Very clear, David. Many thanks. The wheels will turn starting today. :)
On 8/20/12 9:17 AM, dtbw wrote:
> Not exactly the correct theory of operation. The Atwood water heaters have a simply operation cycle. There are two bi-metal thermodiscs connected in series that sense the tank metal temperature (an analog to the actual water temp). One of them has a higher temp rating than the other and acts as a emergency overheating sensor (it is normally always closed). When the tank temp is below the temp rating on the lower rated thermodisc, it closes as well and this begins the ignition cycle. With the two thermodiscs now completing an electrical circuit, the control module supplies voltage to the two solenoids on the gas valve which allows gas to start flowing and at the same time starts firing the ignitor (that clicking sound you can hear). Now the control module starts an internal timer and if the module does NOT sense that the flame on the burner has ignited (detected by the flame engulfing the ignitor/flame sensor) , it closes the gas valve, waits for a period and tries again to fire up the water heater. It will do this 3 times before going into a fault state and turning on the red indicator light located by the water heater power switch (ie the one you are seeing turn on). As long as the control module senses that the flame is present it continues to keep the gas supply flowing until such time as the lower temp thermodisc opens due to the tank being at the thermodisc's temperature rating. Once this happens, the control module immediately shuts off the gas value solenoids. That is how the system works when all is well.
>
> Now what you are describing is obviously abnormal. I had exactly the same situation occur. On a cold tank, everything worked perfectly all the way through the shutoff due to the tank being sufficiently heated. It was the from that point on, the water heater would attempt to light, fail, and finally fault. There are TWO potential causes for this. The first is that the ignitor/flame sensor may be defective. As this device is in the direct path of the flame, it gets VERY hot. That heat can lead to an intermittent problem. The only way to test this is to replace it. If you have a buddy with the same water heater, see if you can borrow the ignitor and install it on your water heater. The other potential cause is the control module itself. Like the ignitor/flame sensor, the only way to test is to replace and again, if you can borrow a working module, do so before ordering one as the control module is expensive. In my situation, it was unfortunately the control module.
>
> FYI, the control module is that plastic looking box located in the upper right corner of the water heater behind the exterior door. As your WH works for the first cycle, that pretty much clears the gas valve and the thermodiscs as suspect.
>
> David T.
> On Aug 20, 2012, at 7:28 AM, Stephen Syrotiak <stephen.s1@comcast.net> wrote:
>
>> Flick the HWH on. Ignition, followed by ignition light shutting off.
>> Water gets hot. Gas shuts off. All well at this point. But the
>> ignition cycles several times. heater does not light. Ignition light
>> stays on until I shut it off. Because the ignition light is on, as the
>> water is used (cooling) the gas does not flow. Manually recycling the
>> switch does bring sustained ignition and relighting.
>>
>> My theory: The heat sensor shuts the gas off, and the ignition switch
>> remains on, cycles thru its Lets light the gas thing,then quits.
>>
>> What controls all of this? Where should I look for the culprit.
>>
>> S
>>
>>
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