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Rheem hot water heaters have a device in the combustion chamber called Thermal Release Device (TRD). It contains a vial filled with liquid that seems to be mounted in a flex metal plate. It is a safety device that is supposed to trip when the temperature inside the combustion chamber goes above safe limit. This could happen due to "Flammable vapor incidence" or for variety of other reasons. There is not much information about this online. This device tripped off for the gas water heater and the first plumber did not even know anything about it. He thought that it was thermocouple gone bad. Another plumber also thought the same but with GE support he could at least figure out and he knew about it. However, he could not explain how this device worked. I also could not find much online especially exactly how it trips off the burner when it just seem to sit on the base of the heater inside the combustion chamber.

EDIT

American Water Heater technical bulletin - they call is Flammable Vapor Ignition Resistant (FVIR)

EDIT

Got some pictures of the device

TRD-01 TRD-02 TRD-03 TRD-04

and instructions

TRD-Instructions-01 TRD-Instructions-02 TRD-Instructions-03

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Great question, Sure got my curiosity peaked. –  shirlock homes Dec 30 '11 at 11:58
    
Thermal Release Device- A small glass bubble or vial that is designed to cut off the fresh air entering the combustion chamber as during an extreme overheat situation. Not exactly sure how it works, but that's what I could dig up so far. –  Tester101 Dec 30 '11 at 18:14
    
Also sounds like if this thing trips, you'll have to get a new water heater. 17. Can the thermal release device be replaced or reset? Answer: No. If the unit trips the thermal release device for any reason we want to be aware of it. It means combustion chamber temperatures exceeded what we consider to be normal. The water heater must be replaced Source –  Tester101 Dec 30 '11 at 18:38
    
Posted the picture of device and the instructions that came with it. –  amit_g Dec 31 '11 at 2:01
    
@Tester101, thanks for the research and information. As per the GE technical support TRD is federally regulated device and can be replaced at most once. If this happens again, the water heater has to be replaced. –  amit_g Dec 31 '11 at 2:01
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2 Answers

up vote 4 down vote accepted

Either expansion/pop or fusible link melt are the two methods used. Fire sprinkler heads also use these devices.

The melting method is usually two pieces of metal joined by a calibrated alloy similar to solder that has a specific temperature where the pieces of metal debond allowing the safety mechanism to trip.

Expansion usually is a capsule filled with a liquid that's calibrated to burst at a set temperature allowing the safety mechanism to trip.

In fire sprinklers, the safety mechanism is allowing the seal to blow and water to flow. In furnaces, fuel flow cut, in this water heater, a damper to cut off air supply.

Here's a link to a safety bulletin (Rheem/Ruud)

Visually, it's pretty self-explanatory in the sequence below. Bulb pops, spring jumps out of the way and pin on spring loaded damper no longer is obstructed from allowing damper plate to seal combustion chamber.

excerpt from safety bulletin

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I couldn't find any specific information on how this particular device works, but in general the device is made up of two parts. The first is the anchor, which is in a fixed stationary position. The second part is movable, and bonded to the first part using a melt-able substance. When the bonding substance reaches it's melting point, the movable portion of the device is released from the anchor.

In the case of a water heater, it seems the movable portion is used to block the combustion air from entering the combustion chamber. Once the air supply is cut off the flame is extinguished, and the heater is rendered inoperable. From what I could find, the device is activated in the water heater somewhere above 375 degrees.

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