it will encounter pressure loss due to friction in
the piping. The circulator needs to produce the
required amount of flow while overcoming that
pressure loss.
Pressure is generally measured in pounds per
square inch, or psi. In hydronics, head loss is
the common term. Converting psi to head loss
is very simple. A column of water 2.31 feet, or
28 inches high, will have a gauge pressure at
the bottom of 1 psi. Therefore, 1 psi of pressure
drop in a system equals 2.31 feet of head.
It’s important to remember when sizing a
circulator there is no need to take into account
the height of the building. In in a closed loop
system, the circulator does not need to lift the
water to the top of the building, it only has to
overcome the friction loss of the piping and
components or the head loss of the system.
To size a circulator properly, start with a heat
loss analysis of the structure. The flow rate
needed for a particular job or a particular zone
is based solely on the heat loss. The heat loss
analysis should be conducted on a room-byroom
basis, since each room will have its own
unique heating requirements and will require
terminal units (typically baseboard) sized for
its unique needs.
After each room is calculated, group the
rooms together into zones to determine what
the zone's total heat loss is. This is important
when zoning by circulator, since each circulator
will be based on the zone flow requirement.
When zoning with zone valves, it will be important
to know the flow rate for the entire job
so the circulator will be sized correctly.
Once the Btuh load of the zone or of the
entire job is known, depending on whether
zoning will be accomplished with zone valves
or circulators, calculate the actual flow rate using
the Universal Hydronics formula:
gpm = Btuh/(ΔT x 500).
26 National Oilheat Research Alliance
For example, when zoning with circulators,
and there is a zone that has 27,000 Btus’ worth
of baseboard, or roughly 45 feet. Since it is
baseboard, use a 20° ΔT, (100% water—no
glycol). To determine the flow rate, divide the
load, 27,000, by the ΔT of 20 times 500.
GPM = Btuh/ΔT x 500
GPM = 27,000/20 x 500 = 2.7
The flow rate for the zone is 2.7 gallons per
minute.
What size pipe should be used for this zone?
The rule of thumb for pipe sizing is two to four
gallons per minute of flow, so use ¾" M copper
pipes.
These will keep flow velocities at no less
than two feet per second and no more than four
feet per second. At velocities greater than four
feet per second, the system will produce unacceptable
noise. At a velocity less than two feet
per second, dissolved oxygen may be released
and cause air problems within the system.
Head Loss of a Zone
To determine the head loss of a zone, measure
the total length of the zone, including both
piping and heat emitters.
Fittings and valves produce pressure drop
in a system equivalent to a few feet of pipe.
To accommodate basic fittings and valves,
multiply the total length of pipe by 1.5. This is
the total developed length of the circuit. Other
head loss items, such as flo-valves (aka flowchecks)
three-way valves and other high head
loss items may be evaluated later.
Next, take that number and multiply by .04.
This number represents four feet of head per
100 feet of copper pipe. That head number applies
as long as the pipe has been sized according
to the velocity guidelines discussed above.
The end product is the head loss for the zone.