A.O. Smith 705 Series 100, 505 Series 100, 305 Series 100 Pump Sizing for Circulation, Table B

Pump Sizing for Circulation

1.Use the chart below or one appropriate for your conditions to determine the heat loss in the length of the hot water supply and return piping. For example, 100 ft of 1-1/2 in bare copper tubing results in a heat loss of 5300 Btu/h.

Approximate Heat Loss from Piping at 140 ºF Inlet, 70 ºF Ambient *

Nominal Size, in.	Bare Copper Tubing,	1/2 in. Glass Fiber Insulated
	Btu/h-ft	Copper Tubing, Btu/h-ft
3/4	30	17.7
1	38	20.3
1-1/4	45	23.4
1-1/2	53	25.4
2	66	29.6
2-1/2	80	33.8
3	94	39.5
4	120	48.4

* Source: 2003 ASHRAE Handbook HVAC Applications

Table “B”

2.Determine the acceptable temperature drop at the last fixture in the loop. For example, if the supply temperature from the water heater is 120 ºF (49 ºC) and an acceptable temperature at the last fixture is 100 ºF (38 ºC) then the acceptable temperature drop is 20 ºF (7 ºC).

3.Calculate the required pump flow rate using the following formula:

FLOW RATE (gpm) =	HEAT LOSS (BTU / h)
	500 X ACCEPTABLE TEMPERATURE DROP (ºF )

4. Based on the above calculations select a pump for the type of circulation system you will be utilizing:

A). Preferred Method (reference drawing H-1-RP) - Reference pump manufacturers flow vs. pressure specifications to select a pump that can provide the flow rate calculated above while overcoming the pressure loss through:

•Tank water heater (reference manufacturer’s information)

•All building supply and return plumbing in the circulation loop (reference local plumbling codes, standards, or practices)

B). Optional Method (H-1-RO) - Reference pump manuf acturers flow vs. pressure specifications to select a pump that can provide 3 gpm of flow or the flow rate calculated above, whichever is greater, while overcoming the pressure loss through:

•On demand water heater (reference flow vs. pressure curve of the model being used)

•Optional storage tank (reference manufacturer’s information)

•All building supply and return plumbing in the circulation loop (reference local plumbling codes, standards, or practices)

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.

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