Updated October 2026. Steps checked against the Intermatic T100 series timer guide and Texas TCEQ aerobic system guidance; panels vary by brand.
The sprinkler timer on an aerobic septic system is inside the control panel. To set it, turn the clock dial until the current time lines up with the pointer, then set the spray window with the ON and OFF trippers or the push pins, and leave the mode switch on automatic. Most systems are set to spray overnight or very early in the morning, when nobody is in the yard. Your permit or maintenance provider sets the exact window.
Two common aerobic sprinkler timer types
| Timer style | How you recognize it | How the spray time is set |
|---|---|---|
| Tripper timer (Intermatic T100 style) | A round clock dial with separate ON and OFF clips (trippers) screwed to its edge | Move the ON tripper to the start time and the OFF tripper to the stop time |
| 24-hour segment or pin timer | A dial ringed with small pins or tabs, one per 15 or 30 minutes | Push the pins for the spray window to the ON position; the rest stay off |
How to set the time on an aerobic septic sprinkler timer
- Open the control panel cover. Do not touch wiring or remove inner panels. If anything looks burned, wet or damaged, stop and call your provider.
- Find the time pointer. It is a small arrow or triangle at the edge of the dial.
- Turn the dial to the current time. On Intermatic T100 style timers, pull the dial outward and turn it until the current time lines up with the pointer. On most pin dials, turn the dial in the direction of the arrow on its face. Check AM and PM: the dial covers 24 hours.
- Set the spray window. Fasten the ON tripper at the start time and the OFF tripper at the stop time, or push the pins for those hours to ON.
- Put the manual switch on automatic. Leave any manual lever or HAND/OFF/AUTO switch in the automatic or timer position.
- Close the cover and check the next morning that the system sprayed during the set window.
Reset the clock after any power outage and when daylight saving time starts or ends. A clock that is hours off is a common reason sprinklers run in the middle of the day.
Timer mode vs on-demand mode
Many aerobic control panels have a switch or float setting that decides when the spray pump runs. In timer mode, the pump sprays only during the set window, as long as the pump tank has enough water. In on-demand mode, a float turns the pump on whenever the pump tank fills, at any time of day. Leave the switch where your maintenance provider set it. Turning the system off stops dispersal, and the high-water alarm will eventually sound.
Sprinklers running at the wrong time or not at all?
A timer that will not hold time, a stuck pump or an alarm needs a service visit. Talk to a local septic company about aerobic system service.
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Aerobic sprinkler care tips
- Keep the chlorinator stocked. Texas TCEQ says to use calcium hypochlorite tablets certified by the EPA for wastewater disinfection, and never swimming pool tablets.
- Keep spray heads clear of grass, toys and lawn furniture. See how to test aerobic sprinklers and how many sprinkler heads you need.
- Spread laundry across the week so the system is not flooded at once.
- Protect lines and heads from freezing. See how to keep a septic system from freezing.
- Keep the maintenance contract current. The EPA notes that aerobic units need regular maintenance for their whole life.
Quick answers
What time should aerobic septic sprinklers go off?
Usually overnight or very early in the morning, when people and pets are not in the spray area. Follow the window on your permit or the one your provider set.
Why are my septic sprinklers going off during the day?
The timer clock is usually wrong after a power outage or a time change, or the panel is in on-demand mode and the pump tank filled up. Reset the clock first.
How long do aerobic sprinklers spray?
Until the pump tank is drawn down to the off float. Long or constant spraying can mean a stuck float or too much water going into the system.
Can I turn off my aerobic sprinklers when it rains?
Do not switch the system off. The pump tank keeps filling, and the alarm will sound. If wet weather is a problem, ask your provider about the timer window.
For how the whole spray setup works, see aerobic septic sprinklers explained and what an aerobic septic system is.
Sources
- Inyo Pools, How to program an Intermatic T100 series timer
- Texas Commission on Environmental Quality, FYI on OSSFs
- U.S. EPA, Types of septic systems