to the boiler where it can be reheated. It
circulates water in a closed loop system
by creating a pressure differential, meaning
it takes the fluid that comes into it at a
given pressure and sends it out at a higher
pressure All the circulator has to do is
create enough of a pressure differential
to produce adequate flow through the
system.
To produce the necessary pressure differential,
an impeller is used. How this
impeller is designed and installed is critical
to the operation of the circulator, and
should be understood by anyone installing
or servicing hydronic systems.
When the circulator is in operation,
water flows into the suction side of the
volute—a part of the circulator which is
always wider and larger than the discharge
side—and then into the eye of
the impeller. As the impeller rotates, the
vanes “slap” the water from the inside of
the impeller to the outside of the impeller.
This adds velocity to the fluid. The
fluid then moves to the discharge side of
the volute, which is smaller and narrower.
The collection chamber on the discharge
side turns the kinetic energy of the fluid
– (energy due to velocity) into pressure.
Thus water comes into the circulator at
a certain pressure and leaves at a higher
pressure.
The thickness, diameter and construction
of impellers all play a role in the
performance of the circulator. The thickness
of the impeller determines its flow
capacity—the thicker the impeller, the
more flow the circulator will be able to
produce.
The diameter of the impeller
is also important. The
larger the diameter, the more
velocity the circulator can
impart to the fluid. The more
velocity, the more pressure
the circulator can produce.
Open vane impellers are
used in circulators designed
to provide high flow and relatively
10 National Oilheat Research Alliance
low head. These types of circulators
are called flat curve circulators – as
will be apparent when looking at performance
curves.
Flat curve circulators are used in radiator
and baseboard applications which
require higher flow rates but have lower
overall head losses.
Closed vane
impellers are
used for higher
head and medium
to high
flow circulators.
They
are designed
for lower flow
systems that
produce higher head losses, which are
typically found in most radiant applications.
10 Zones—A zone is an area
of the house controlled by
a dedicated thermostat. A
common residential zoning system has
one zone for the first floor and one zone
for the second floor. This system requires
either two thermostats and two circulators
OR two thermostats, two zone valves and
one circulator.
11 Zone valves—Zone valves
are two-position valves: they
are either open or closed and
are wired to the thermostats in the house.
The valve is usually closed and will open
only when there is a call for heat from a
thermostat in a particular area of the home.
Once the valve is fully open, the circulator
will start and water will begin flowing to
the rooms that need heat. Figure 12.
12 Flo-Control Valve —Some
systems have a dedicated circulator
for each zone and do
not use zone valves. These systems utilize
flo-control valves that are installed in the
supply piping to prevent gravity circula