Pump Sizing For Storage Tank Applications - A.O. Smith On Demand 305 Series 100 Design Manual

Pump Sizing for Storage Tank Application
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.
Number of
On-Demand
Water Heaters
Pump Flow Requirements
705/505
1 6 gpm @ 30' head
2 12 gpm @ 30' head
3 18 gpm @ 30' head
4 24 gpm @ 30' head
5 30 gpm @ 30' head
6 36 gpm @ 30' head
7 42 gpm @ 30' head
8 48 gpm @ 30' head
9 54 gpm @ 30' head
10 60 gpm @ 30' head
11 66 gpm @ 30' head
12 72 gpm @ 30' head
13 78 gpm @ 30' head
14 84 gpm @ 30' head
15 90 gpm @ 30' head
305
5 gpm @ 25' head
10 gpm @ 25' head
15 gpm @ 25' head
20 gpm @ 25' head
25 gpm @ 25' head
30 gpm @ 25' head
35 gpm @ 25' head
40 gpm @ 25' head
45 gpm @ 25' head
50 gpm @ 25' head
55 gpm @ 25' head
60 gpm @ 25' head
65 gpm @ 25' head
70 gpm @ 25' head
75 gpm @ 25' head
11