Install Instructions

31
IM-PR 566442 1114 (Design Manual)
1.9 Circulator pumps
Selection of a system’s circulator pump(s) is
an important step in the overall system design
because it directly affects the system’s heating
capacity and its electrical energy use. Under-sizing
a circulator can result in failure to satisfy heating
loads, and over-sizing a circulator can result in
wasted energy and increased expenses for the
building owner, as well as premature failure and
unwanted noise. Proper selection of circulators is
based upon the system design flow rates and the
relevant pressure drop.
1.9.1 Flow rates
Circuit flow rates are determined based on the heat
load of a space, the composition of the radiant
panel assembly and the temperature drop of the
system. To estimate the flow rate required, you may
use the following equation:
Total Design Heat Load (Btu/hr)
∆T • 8.01 • C
p • P
GPM =
where
∆T is the system temperature drop from supply
to return.
C
p
is the specific heat of the fluid, taken at the
average of the supply and return temperature (Btu/
lb/°F). For 100% water solutions in hydronic heating
applications, use a value of 1.0 for the specific
heat. For the specific heat of glycol solutions, see
Table 1-9 for propylene glycol or Table 1-12 for
ethylene glycol.
8.01 is a constant to convert between units.
P is the density of the fluid, taken at the average
of the supply and return temperature (lbs/ft
3
). For
the density of a 100% water solution in hydronic
heating applications, see the 0% glycol column in
Table 1-11 or Table 1-13. For the density of glycol
solutions, see Table 1-11 for propylene glycol or
Table 1-13 for ethylene glycol.
For quick approximations in all-water systems, you
can use the following equation:
Here, the constant of 500 is calculated
by multiplying 8.01*1.0*62.4 (conversion
constant*specific heat of water*density of water
at 68°F).
For primary and secondary loops or distribution
piping, ensure that the targeted flow rate
corresponds to a velocity of between 2 and 4
feet per second by selecting a piping diameter
to accommodate the design flow rate within this
velocity range. (See Table 1-5 for more details.)
1.9.2 Pressure drop
Circulators must be sized for the pressure drop
that they will experience at their design flow rate.
The first step in calculating the pressure drop
associated with a circulator is identifying the
network of piping and accessories (valves, fittings,
mixing devices, expansion tanks, air separators,
etc.) that corresponds with each circulator.
Total Design Heat Load (Btu/hr)
∆T • 500
GPM =