Best Thermostat Settings for an Arizona Summer
Hold 78 to 80 degrees while you are home and let the house drift to 82 or 83 while you are out. Going higher than about 85 rarely pays here, because the system cannot pull the house back down in the afternoon without running for hours through the most expensive part of the day. Pick one realistic number and leave it alone.
Every May somebody tells you that 78 degrees is the magic number, and every July somebody else tells you to shut the system off while you are at work. Both pieces of advice are missing the thing that matters in Mesa, which is how long your system takes to recover once the house has been allowed to get hot.
Why does a deep setback backfire here?
In a mild climate you can let a house drift ten degrees and pull it back in twenty minutes. In Mesa in July, with 110 outside and a roof that has been soaking up sun since nine in the morning, the walls, the tile, the drywall and the furniture are all holding heat. Your air conditioner has to remove that stored heat before the room temperature moves at all.
Set 88 while you are out and the system starts its recovery run at four or five in the afternoon, which is the hottest hour and the worst hour on most rate plans. It runs flat out for two or three hours, at the point when it is least efficient and electricity costs the most. The setback saved you something. The recovery gave a chunk of it back.
A moderate drift behaves differently. Two or three degrees of drift lets the system loaf through the afternoon instead of sprinting through the evening.
What should you actually set?
| Situation | Setting | Why |
|---|---|---|
| Home and awake | 78 to 80 | Comfortable with a ceiling fan running |
| Home and asleep | 76 to 78 | Bodies and bedding add heat, and overnight is cheap to run |
| Out for the workday | 82 to 83 | Enough drift to save, close enough to recover by evening |
| Away for a week | 85 | Protects the house without a punishing recovery |
| Vacant all summer | 85 with the fan on auto | Keeps humidity and heat off furniture, floors and electronics |
Snowbird owners often ask whether they can just switch it off. The answer is no. A closed house in a Phoenix area summer will pass 120 degrees inside. That warps doors, lifts vinyl, cracks candles into carpet and cooks anything with a battery. Holding 85 costs money. It costs less than the damage.
How does your rate plan change the answer?
This is where most generic advice falls apart, because SRP and APS do not use the same windows.
- SRP residential time of use plans put the expensive window in the evening, and the exact hours depend on which plan you are on. The current plans use windows like 5pm to 10pm or 6pm to 9pm on weekdays.
- APS time of use plans center on 4pm to 7pm on weekdays.
- SRP also runs a super off peak window in the middle of the day on some plans, which is unusual and useful.
Pull up your own plan on your utility’s site before you build a schedule. Then do one thing: pre-cool. Drop the setpoint two degrees an hour or so before your peak window starts, let the house coast through the expensive hours, and bring it back after. You are moving run time rather than adding it.
What about ceiling fans?
Fans cool people. They do nothing for an empty room. Run them where you are, turn them off where you are not, and let them buy you the two degrees that make 79 feel like 77. In the two story homes around Ocotillo in Chandler, where the upstairs runs hot by mid afternoon, a fan upstairs is often the difference between a comfortable evening and a thermostat war.
Is a smart thermostat worth it?
It helps if it does something your old thermostat could not. ENERGY STAR certified smart thermostats handle schedules, remote changes and occupancy sensing, and many utilities run demand response programs that pay a credit for letting them nudge your setpoint during peak events.
Two cautions. A smart thermostat cannot fix a dirty filter, a low charge or crushed ductwork. And a learning thermostat that keeps drifting the setpoint around will frustrate you in a climate this hot. Set a schedule, then stop fiddling with it.
The small things that let you hold a higher number
- Close blinds on west and south glass before noon, before the glass heats up
- Keep the filter clean through dust season, monthly on a one inch filter
- Seal the return plenum if you can hear it whistling
- Keep furniture off supply registers, especially in bedrooms
- Turn the oven off and cook outside or late in the day
- Check that the outdoor unit has clear air on all sides and a clean coil
What if the house will not hold 78 at all?
Then the thermostat is the wrong thing to be adjusting. A system that cannot reach setpoint on a normal August afternoon is telling you something: low charge, a failing capacitor, a dirty condenser coil, undersized return air, or ductwork leaking cooled air into the attic. Run a temperature split check yourself as a first pass. Put a probe thermometer in a return vent and a supply vent after twenty minutes of run time. A healthy split is roughly 16 to 22 degrees. A split under 12 usually means airflow or charge.
If you get a big split with weak flow, look at ducts and filters. Our ductwork and airflow page explains what static pressure testing finds.
Where a maintenance visit fits
A system that has been cleaned, charged correctly and checked for airflow will hold 78 on a bad day without drama. That is really all a tune-up buys you, and in this climate it is worth a lot. Our maintenance plans cover two visits a year, one before summer and one before heating season, with priority scheduling when it gets hot.