A.O. Smith 186965-000, Design Manual

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The following applies when using on-demand water heaters to recover a storage tank.
Drawing H-1-TC is an example.
On-Demand water heaters have a pressure loss which must be considered in the system design.
Reference the pressure loss curve for the heater model being used to determine the pump size for the
desired recovery rate.
For recommended pump sizes use the table below. Additional pressure losses in plumbing between the
Heater(s) and the storage tank must also be taken into consideration.
The specified pump size is to provide maximum recovery of the storage tank. A smaller pump size may
be used, but could result in longer recovery time of the tank. Please contact the Tech Services department
with any questions on pump sizing.
NOTE: Only use pumps of brass or stainless steel construction. Do not use pumps of iron
construction as they will oxidize and clog the inlet filter on the appliance.
Pump Sizing for Storage Tank Application
11
Pump Flow Requirements
Number of
On-Demand
Water Heaters
705/505
305
1 6 gpm @ 30' head 5 gpm @ 25' head
2 12 gpm @ 30' head 10 gpm @ 25' head
3 18 gpm @ 30' head 15 gpm @ 25' head
4 24 gpm @ 30' head 20 gpm @ 25' head
5 30 gpm @ 30' head 25 gpm @ 25' head
6 36 gpm @ 30' head 30 gpm @ 25' head
7 42 gpm @ 30' head 35 gpm @ 25' head
8 48 gpm @ 30' head 40 gpm @ 25' head
9 54 gpm @ 30' head 45 gpm @ 25' head
10 60 gpm @ 30' head 50 gpm @ 25' head
11 66 gpm @ 30' head 55 gpm @ 25' head
12 72 gpm @ 30' head 60 gpm @ 25' head
13 78 gpm @ 30' head 65 gpm @ 25' head
14 84 gpm @ 30' head 70 gpm @ 25' head
15 90 gpm @ 30' head 75 gpm @ 25' head
Table “C”