HVAC523
Expansion Tanks
Expansion Tanks

All fluids used in hydronic systems
expand when heated.

This thermal expansion is an
unavoidable and extremely powerful act
of nature.
Expansion Tanks

The molecules in the fluid take up more
space when they are heated.
Expansion Tanks

Any container that is filled with a liquid
and sealed from the atmosphere will
experience a rapid increase in pressure
as the liquid is heated.

If this pressure is allowed to build, the
container will burst, and in some cases
violently.
Violently bursting container
Expansion Tanks

To prevent this from happening, all
hydronic heating systems must be
supplied with a means of accommodating
the volume increase of the fluid as it is
heated.

The device that is commonly used in the
closed loop hydronic system is the
diaphragm expansion tank.
Diaphragm expansion tank
Diaphragm Expansion tank

A diaphragm type expansion tank has a
flexible rubber membrane installed inside
the tank.

On one side of this membrane, there is a
captive air volume which is prepressurized by the manufacturer.
Diaphragm Expansion tank
Diaphragm Expansion tank

On the opposite side is a chamber for
accommodating the expanded volume of
fluid.

As more fluid enters the tank, the
diaphragm flexes, allowing the air
volume to be compressed.
Diaphragm Expansion tank
Air cushion
Rubber diaphragm
Air valve
Fluid reservoir
Liner
System connector
Diaphragm type expansion
tank

Since the diaphragm separates the fluid
from the air, the air can not be
reabsorbed by the fluid.

Diaphragm type expansion tanks do not
have to be periodically drained to
prevent water logging.
Diaphragm type expansion
tank

By avoiding water logging, the possibility
of accelerated corrosion caused by
addition of fresh water to make up for
relief valve losses is no longer a factor.

The air pressure in the tank can be
adjusted to match the static pressure of
the system before the system is filled
with water.
Diaphragm type expansion
tank

No significant amount of water enters the
tank until the system warms up resulting
in a small lightweight tank.

Because the air volume is captive, the
tank can be theoretically mounted in any
position.
Diaphragm type expansion
tank

A properly sized diaphragm type
expansion tank should reach a pressure
of about 5 psi lower than the relief valve
setting when the system reaches its
maximum operating temperature.

The safety margin prevents the relief
valve from leaking just below its rated
opening pressure.
Air side pressurization

The proper air side pressurization is
equal to the static fluid pressure at the
inlet of the tank, plus an additional 5 psi
allowance at the top of the system.

The pressure on the air side diaphragm
is always adjusted to the calculated
value before the fluid is added to the
system.
Air side pressurization

Adjusting the air pressure is done by
adding or removing air through the
schrader valve on the shell of the tank.

A small air compressor or bicycle tire
pump can be used when air is needed.
Air side pressurization

Proper air side pressure adjustment
ensures the diaphragm will be fully
expanded against the shell of the tank
when the system is filled with fluid, but
before it is heated.

The failure to make this adjustment can
result in the diaphragm being partially
compressed by the fluids static pressure
before any heating take place.
Pressure and temperature
ratings

Expansion tanks have a maximum
pressure and temperature ratings.

This information is stamped on the label
of the tank.

Typical ratings are 60 psi and 240
degrees.
Selection, mounting and
service

Tanks are available in a wide range of
shapes and sizes.

Tanks with volumes of 1 gallon to 14
gallons are usually adequate for
standard residential and light commercial
hydronic systems.
Selection, mounting and
service

These tanks are usually equipped with a ½”
inch pipe connection from which they are
designed to hang vertically.

One common mounting method is to hang a
diaphragm tank from the bottom of the
system’s air separator.

Many air separators have a ½” bottom tapping
for this purpose.
Selection, mounting and
service

Although the captive air volume would
theoretically allow it to function in any
orientation, there are other concerns to
address whenever the tank is hung in any
other position than vertically from its inlet
connection.

One is that air bubbles could form on the
water side of this system and promote
corrosion.
Horizontal installation?
Selection, mounting and
service

When very large expansion tanks are
required, floor mounted models are
available.
Floor mount expansion tanks
Diaphragm expansion tank
failure

Most diaphragm expansion tanks will
retain their captive air volumes for
decades.

However, there is always the possibility
that a diaphragm could leak.
Diaphragm expansion tank
failure

If this occurs, the tank will eventually
lose it’s air and fill the system with fluid.

Such a failure is easy to detect during a
routine service check.

Most complaints start with a “release of
fluid from the pressure relief valve”.
Diaphragm expansion tank
failure

During the service check, simply press in
on the schrader valve stem on the tanks
air valve.

If fluid comes out, the diaphragm has
failed and the entire tank has to be
replaced.
Diaphragm expansion tank
failure

Tapping on the side of an intact
diaphragm usually produces a hollow
sound.

If a “thud” is heard, most likely the
diaphragm has failed and the tank is full
of fluid.
Diaphragm expansion tank
failure and replacement

Care should be taken when removing a
failed expansion tank.

First of all, the pressure on the tank must
be reduced to zero psi or you will end up
being showered with potentially very hot
fluid.

Secondly, a tank filled with fluid is
surprisingly heavy!
Diaphragm expansion tank
failure and replacement

As stated earlier, the air side pressure on
the new expansion tank must be adjusted
before fluid is allowed to enter the tank.
Diaphragm expansion tank
sizing chart
Compression Tank

Still in service today, but is slowly being
replaced with the modern diaphragm
tank, is the steel compression tank.

These tanks were often hung from the
underside of the floor joists just above
the boiler.
Compression Tank
Compression Tank

The air in this type of tank is initially at
atmospheric pressure.

When the system is filled with fluid, the
air is trapped in the top of the tank and
partially compressed.
Compression Tank

As the water expands upon heating,
additional fluid enters the tank, further
compressing the air.

An inherent problem with this design is
that the air and water they contain are in
direct contact with each other.
Compression Tank

As the fluid in the tank cools off, it has
the ability to reabsorb some of the air
back into solution.

Upon reheating, the dissolved air will
again come out of solution, but now on
the system piping.
Compression Tank

Eventually this air will be removed by the
air vents.

An automatic feed water valve will then
admit a small amount of water to make
up for the lost air.
Compression Tank

The result of this cycle of venting and
filling will eventually lead to a water
logged tank.

Which means the tank has become
completely filled with water.
Servicing the Compression
Tank

Compression tanks typically need to be
drained and refilled two times a year to
prevent the problems associated with
water logging.

Special drain valves that allow air to
enter the tank the same time water is
being drained are available for this
purpose.
Specialized drain fitting