Friction Loss - Gutter Pumper

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Fitting Instructions
Calculating Friction Loss to estimate maximum pipe length to second vertical drop
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Calculating whole system flow rate

Nearly all Gutter Pumpers are installed as per Method A or Method B



METHOD A plumbs the initial horizontal pipe directly to the second vertical drop.

METHOD B plumbs the initial horizontal pipe along the wall to the second vertical drop.

NOTE that both methods have an elbow at the bottom of the second drop. The elbow provides back pressure and must be fitted.

Gutter Pumper will instantly prime the initial vertical drop when the anti vortex baffle is covered, regardless of the drop’s length.

When installing a Gutter Pumper as per Method B, you need to calculate the length of the initial vertical drop to determine the recommended maximum total friction loss (expressed as pipe length) between the Gutter Pumper and the top of the second vertical drop.

The initial vertical drop is the distance between the gutter and the top of the initial horizontal pipe PLUS 25 mm for the Gutter Pumper inside the gutter.
The minimum initial vertical drop possible is 70 mm when the elbow is fitted directly under the Gutter Pumper.

Using the 45° elbow minimises friction loss

Black fitting is a 90° threaded poly elbow

Friction loss between the Gutter Pumper and the second drop = the pipe length between the top of the Gutter Pumper to the second drop PLUS the equivalent pipe length for each elbow including the 90 degree elbow above the second drop.

90° elbow = 0.6 metres.

45° elbow = 0.3 metres.

A minimum 70 mm initial vertical drop will generate a flow rate sufficient to prime the second drop after flowing through a pipe with 3.5 metres of friction loss but there must be sufficient velocity generated by longer first drops to prime the second vertical drop and boost the system’s flow rate when longer initial horizontal pipes are used.

The charts below show the recommended maximum length of pipe (expressed as friction loss) between the Gutter Pumper and the second drop for different lengths of initial vertical drops. These charts need to be consulted when plumbing long lengths of initial horizontal pipe along walls. The Method B installation shown in the second photo is explained below.


ELBOWS: 2 x 90° = 1.2 metres equivalent pipe length

1 x 45° = 0.3 metres equivalent pipe length

TOTAL PIPE LENGTH TO SECOND DROP: Approximately 2.2 metres

TOTAL FRICTION LOSS: 1.2 m + 0.3 m + 2.2 metres = 3.7 metres


If we look at Chart A and check the 110 mm vertical drop’s line intersect, we see that the maximum recommended friction loss to the second drop is 5.5 metres.

RESULT: 3.5 metres is ok to use.

Question: But what if we need to run the pipe 5 metres along the top of the wall and the friction loss (+ elbows and the initial drop) is then 6.6 metres, what can we do then?
Answer: Increase the length of the initial drop to 135 mm, see Chart A.

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Fitting a 20 x 15 mm PVC reducer near the bottom of the second drop will quickly prime the second drop and allow an additional 33% of friction loss. A nominal 15 mm class 18 pressure pipe has an internal diameter of 17.8 mm and the smaller pipe can either be plumbed to a nearby point of discharge or a second reducer fitted that reconnects to a nominal 20 mm pressure pipe (the prime will break at that point). The reducer and short length of 15 mm pipe will reduce the calculated flow rate by about 5-6 litres per minute and are often fitted to retard the velocity from high gutters.

Checking the 110 mm intersect on Chart B, we find that the recommended maximum length of friction loss for a 110 mm first drop is 7.25 metres.

RESULT: 6.6 metres is ok to use.

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