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I have an AquaMax 205 water heater. Recently the pilot started going out, so I replaced the thermocouple and cleaned the burner.

After replacing the thermocouple, the pilot would go out sometime through the night. I would like the pilot in the morning and people would use hot water during the day and cause the burner to turn on and turn off and the pilot never goes out. However, when I wake up in the morning, the pilot is always out.

Nighttime temperatures are around 11 to 16 Celsius over the last few weeks.

What could be causing this?

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The problem ended up being my gas supply. I was sure the thermocouple was installed properly and the whole assembly was cleaned properly as well. The other thing being that the pilot would always go out in the morning and would be fine during the day was another reason.

I called my gas distributor and they sent out some guys to pump the water and condensation out of the gas pipe entering my property. Unfortunately, I live in an area where they still use low-pressure gas which are susceptible to condensation. A lot of pipes are also very old, and have small leaks and cracks, enabling water to enter.

After the water was pumped out, my pilots have been rock solid ever since. They are going to upgrade our area to high pressure soon, so hopefully this should not be a problem for long.

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In my experience, the most likely causes for this are:

  1. The water temperature setting is too hot; as a result, the thermocouple cannot sense a temperature gradient so it cannot generate voltage to hold open the gas valve.
  2. The thermocouple bracket is positioned so that the probe is too close to the pilot light glow-plug; the result is similar to the above -- the thermocouple cannot control the fuel supply.

In case 1, try adjusting the water temperature down (cooler) to just above 60C. Remember that is the recommended minimum in Australia in order to kill potential bacterial (Legionara) growth.

In case 2, try re-positioning the thermocouple (by slightly bending the bracket) so that the only tip of the thermocouple is 10-15mm above the pilot glow-plug. This way, the thermocouple will properly sense the pilot heat and keep the fuel flowing.

Please report back after trying each of these resolutions (one at a time!)

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    I have never seen a problem with the tank temp causing the pilot to go out. Thermocouples work with dissimilar metals as long as they are hot they generate a voltage, the voltage keeps the valve open. – Ed Beal Dec 19 '16 at 22:51
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    I agree with @EdBeal it's either positioned wrong in the flame, the flame needs adjustment, or the gas valve end of the thermocouple is loose or not seated properly. Be careful on the last one, don't mess up the threads on the gas valve by over tightening. – Tyson Dec 19 '16 at 22:56
  • I've just checked the temperature controls and it is currently at around 60 degrees Celsius. It's been at this setting for the life of the heater (since it was installed more than 10 years ago). I will try adjusting the thermocouple on the weekend. The pilot is still on after lighting it this morning, but I am guessing it will go out tonight or early tomorrow morning again. – F21 Dec 20 '16 at 7:39
  • Is the pilot light flame sufficiently large to keep the thermocouple heated properly? How is the orifice of the pilot light to be cleaned? with a poker wire of the right diameter? If the pilot is going out after a long period in which the main burner has not been activated, then the thermocouple depending on the incremental heating from the main burner, or the inactivity of the main burner is causing the thermocouple to shift out of the location it must be in for the pilot flame to heat it properly. – Jim Stewart Dec 22 '16 at 11:23

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