now if you have a sealed unit cold pressure at 12 psi and only goes up in psi when on why do u need a expansion tank when the pipes can handle well over, yah i know the relief velve lets loose at 30 psi
SJI has it. But why do you really care? Shut off the expansion tank, then turn on the boiler and see how high the pressure climbs. Thats one reason you need the tank.
If you didn't have the expansion tank the pressure would go above 30 pounds every time the boiler heated.Which would open the relief valve and dump water. That would have to be made up to maintain the correct pressure when it cools.The relief valve is in place so that in the event of a control failure and the boiler continues to heat it can never build up enough pressure to blow seals out or rupture pipes.All this safety equipment is in place because many years ago someone learned some lessons the hard way.
Cushion of air for the expanding water when heated so your pipes don't take the shock 30 times a day and another 30 times when the water cools and creates a vacuum . The effects of shirk and swell. The pipes can handle well over that pressure asme code is (factor of safety) if pipes fail at 125 psi then their (mawp) maximum allowable working pressure is 25 psi which means their factor of safety is 5. different for hot water heaters, hot water boilers, steam boilers, power boilers, critical boilers etc.
