The supply side
POINT OF NO PRESSURE CHANGE
Why can’t the circulator change the system’s pressure
at the point where the expansion tank connects to the
system? This is a question asked often, both in the
field and at our hydronic seminars. The answer is
based on Boyle’s Law of Perfect Gases, which the science
community has referenced for hundreds of years.
Boyle’s Law says that if you have a gas (air) trapped
in a tank (expansion tank), its volume will shrink if
you add pressure to it. Likewise, its volume will expand
if the pressure is lowered. In other words, if you
squeeze a gas, its pressure will rise and conversely,
let the gas expand and its pressure will drop. How does
all of this relate to hydronic systems?
To change the pressure in the expansion tank (diaphragm
or steel), you have to squeeze the air that is
in there. When the boiler heats the water in the system,
the water expands, compressing the air in the
expansion tank. This causes an increase in the system’s
pressure, which you can see on the gauge of the
boiler. When you open the fill valve to add more water
to the system, because the piping is already filled
with water, the additional water enters into the expansion
tank, compressing the air, causing a rise in
system pressure which is indicated on the boiler’s
When a circulator turns on, does it heat the water?
Does it add more water to the system? Or does it just
circulate the water? Can you see the difference? If
there is no change in volume or temperature of the
water, there can’t be a change in its pressure.
If the circulator is pumping away from the expansion
tank, can it take water out of the tank? If you
think yes, where would you put it? The piping is already
filled and you can’t compress water. If the circulator
is pumping towards the expansion tank, can
it put water into the tank? Air is compressible, so water
can enter the tank.
But where would the water come from? If you think
it can come from the piping circuit, that means there
would be a void in the piping circuit. And Mother Nature
hates a vacuum (void)! No, when a circulator
comes on, it can neither add nor remove water from
the expansion tank. And if it can’t change the volume
of the water in the tank, it can’t change the volume of
the air (gas) in the tank. This means it can’t change
the pressure in the tank or the piping connecting the
tank to the system.
Every time a circulator turns on, it “looks” for this
point. Depending upon whether it’s pumping away or
towards the expansion tank, it either increases its discharge
pressure or drops its suction pressure. And this
is why since the 1960’s, when Gil Carlson wrote his
famous paper on the “Point of No Pressure Change”,
circulators should be located on the supply “pumping
away” from the expansion tank.