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Dueling T & P Valves


hausdok
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Hi All,

Meant to seek input from the brethren last night and forgot to do so. Looking for opinions or codes re: T & P's from two water heaters piped into a single, otherwise properly configured discharge pipe.

Seems to me that if only one valve popped off it would be fine, but if both popped off at the same time it would be like having a reduction in diameter of the discharge line, wouldn't it?

My curmudgeon's mind is refusing to lock in on this one. When that happens, it usually means I know the answer, have forgotten it and can't dredge it back to the fore again.

What's everyone think?

ONE TEAM - ONE FIGHT!!!

Mike

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I don't know the By-the-book answer either, but plain logic says the full discharge of two won't fit in the space of one. Of course the same logic would say that there is virtually zero chance they would ever both go at once, if you forget about By-the-book. Wise home inspectors stay very near to the book, whenever there is one.

Brian G.

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Originally posted by hausdok

Seems to me that if only one valve popped off it would be fine, but if both popped off at the same time it would be like having a reduction in diameter of the discharge line, wouldn't it?

Mike...Don't know about the code for this but from a purely practical point of view....

The chances of both heaters controls failing at exactly the same time are sort of remote...(10,674,527,123 to one to be exact). I could see both valves popping at once as the pressure from one tank could affect the pressure in the other. However...the excess pressure in that case is still only being created by the one heater and it would seem that a single full size outlet would be sufficient.

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Originally posted by hausdok

As it turned out, apparently their own engineers had quite a debate about it and eventually fell back on Art 8 ASME (HLW-801.7) that says that the inside cross sectional area of the discharge pipe serving the two, can't be less than the total inside cross sectional are of the two outlets combined, while acknowledging odds like Richard's.

That sounds reasonable. If I ever see this, I estimate the odds of them having used such a pipe as approximately the same as both valves going off at once. But I'll cite Art. 8 ASME (HLW-801.7) when I write it up, and look like a genius.

Brian G.

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