Hot Water Circulation

Based on the hot water supply system characteristics, the following methods can be used. Method A and Method B are basically for small systems. The heat loss method (Method C) is more accurate. The calculation procedure is described in detail in the book, "Engineered Plumbing Design", by Alfred Steele. It is especially useful for larger systems. However, because of its complexity for manual calculation, it is not widely used in plumbing system design. Then, Method D is introduced as an Excel template by using VBA.

A. Estimating Flowrate by Number of Risers:

Size of Riser	No. of Risers	Flow, gpm
1" or less		0.5
1-1/4" and 1-1/2"		1.0
2" and larger		2.0

B. Estimating Flowrate by Fixture Units (FU):

FU	Flow, gpm
	1.0

C. Estimation of Circulation Flowrate by Heat Loss:

Table 1. Piping Heat Loss
(Btu/hr per lineal foot for 140°F water and 70°F room temperature.)
Nominal Pipe Size	Insulated Pipe (½’ Fiberglass)	Bare Pipe
Nominal Pipe Size	Insulated Pipe (½’ Fiberglass)	Sch. 40 Steel	Brass, Copper, T.P.	Type K Copper
½”	15	35	26	19
¾”	17	43	32	26
1”	19	53	38	32
1¼’	21	65	46	39
1½”	25	73	53	46
2”	28	91	65	58
2½”	32	108	75	68
3”	38	129	90	81
4”	46	163	113	103
5”	55	199	138	127
6”	63	233	161	149
8”	80	299	201	188

Source: Engineered Plumbing Design, Alfred Steele, PE, CIPE

D. The Computer-Aided Hot- Water Circulation System Sizing

This VBA based template was introduced by Anjian Lu, CPD, and published on Plumbing System & Design magazine, Dec. 2004. By using this method, the required circulation flow rate and friction loss in the system are both calculated, so are the circulation pipe sizes and pump capacity.

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