Before I reveal the proper setback temperature in Arizona, let me make the disclaimer that my job is to help reduce homeowner’s utility bills, and I am a lifelong student and love the building science arena. During college, I dove into the exhilarating subject of psychometrics and aerospace classes, which is the basis for of how we stay comfortable in homes and how heat pumps work to accomplish this goal.
I’ve been in many discussions (sometimes very passionate) about this topic with my air conditioning friends and like you, have done a little research on this topic. I am also doubtful of friends of friends that have worked at an air conditioning company for the last 20 years or even air conditioning techs that proudly say the thermostat should be set no more than 5 degrees from your normal setpoint. That is simply not true, and here is why:
Your heat pump takes 5 – 10 minutes to run at its highest efficiency, and when it is constantly cycling on and off you are using a lot more energy than if the heat pump was continually running. So, if the thermostat is bumped up to 83 degrees while you are at work, and you set it to 78 when you come home, the heat pump does not have as much heat to remove as a 88 degree house and will run less to get to 78.
Plus if your air conditioning system is like 90% of the ACs in Phoenix, it is oversized and will remove the heat even faster than designed. Now, it is a good idea to pre-set the thermostat lower an hour before you come home so it does not take as long to cool your home at 88 degrees. SRP also addresses this old wives’ tale and has a great Q&A here.
Also check out Energy Savers tips on thermostat settings.
Feel free to post your comments and thoughts below and let’s save some energy!