vvEPA
United States
Environmental Protection
Agency
Calculated Flush Time (CFT) or Calculated Flush Volume (CFV) Approach
for Representative Distribution System Chlorine and Disinfection Byproduct (DBP) Sample Collection from Building Taps
The contents of this guidance document do not have the force and effect of law and the Agency does not bind the public in any way. It intends only to provide clarity to
the public regarding existing requirements under the law or Agency policies, except as authorized by law or as incorporated into a contract. When a guidance document
is binding because binding guidance is authorized by law or because the guidance is incorporated into a contract, the statement will reflect that.
Objective: This guidance document provides details on best practices to consistently and accurately assess drinking water disinfectant residual and disinfection
byproduct (DBP) concentrations in samples collected from building taps in order to characterize water quality in the distribution system main. Under-flushing
prior to sampling may result in characterizing water quality from the service line, which may not be representative of the water quality in the distribution system
main, and this is a common issue in some businesses or larger buildings with larger diameter and longer length service lines. Over-flushing may unnecessarily
waste a consumer's water. The CFT/CFV approaches were developed to ensure a consistent, fundamentally sound approach to collecting samples from building
taps.
The CFT/CFV is determined based on estimates for the total length of line (including exterior service line and indoor plumbing), the internal diameter (ID) of
line(s), and flow rate through the tap using the following approach.
1.	Estimate the ID of each segment of the line (both inside and outside of the building), considering the following:
a.	Commonly, the ID of a residential service line is %"
b.	If the water meter is visible, it will typically indicate the ID of the buried service line and premise plumbing (e.g., 1", 2"); assume the ID of the service
line and premise plumbing are the same unless a visible difference is observed.
c.	If the water meter is not visible, locate where the service line enters the building (e.g., basement, utility closet) and estimate the ID of the line.
d.	The wall thickness of pipes varies based on pipe material (e.g., lead pipes are typically thicker than copper pipes).
2.	Estimate the total length of the line outside of and inside the building, possibly utilizing a distribution system map:
a.	Generally, the line runs from the main, through a curb stop (shutoff valve), and directly into the building or meter.
i.	Sometimes the service line will enter the building through the back or side.
ii.	A map may indicate the main location in the absence of other information (i.e., visible curb stop, hydrant orientation, operator knowledge).
b.	Select a sample tap near where the service line enters the building to minimize the amount of indoor plumbing to be flushed. Determining the length and
diameter of indoor plumbing can be difficult which is why a safety factor is applied to the calculation.
c.	Measure the distance between the water main and where the line enters the building and the additional distance to the sample tap.
3.	Determine the CFT (at 2 gpm) or CFV for each segment of pipe using Table 1 (time) or Table 2 (volume), respectively, sum the times (or volumes) and apply
a 2-fold safety factor to account for any unidentified line, as shown in the examples below.
Round the pipe length to the nearest value and extrapolate or interpolate for any lengths not specifically listed in the table.
CFT
100' + 25' of 3/4" ID line	1.1 + 0.3 minutes = 1.4 minutes
+ 52' of Vz" ID line (round to 50') + 0.3 minutes
= Total CFT @ 2 gpm = 1.7 minutes x 2 (safety factor) = 3.4 minutes
CFV
100' + 25' of 3/4" ID line ^ 2.3 + 0.6 gallons = 2.9 gallons
+ 52' of Vi" ID line (round to 50') ^ + 0.5 gallons
= Total CFV = 3.4 gallons x 2 (safety factor) = 6.8 gallons
Office of Water (MS140)
EPA 815-B-21-001
January 2021

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4.	Flush the tap, removing aerator before sampling from the tap (if applicable)
a.	If flush time is used, turn on the tap and start a timer. Then check the flow rate. If it is significantly different than 2 gpm, adjust the CFT using the
following equation.
2 gpm
CFTactualflowrate = Total CFT@ 2gpm x actual flow rate> gpm
		2 QVTtl
Adjusting the earlier CFT example for 5 gpm flow rate 4 CFT5 gpm = 3.4 minutes X g m =1.4 minutes
b.	If volume measurement is used, flush the line for the proper amount of volume using an appropriate measuring device.
5.	After the tap has flushed for the CFT/CFV, collect and analyze the sample for the desired parameters (e.g., chlorine, pH, temperature). Note: the CFT/CFV
only has to be determined once for each sampling location and the result can then be used routinely as long as the flow rate during the flushing is relatively
consistent from one sample event to another.
Equipment Needed to Determine CFT/CFV
	Measuring device (i.e., cup, bucket, meter setup) - to measure flushed volume and/or flow rate; recommend 4-cup (32 oz.) or larger to allow sufficient fill
volume over a reasonable time-frame (i.e., at a 2 gpm flow rate, the tap fills 32 oz. in 7.5 seconds)
	Stopwatch/Timer - to measure flush duration
	Tape measure or distance wheel - to measure length of line
Table 1: Calculated Flush Times (minutes) at 2 gpm for Various Line Sizes (inner diameter, ID, provided in inches)
Line Length (feet)
3/8" ID
v" id
%" ID
1" ID
i v" id
2" ID
2 v" id
3" ID
4" ID
5
0.01
0.03
0.1
0.1
0.2
0.4
0.6
0.9
1.6
10
0.03
0.1
0.1
0.2
0.5
0.8
1.3
1.8
3.3
25
0.1
0.1
0.3
0.5
1.1
2.0
3.2
4.6
8.2
50
0.1
0.3
0.6
1.0
2.3
4.1
6.4
9.2
16.3
100
0.3
0.5
1.1
2.0
4.6
8.2
12.7
18.4
32.6
Note: some water systems and state primacy agencies have applied a 5-minute flush time before sample collection. This may be excessive, appropriate, or insufficient depending
on the circumstances at that sample point.
Table 2: Calculated Flush Volume (gallons) for Various Line Sizes (inner diameter, ID, provided in inches)
Line Length (feet)
3/8" ID
V" ID
%" ID
1" ID
i y2" id
2" ID
2 y2" id
3" ID
4" ID
5
0.03
0.1
0.1
0.2
0.5
0.8
1.3
1.8
3.3
10
0.1
0.1
0.2
0.4
0.9
1.6
2.5
3.7
6.5
25
0.1
0.3
0.6
1.0
2.3
4.1
6.4
9.2
16.3
50
0.3
0.5
1.1
2.0
4.6
8.2
12.7
18.4
32.6
100
0.6
1.0
2.3
4.1
9.2
16.3
25.5
36.7
65.3
Note: this approach is not dependent on the sample tap flow rate, but an approach for measuring volume is needed (i.e., appropriate measuring device).
Office of Water (MS140)	EPA 815-B-21-001	January 2021

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