Question about point of no pressure change

charliechicago

Hello all,

I'm an HVAC tech and I bought Pumping Away (along with some of Dan's other books) over a decade ago and have been trying to understand the PONPC and other related concepts ever since. I had always though the system pressure was based only on street pressure. Open the valve, let the pressure reducing valve do its job, it fills to 12 PSI and that's it. Dan writes in Pumping Away (pg 14) "You filled the system with water until the static pressure reached 12 PSI…The air in the compression tank will make sure the pressure remains on the system once you disconnect the hose. Without a tank, the pressure would have dropped instantly when you disconnect the hose." If Dan is correct, and I know he is, why is it that when you have a garden hose and you turn off the spigot you still have pressure in the hose? If you want to get the nozzle off the hose you have to get the pressure out of the hose first.

jesmed1

It's maybe not a perfect analogy because Dan was using simple language to illustrate a concept. His point was that, since water is incompressible, as soon as you disconnect your hose from the spigot, or open the spray nozzle with the spigot off, the pressure trapped inside the hose is instantly lost.

By contrast, the air in the expansion tank is compressible and will "store" the energy of the 12 psi much like a spring will store mechanical energy. So that, if you opened a boiler drain valve briefly (like opening the hose spray nozzle with the spigot off), you wouldn't instantly lose all the pressure in the system. You'd lose a cupful of water, but the system pressure would still be roughly 12 psi.

So while you're charging the system to 12 psi with street-pressure water that's been regulated down, you're pressurizing the air in the expansion tank to 12 psi, storing pressure energy in the tank just like storing mechanical energy in a spring. So that pressure will remain more or less the same even if you open a drain valve and drain a few cups of water. Whereas with the pressurized garden hose, as soon as you open the nozzle and drain a few cups of water, the pressure goes to zero.

EBEBRATT-Ed

I don't know what the "hose" thing is all about. But in a normal 1-2 story house the bladder on the ex tank should be pressurized to 12-15 psi while it is off the boiler (no water pressure on the bladder). Then install the tank. Fill the system and adjust the PRV to 12-15 psi (the same as the tank). The MU water should be piped to or close to the expansion tank.

When the circulator runs and your pumping away from the expansion tank (best method) your suction pressure will always remain at the 12-15 psi. The circ discharge pressure will show the system resistance as whatever the gauge reads-12-15 psi.

If you pump towards the expansion tank as some older system are you will have a constant 12-15 psi on the pump discharge and the pump suction pressure will drop by the amount of the system resistance.

The danger in pumping toward the ex tank is the suction pressure can drop into a vacuum if the system resistance is high enough

109A_5

" why is it that when you have a garden hose and you turn off the spigot you still have pressure in the hose? "

The garden hose is elastic which stores the pressure.