Zone Valves
When one circulator is serving multiple
zones, the gpm requirement and head loss for
each zone must be determined.
To find the total gpm, add all of the zones
together. However, to determine head loss, size
the largest head loss.
Example:
Zone 1 requires 2.5 gpm with 6 foot of
head loss
Zone 2 requires 3.5 gpm with 4 foot of
heat loss
Circulator demands will be 6.0 gpm with 6
foot of head loss. GPM is cumulative; head
loss is not cumulative—if it can overcome a
high head loss zone, then the same circulator
can overcome a lower head loss zone.
Cv—Coefficient of Flow
An additional factor in determining total
head that a circulator may need to overcome
is consideration of the different components
in the system. In assessing those components,
coefficient of flow (Cv) through those components
must be determined.
Cv is a function of the pressure drop in
28 National Oilheat Research Alliance
devices installed in hydronic systems. Every
component has an associated Cv value. The
higher Cv rating for the device, the lower the
pressure drop. This is important to understand
because it represents the pressure drop in that
device for a certain flow rate.
Always check the Cv of the
specific device being used.
Cv is the gpm that can pass through a device
with a pressure drop of 1 psi. For example,
a device with a Cv value of 3.5 will cause a
1 psi pressure drop when 3.5 gpm are flowing
through it. To equate that to foot of head
loss, multiply the psi pressure drop by 2.31. A
device with a Cv of 3.5 and with a design flow
rate of 3.5 will have a pressure drop of 2.31
foot of head:
(Flow/Cv)2 x 2.31
From the previous example, Zone 2 has a
flow requirement of 3.5 gpm and the circulator
has to overcome 6' of head loss.
If the system has two ¾" zone valves that
each have a Cv of 3.5, the additional head loss
will be:
(3.5/3.5)2 X 2.31 foot of head for the device
at that flowrate.
The Cv of a flow rate valve can be
significantly different from the
Cv of a zone valve
that is the same size.
For example, one manufacturer's
¾" flow valve has a Cv of 5 and
their ¾" zone valve has a Cv of 6.1