A. O. Smith Water Heater Blinks 3 Times: What It Means

The Short Answer: Three flashes and a pause on an A. O. Smith gas heater means the control could not confirm the venting was ready to fire. On Ultra Low NOx residential models, A. O. Smith defines it as the flue damper’s end switch failing open (the damper did not open). On BTR 500 commercial models it means the blower’s air-pressure switch never closed. Standing-pilot models have no three-flash code.

First, make sure it really is three

Watch at least two full cycles. A code is a group of flashes followed by a pause of about three seconds, then the same group again. Two easy mistakes:

• A single slow flash or a bright-dim pulse is a normal heartbeat, not a code. See A. O. Smith blinking white light.
• On Ultra Low NOx models, some codes come in two parts, such as six flashes then one. Count the whole sequence before the pause.

Then find which kind of heater you have from the rating plate and the heater itself.

Other A. O. Smith, State, and American models, including residential power-vent and electric models, use their own tables. If yours is not in the list, read the code label on the heater or the manual for your model number.

Ultra Low NOx models: the flue damper

These heaters have a small motorized damper in the flue pipe on top of the tank. It closes when the heater is idle to keep heat from escaping up the vent, and must open before the burner lights. A switch tells the control the damper is fully open. Three flashes means that switch did not confirm “open,” so the control will not light the burner. (Two flashes on the same control means the opposite: the switch reports the damper did not close.)

A. O. Smith’s troubleshooting for three flashes is to turn the gas control’s power switch off and unplug the heater, then follow the lighting instructions and watch whether the damper opens when the heater calls for heat. If it does not open, A. O. Smith points to wiring connections and then to replacing the damper.

What you can do safely:

• Look at the damper on top of the heater from a step stool, without touching the flue pipe, which can be hot. Note whether anything is resting on or against it.
• Turn the gas control’s power switch off, unplug the heater for a short time, then plug it in and follow the lighting instructions on the label once.
• If three flashes returns, leave the heater off and book a technician. Note whether you heard the damper motor run.

Do not force the damper open or wedge it. The damper switch is a safety: the control must know the flue is open before it lights, or combustion gases can spill into the room.

BTR 500 commercial models: the blower prover switch

On these commercial heaters, the blower must create a set amount of suction before its pressure switch closes and the board allows ignition. A. O. Smith lists the causes a technician checks for a three-flash code: the switch itself, an obstructed vent, too many elbows in the vent, and air pressure differences between the equipment room and outdoors. Room exhaust fans or a sealed equipment room can cause that last one; A. O. Smith suggests opening doors to the outdoors as a test. Building staff can check for a blocked vent termination outside and report any recent changes to the room’s ventilation.

What the technician will check

On an Ultra Low NOx heater, the technician watches the damper through a heating cycle, checks the wiring harness and connectors to the damper, and replaces the damper if it does not open or its switch does not report correctly. On a commercial heater with a blower, they measure the suction at the pressure switch, inspect the vent for blockage and excess length, and test the switch.

Both are part repairs. Neither alone is a reason to replace the water heater. Related guides: blower not working and the five-flash code by brand.

Frequently Asked Questions

Q: Can I run the heater with the damper propped open? A: No. The damper’s switch is part of the safety sequence, and tampering with it or the damper can let the heater fire in unsafe conditions. Leave it off until the damper is repaired.

Q: My heater flashed three times once and then worked fine. Should I worry? A: A single event after a power interruption can happen. If it recurs, the damper motor or switch is likely starting to fail and should be checked before it leaves you without hot water.