Domestic Hot Water Circulation - Bosch 520PN Applications Manual

8 | THERM/GREENTHERM Tankless Water Heaters

5.2 Domestic hot water circulation

5.2.1 Ariston mini-tank DHW recirculation
Reference drawing 9.2.3 on page 20.
If recirculating through an Ariston mini-tank only, use the
smallest pump that will overcome the pressure of the loop.
Recommended ow rate within the range of 1 to 2 GPM.
This method of recirculation maintains warranty.
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The tank and the Therm should be set at roughly the
same temperature.
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An aquastat is required to control the recirculation
pump operation. An aquastat temperature setting of
110° F with a 10° F differential is recommended.
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The recirculation flow must be <2 gpm. Higher flows
will waste energy and can erode the Ariston anode,
requiring frequent anode replacement to maintain its
warranty.
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To minimize energy costs:
1. Insulate all pipes in the recirculation loop.
2. Install a timer or sensor system that will shut the
pump off when hot water will not be used.
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Recirculation loops over 200 feet in total length will
require an electric tank with a larger heating capacity
than the Ariston 8 gallon model.
5.2.2 Direct DHW recirculation
Reference drawing 9.2.4 on page 21.
If the Bosch tankless water heater provides the
heat source for recirculated water , the heat
exchanger warranty is decreased. Please refer
to the installation manual for detailed warranty
period information.
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Direct recirculation flow through the tankless water
heater must be above 1.7 gpm for best performance.
Flow can be verified in the diagnostic menu, mode
P4-3d. Refer to the Diagnostic Menu section of your
Therm installation manual for details.
System conditions vary and each pump must be
sized by a professional to insure performance.
Refer to pump manufacturers ow vs pressure
speci cations to select a pump that can provide ow
between 1.7 gpm and 3 gpm while overcoming the
pressure loss through the Therm and the recirculation
loop. Refer to gure 1 and table... to determine pressure
loss.
Data subject to change
Use only bronze or stainless steel pumps. Do not
use pumps of iron construction as they will oxi-
dize and clog the inlet lter on the appliance.
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When properly installed in a direct recirculation
application the Therm will regulate water temperature
in a recirculation loop within a 20° F range. A
thermostatic mixing valve is required after the water
heater if no other temperature control is used.
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Closer temperature control can also be accomplished
with a recirculation pump aquastat. The Therm
temperature setting must always be at least 14° F
higher than the aquastat cut-out setting.
EXAMPLE:
— Therm temperature setting: 122° F
— Aquastat cut-out temperature setting: 108° F
— To minimize energy costs:
1. Insulate all pipes in the recirculation loop.
2. Install a timer or sensor system that will shut
the pump off when hot water will not be used.
Recirculation Loop Pressure Drop at 2GPM (Feet of Head)
10 ft Pipe
½
" Type L Copper 0.85
¾
0.15
" Type L Copper
¾
0.25
" Pex
0.53
" Pex
½
1.3
" Pex
Source: 2009 International Plumbing Code
EXAMPLE:
Sizing a pump for a 2gpm direct DHW recirc system with a
THERM 940ES and have 150 ft total loop length.
Head Loss Loop Component
9.00 ft 940ES (See Fig.1, page 7)
1.20 ft 80 ft of ¾" copper
5.90 ft 70 ft of ½" copper return
0.24 ft 8 x ¾" 90 elbows
0.10 ft 2 x ¾" Tees (branch)
0.72 ft 8 x ½" 90 elbows
+ 0.18 ft 1 x ½" Tee (branch)
17.34 ft Minimum Pump Head at 2.0 gpm
Once the loop head loss has been calculated, use the
pump manufacturer's performance curves to select the
proper potable water circulator at the required ow rate.
Applications Manual
Tee
90 Elbow 45 Elbow
Branch
0.09 0.05 0.18
0.03 0.01 0.05
0.02 N/A 0.03
0.04 N/A 0.06
0.12 N/A 0.18
Bosch Thermotechnology Corp.