hot water heater lights but won't stay lit

Chris Bryant wrote:
As spadoctor pointed out, these DSI units do not use a thermocouple to detect the flame. They run a very small current *through* the flame to detect it. The current must go through the electrode wire, then the electrode, then the flame, then back through the ground to the circuit board. Any connection in the path can keep it from working, as can a bad electrode.

A thermocouple, on the other hand, generates a small voltage when heated, using dissimilar metals. Completely different in design and operation.

I stand corrected.

Sounds like DSI reinvented the wheel.

As spadoctor pointed out, these DSI units do not use a thermocouple to detect the flame. They run a very small current *through* the flame to detect it. The current must go through the electrode wire, then the electrode, then the flame, then back through the ground to the circuit board. Any connection in the path can keep it from working, as can a bad electrode.

A thermocouple, on the other hand, generates a small voltage when heated, using dissimilar metals. Completely different in design and operation.

The light goes out & I can hear it light, but in a few seconds the light comes back on & the burner goes off.

Take the thermocouple out and burnish it with a wire wheel to clean it off.

worked on mine.

spadoctor wrote:
-- snip --
...rather a small current is generated back thru the electrode when in the flame. --snip--

You have just described the operation of a thermocouple.

If it is a DSI unit it does not have a thermocouple...rather a small current is generated back thru the electrode when in the flame. If it is DSI when it does light does it continue to spark? If so open the outside service door....lok in the upper right hand corner. You eill see 2 1/4" hex head screws. back these out and retighten a couple times and see what happens. This is the ground wire mounting point inside the unit. If it loses ground it will ignite but shut down in about 3 to 5 seconds. A reset of the switch will allow ignition again with the same results....very commom. Also a poor ground between the electrode and the gas valve will cause delayed ignition resulting in a lould boom when the gas lights...also common

Don't think OP is talking about 'indicator light' but burner not staying lit ie: "It will light, but goes out in a few seconds"

Dirty or faulty electrode.....remove electrode and clean it with some emery cloth to remove carbon. If it still doesn't stay lit..then replace electrode.
Make sure the electrode is 'engulfed' in flame.
While you have electrode out.....run a piece of wire thru combustion chamber and out the exhaust then attach a rag and pull it back thru. Compressed air is OK but doesn't remove spider webs/debris....pulling the rag thru will clean out the 'U' tube better.

Normally the light should go OUT after the burner fires. If the light stays on the burner is not running.

That's classic "bad thermocouple" behavior. It is also possible that blowing out the burner with compressed air pushed the thermocouple out of the place it is supposed to be. It might also have blown the tip off the thermocouple causing the "bad" response. (If so, it was almost dead anyway.)

I'd check the positioning of that thermocouple first, and if it is ok, then replace it. They are cheap and easy to replace.

The proper position will be such that the tip is actually IN the burner flame when the burner is lit, so that it can sense the flame. Of course, this causes it to eventually burn out, but in the meantime, it is a safety feature that shuts off the gas if the burner doesn't light.

On mine, the red light is a "fault" light. Once the burner fires, the light goes out but the burner is on. After the light goes out, go check to see if the burner is working.