“SU” on a thermostat almost always stands for “Sunday.” It appears as part of the day-of-week display used in programmable scheduling. On some models, particularly Honeywell units, you may also see “SU” as part of a setup or installer menu code. The meaning depends on where it shows up on your screen and what brand you own.
SU as a Day of the Week
Most programmable thermostats abbreviate the days of the week across the top or side of the display: Mo, Tu, We, Th, Fr, Sa, Su. In this context, “Su” simply means Sunday. You’ll see it highlighted or flashing when you’re setting your weekly heating and cooling schedule.
Braeburn thermostats use a slightly different convention. On models with a 5-2 day schedule (five weekdays, two weekend days), Saturday is abbreviated as “S” and Sunday as “SU.” So when you’re programming weekend temperatures on a Braeburn unit, the display walks you through the “S” settings first, then the “SU” settings. Venstar thermostats follow the more standard pattern, listing the full week as SuMoTuWeThFrSa.
SU in Setup or Installer Menus
On Honeywell and Honeywell Home thermostats, “SU” can appear as part of the Installer Setup menu, often labeled “ISU” on the display. This is a configuration menu typically used during initial installation to set system type, fan behavior, and other hardware-level options. On first power-up, the thermostat enters this menu automatically. After that, you can access it by pressing and holding two buttons simultaneously for about five seconds (the exact combination varies by model, but it’s commonly the bottom-left button and the up arrow).
If your Honeywell thermostat is showing “ISU” followed by a number, you’re inside the installer configuration. Each number corresponds to a different setting. You can press “Next” to cycle through the options and “Save” when finished. If you accidentally entered this menu and don’t know what the settings do, pressing Save without changing anything will exit safely with the existing configuration intact.
Stuck on “SU” or “Sun Temporary” Mode
Some readers searching this term are seeing “Su” on the display alongside an unexpected temperature, and the thermostat won’t respond normally. This typically means the unit is in a temporary override for Sunday’s schedule. In temporary mode, the thermostat holds a manually set temperature instead of following your programmed schedule. It will usually revert on its own at the next scheduled time change, but if it seems stuck, here’s what to try:
- Press “Cancel” or “Run Schedule” to exit the temporary override and return to your normal programming.
- Check the batteries. Low battery power can cause display glitches and unresponsive controls. Swap in fresh AAs or AAAs (check your model) and see if normal operation resumes.
- Power cycle the thermostat. Remove it from the wall plate or pull the batteries for 30 seconds, then reattach. This clears most temporary states and minor software hiccups.
If the display still won’t change after a power cycle and fresh batteries, your thermostat’s internal clock may have lost its time and date settings. Re-enter the current day and time through the menu, and the weekly schedule should pick back up correctly.
How to Tell Which Meaning Applies
Look at where “SU” appears on the screen. If it’s part of a row of two-letter abbreviations across the top (Mo Tu We Th Fr Sa Su), it’s Sunday. If it appears with a number next to it or shows up after you’ve held down buttons, you’re in a setup menu. And if it’s flashing alongside a temperature reading, you’re likely in a temporary schedule override for Sunday.
Your thermostat’s manual will have the definitive answer for your specific model. If you don’t have the paper copy, search the model number printed on the thermostat (usually on the front or behind the faceplate) along with “manual PDF” to find the manufacturer’s documentation online.