BTH Electric Boilers
USE & CARE MANUAL
(Revision August /06)
, Page
20.
PUMP AND PUMP SIZING
Boiler water temperature drop (BWTD)
through the heating loop
A simplified design method based on a 20°F
temperature drop (BWTD) between boiler outlet
and inlet is commonly used. Although such a
method is widely used and generates
satisfactory system performance when applied
properly, it does not determine the system
operating point. The pipe size is often
uneconomically large, and the actual system
flow rate is likely to be much higher than
intended. Such design methods seldom
consider temperature drops higher than 20°F,
which results in overdesign.
Another method by which the boiler water
temperature drop (BWTD) could be calculated
is to assume a constant supply boiler water
temperature minus the return boiler water
temperature. For example a boiler might have a
return temperature of 140 °F. Assuming a
constant supply boiler temperature of 180 °F,
the BWTD would be 40 °F ( = 180 °F – 140 °F).
Second example: If the boiler water has a return
temperature of 120 °F and the boiler supply is at
140 °F, then the temperature drop is 20 °F (=140
°F – 120 °F).
By design the boiler water supply temperature
cannot be higher than 190°F. Also, when the
boiler surfaces are hot due to previous loads
such as domestic hot water generation, the large
temperature difference between the boiler and
its return water can cause the boiler to become
thermally shocked.
An experienced designer could work with other
values than those proposed by looking into the
performance tables and using the guidelines
stated above to design a state of the art system.
The following chart suggests the temperature
drop (BWTD) to be used to calculate the pump
flow rate.
PROPOSED BOILER WATER
TEMPERATURE DROP THROUGH THE
HEATING CIRCUIT (BWTD)
System
type
Boiler
water
Supply
tempera-
ture
Boiler
water
Return
tempera-
ture
BWTD
Baseboards
190°F to
140°F
170°F to
120°F
20°F to
40°F
Cast Iron
Radiators
160°F to
130°F
140°F to
110°F
20°F to
40°F
Radiant
In-Floor
130°F to
90°F
110°F to
70°F
20°F to
40°F
Pump flow rate calculation
The boiler’s output rating must correspond to the
heating load calculations or be within the sizing
guide recommendations. Use the equation
below to calculate the pump flow rate.
Pump flow rate = Boiler output
÷
BWTD
÷
500
•
Pump flow rate is express in U.S. gallons
per minute or GPM.
•
The Boiler output ( in net BTU per hour) is
the maximum heat to be transferred through
the heating loop to meet the heating
demand.
•
BWTD is the boiler water temperature drop
through the heating loop.
For example, a BTH 24 as an output rated at
81,888 BTU per hour. The system is designed
for a temperature drop (TD) of 20°F.
Pump flow rate = 81,888
÷
20
÷
500 = 8.2 GPM.