TURBOMAX Use and Care Manual with Installation Instructions July 2014 Page
12
PUMP & PIPE SIZING
Boiler water temperature drop (TD) through
the water heater
Simplified design methods based on a 20°F
temperature drop (TD) of boiler water going
through the water heater to heat up domestic
water are commonly used. Although such
methods are widely used and generate
satisfactory system performance when applied
properly, they do 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 over-design.
Another method by which the boiler water
temperature drop (TD) could be calculated is to
assume a constant supply boiler water
temperature minus the domestic water final
temperature. For example a domestic water
heater might have a final temperature of 140 °F.
Assuming a constant supply boiler temperature
of 180 °F, the TD would be 40 °F
( = 180 °F – 140 °F). Second example: If the
domestic water heater has a final temperature of
180 °F and the boiler supply is at 200 °F, then
temperature drop is 20 °F (= 200 °F – 180 °F).
Precautions should be taken so that the boiler
return is above the boiler manufacturer’s lowest
recommended temperature. Most hot water
heating systems use standard, non-condensing
boilers (cast iron or steel), which must be
operated above 140°F in order to prevent the
corrosion that is associated with flue gas
condensation.
Alternatively, 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
TURBOMAX performance tables and use the
guidelines stated above to design a state of the
art system.
The following chart proposes a temperature drop
(TD) that should be used to calculate the pump
flow rate.
Proposed boiler water temperature drop
through the water heater (TD)
Boiler water
supply
temperature
Domestic water
final
temperature
TD
200 °F
180 °F
20 °F
200 °F
160 °F
40 °F
180 °F
160 °F
20 °F
180 °F
140 °F
20 °F to 40 °F
180 °F
125 °F
20 °F to 40 °F
180 °F
110 °F
20 °F to 40 °F
160 °F
140 °F
20 °F
160 °F
125 °F
20 °F
160 °F
110 °F
20 °F
NOTE: The boiler water in TURBOMAX
®
constitutes a store of heat energy ready to heat
fresh domestic water. The volume of boiler
water stored in TURBOMAX
®
tank provides
enough heat to keep your domestic hot water
hot while the boiler heats up. In fact, it acts as a
buffer, which prevents domestic hot water, or
boiler water temperature swings.
Pump flow rate calculation
The boiler’s output rating must be within the
heater’s the heat loads calculation or the sizing
guide recommendations. Use the equation
below to calculate the pump flow rate.
Pump flow rate = Boiler output
÷÷÷÷
TD
÷÷÷÷
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 water heater to meet the hot water
demand.
•
TD is the boiler water temperature drop
through the TURBOMAX
®
.
For example, a cast iron boiler has an output
rated at 90,000 BTU per hour. The system is
designed for a temperature drop (TD) of 20°F.
Pump flow rate = 90,000
÷
20
÷
500 = 9 GPM.
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