Heating - Expansion Tank
Carnation Construction
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Heating - Expansion Tank

Expansion Tank Potable 

http://www.supplyhouse.com/Amtrol-141N43-THERM-X-TROL-ST-12-Expansion-Tank-3715000-p     $56.75      This 4.4 gallon model is suitable for potable water systems.

On a closed loop radiant heating system it is not actually necessary to use a potable rated expansion tank but as it is small money I use a potable version even for that.

You need an expansion tank to take up some of the pressure and avoid the pressure relief valve needing to operate.

It is good to have an expansion tank that is about 5% of the gallons of the water heater.

Expansion tanks should be mounted with the pipe at the top.