Replacement Thermal Expansion Tank Air Adjustment

Old 03-15-08, 07:02 PM
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Replacement Thermal Expansion Tank Air Adjustment

Woke up yesterday morning to the 7.5 year old Kenmore 2 gal Thermal Expansion Tank on top of the water heater leaking water out of a pinhole.

Yesterday I replaced the tank.

Instructions read to measure the water pressure, then fill the new expansion tank with air pressurized to match the water pressure.

I did that; measured 74 lbs of water pressure so I charged the air to 74 lbs.

My question is this: What happens when the pressure from the city drops, or gets higher? Further, is the system better off having a little "less" or a little "more" pressure in the air tank than the water pressure?

Is it apparent that I don't understand how the pressure in the air tank affects the buffering of the thermal expansion of the water?!

Old 03-15-08, 07:48 PM
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The two pressures are equal, so when the water expands due to heat, it will compress the air in the void, and will contract when it cools. With unequal pressure, the water would over expand in the void area and never spring back when it cools.
Old 03-15-08, 07:58 PM
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You adjust the pressure in the expansion tank to equal the "nominal" (average) pressure in the city water line. This ensures that under "normal" (average) conditions there is no water in the expansion tank.

When you use a quantity of hot water the cold incoming water now needs to be heated. When the water is heated it expands and this expansion raises the pressure in the essentially "closed circuit" of your home's water system. Since water is virtually incompressible if there were no expansion tank the pressure would rise dramatically and often so high as to cause the Temperature and Pressure (T&P) valve on the water heater to open.

However with the expansion tank the pressure instead causes the water to move into the expansion tank and compresses the rubber bladder (or diaphragm) that separates the air charge from the water. The air, being quite compressible, moves into a smaller volume and that allows for an increased volume to contain the expanded water with only a minimal increase in system pressure.

When you open a faucet the pressure is almost instantly reduced back to the city water pressure (or the pressure setting of the Pressure Reducing Valve (PRV) if you have one) and in so doing the air in the expansion tank expands, pushing the water out of the expansion tank and again the system is in equilibrium until the next cycle.

The closer the air pressure inside the expansion tank is to the city water pressure the better the thing works although slight differences in the two pressures won't make a big difference in the overall operation.

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