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Posted by Jeff on November 22, 2007 at 11:33:05:
In Reply to: Re: Laars/Endurance Relief Valve Leak posted by John on November 21, 2007 at 23:44:00:
Here is what I don't understand on how the expansion tank is supposed to work. Cold water pressure in the system is 15PSI, and it fills up the expansion tank, when we install it new. Since the factory set was at 12PSI, it compresses the air a bit.
I understand this much from high school physics, as water heats up (with volume constant), the pressure increases, and therefore air side of the expansion tank further compresse to equalize the pressure. Remember we already have some cold water inside the expansion tank (possibly getting hotter as the furnace cycles).
Now, this where I am getting confused. How does the pressure in the closed loop system remain constant? Is it because, as water gets hotter in the closed loop system, its pressure increases, and wants to go to lower pressure expansion tank (which is guaranteed to take that pressure).
Assuming that is the case, if there is already some water in the expansion tank, is there going to be more water let into the expansion tank, to keep the pressure seen by the close loop system same as the cold water pressure.
If that is the case, how do we know there is enough space?
: I have a similar situation with a Trainco Heatmaker. The pressure changes excessively from cold to hot. I've had the unit serviced several times without any improvement. I think there's more to the expansion tank and it's pressure than meets the eye. It's in the system to prevent this problem we both seem to have. If it comes from the factory with a 15 psig charge and it's installed in a system that adds 15 pounds to oppose it, it seems to me that the pressure on the air side will go above the initial 15# charge. My thinking is that the air charge might need to be increased but I'm not sure.
: I have globe valves on both sides of my 'auto-fill' valve that I keep closed unless I'm adding water.
: Good luck, but don't hold your breath for answers from this web site - I've been very disappointed.
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