PONPC Perplexed

solpanc

Dan Holohan writes in his book "since the pump can't add or remove water from the expansion tank it cannot change the air pressure in the tank" can the pump change the psi of the "water" in the tank? In a loop without a expansion tankna pump cannot either add or remove water from the loop yet it can still change the psi of the water. I am confused. Please help. Thank you.

Jamie Hall

The expansion tank has air in it -- which is very compressible. Therefore a change in the volume of the water with a change in temperature -- which is what is important -- changes the pressure in the tank but only by a very small amount. Without the tank, the temperature change would make very large changes in pressure as the water expanded.

A pump when it is running will change the pressure distribution around a loop of pipe, but not the overall pressure. On the outlet side of the pump, the pressure will rise. As the water flows around the loop, the pressure will drop, and will be lowest at the inlet to the pump.

Now let us suppose that we forget, for the moment, about expansion and just concern ourselves with the pump and an expansion tank. Since there is no temperature change, there is no change in the volume of the water in the system, and there will be no change in the pressure in the tank. If the pump is turned on, the expansion tank will remain at exactly the same pressure as before. Hence it is referred to as the point of no pressure change. Pressure upstream of the tank -- towards the pump -- will be higher, reaching a maximum at the outlet of the pump. Pressure on downstream will be lower, reaching a minimum at the inlet to the pump.

Alan (California Radiant) Forbes

Maybe one day I'll understand. Remember in school, you were taught a "given" and I considered a given to be a truth, whether I understood it or not. It is a basis for further understanding and for me, I now understand hydronic systems when I know where the point of no pressure change is located.

As someone I know says,

"The magic is in hydronics and hydronics is in me."

How can you possibly explain magic?