water cools down and “condenses,” the
system would not have enough water to
operate properly.
Expansion tanks are designed to enable
the expansion and contraction of the
system water so the system can continue
to operate effectively.
There are two main types of expansion
tanks in use today—diaphragm tanks and
steel compression tanks.
Diaphragm Tanks (Figure 10)
These tanks have an internal rubber
diaphragm that separates the heating system
water on one side from compressed
air on the other side.
Before the tank is installed on the
system, the diaphragm is fully expanded
and flush with the inside wall of the tank
at the system tapping. This is because the
air side of tank is normally pre-charged
by the manufacturer to 12 psi. Once the
tank is installed on the system, the water
pressure pushes against the diaphragm. As
the system heats up, the “expanded” water
forces its way into the tank. As the water
cools, the air pressure forces the water
back out of the tank.
The air side of the tank should be
charged to the system’s operating pressure.
If the PRV is set to provide 17 psi,
the air side of the diaphragm should be
pumped up to 17 psi. If the air side pressure
doesn’t match the water pressure,
the relief valve may discharge because
the water pressure might push the diaphragm
all the way back before the water
expands.
Diaphragm tanks work best when installed
on an air separator with an automatic
air vent.
Steel Compression Tanks (Also called
air cushion tanks, Figure 11). Under
9 National Oilheat Research Alliance
normal operation, these tanks are typically
about 2/3 full of water and 1/3 full
of air during the off cycle. As the system
water heats up, it expands by about 5%
and pushes against the air in the top of the
tank compressing it and creating a pressure
increase of about 1 psi. As the water
cools down during the off cycle, the water
condenses and the air expands back to its
original volume.
These tanks can become full of water, or
“water logged.” When this happens there
is no air for the water to compress and no
room for expansion. The water pressure
will increase and the boiler relief valve will
discharge. Once the tank is drained to the
proper level again, the system will operate
normally.
9 Circulator—A circulator’s job
is to take water from the boiler,
move it through the terminal units
where heat is emitted and then return it
Figure 11
Air Cushion Expansion Tank
Air Cusion
Expansion Tank,
Typical
Fill Line
Pressure
Reducing
Valve
Tank Fitting,
Typical
Flow Air Separator, Typical