Diaphram/bladder expansion tank

Mark J Strawcutter

Charge air side to system fill pressure (typically 12psi for 2 story house). Does bladder/diaphram stretch so that the water side is empty (ie doesn't exist)?

Then as water heats and expands, it can exert more than 12psi against the diaphram and allowing water to enter the tank.

So if I understand it correctly, when cold there will be no water in the tank?

Mark

jim lockard

Oh

1st pressure is pressure. If you have 12psi of air or nitrogen(some perfer) with less then 12psi in a water heating system the bladder will fill the tank yes. Should the pressure of the system exceed the pressure of the tank yes there will be some water in the tank. For this reason some guys add 3 to 5 psi to the bladder tank above the static pressure of the system. Hope this helps J.Lockard

scrook_2

bladder tank...

yes, if the air side is filled to 12psi and it is then attached to a system at 12psi (cold) than the tank contains just air and bladder. As the system pressure rises the tank starts to fill w/ water displacing the diaphragm and compressing the air to the same pressure as the system, e.g. at 13psi the tank will be about 3 1/2% full, at 15 psi about 10% full, at 20psi about 23% and at 30psi (just before the relief valve lifts) about 40% full. The air side will always be at the same pressure as the water side, the higher the pressure the more the air compresses and the more water can enter the tank.

A plain steel tank though, because it starts at atmospheric pressure not at cold system pressure, will fill to about 45% full, and upon heating and expanding the water it will fill more from there, e.g at 20psi it will be about 58% full, at 30psi (just before the relief valve lifts) it will be about 67% full -- hence the larger size than a comperable bladder tank. Note that just like the bladder tank the air and water pressure are always the same, but that to get the initial 12psi air pressure when cold filled, some water has already entered the tank. Also the air can dissolve into the water (sometimes causing waterlogging) or air bubbles from an air scoop or purger can be bubbled up through the water into the air space in the plain tank (say using a B&G airtrol fitting), whereas the trapped/eliminated/collected/scooped air in a diaphragm tank system needs to be vented to the atmosphere via a float or other vent.

scrook_2

bladder tank...

yes, if the air side is filled to 12psi and it is then attached to a system at 12psi (cold) than the tank contains just air and bladder. As the system pressure rises the tank starts to fill w/ water displacing the diaphragm and compressing the air to the same pressure as the system, e.g. at 13psi the tank will be about 3 1/2% full, at 15 psi about 10% full, at 20psi about 23% and at 30psi (just before the relief valve lifts) about 40% full. The air side will always be at the same pressure as the water side, the higher the pressure the more the air compresses and the more water can enter the tank.

A plain steel tank though, because it starts at atmospheric pressure not at cold system pressure, will fill to about 45% full, and upon heating and expanding the water it will fill more from there, e.g at 20psi it will be about 58% full, at 30psi (just before the relief valve lifts) it will be about 67% full -- hence the larger size than a comperable bladder tank.

Note that just like the bladder tank the air and water pressure are always the same, but that to get the initial 12psi air pressure when cold filled, some water has already entered the tank. Also the air can dissolve into the water (sometimes causing waterlogging) or air bubbles from an air scoop or purger can be bubbled up through the water into the air space in the plain tank (say using a B&G airtrol fitting), whereas the trapped/eliminated/collected/scooped air in a diaphragm tank system needs to be vented to the atmosphere via a float or other vent.

Mark J Strawcutter

plain tank

Thanks Scrook, good to know I was on the right track concerning the bladder tank.

Your explanation of the plain tank makes sense if you start with an empty (air-filled) tank at atmospheric pressure, close it off from the atmosphere, and then open it to the system to fill.

However, the instructions for B&G airtrol fitting call for a slightly different approach. The ATF has a "dip tube" that is about 1/3 of the tank diameter. They say to leave the tank open to atmosphere (via the dip tube), open it to the system to fill. When water comes out the vent, close it and finish the fill. The remaining air would then compress and water would cover the tube end.

I think your way would result in a larger air cushion and hence more expansion capacity. But perhaps the ATF needs the tube end covered to work properly?

Mark