R.S. 32-1121. A home watcher can change a filter, flush a drain line and hose a coil. A capacitor replacement, refrigerant work or a re-piped drain crosses into licensed territory, and the licence, bond and complaint history of whoever does it are searchable on the ROC's public database. The time to verify that licence and put the contractor on file is before the house is empty, not during the phone call about a warm living room.