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Understanding pipe sizes and flow rates
TO ACHIEVE FULL FLOW
To remain primed, the vertical drop must not be angled or increased in diameter.
FRICTION LOSS
The vertical drop's potential flow rates are reduced by friction losses in the connected pipe work but losses are minimised by connecting the vertical drops to larger horizontal pipes.
This is easy to calculate and is explained with examples in the PIPES & FITTINGS page.
The simple adage is: Larger pipes = less friction loss = higher flow rates.
Polypipe 19mm vertical drops are often connected to 25mm (78% greater volume) horizontal polypipe, depending on intended usage. Polypipe has the advantage of low cost, ease of fitting and is easily altered.
For the recommended class 9 & 12 PVC pressure pipe, the 20mm vertical drops can be connected to 25mm, 32mm, 40mm and even 50mm PVC pressure pipe and fittings. PVC pipes maintain higher flow rates and are more suited to permanent connections than polypipe. Class 9 PVC pipe is cheaper than class 12. Poly and PVC pipe is easily mixed and matched.
PVC pressure pipe is sold as a 'nominal size'… the actual internal diameters (ID) are larger!
For example, 32mm class 12 PVC pipe has an ID of 37.5mm and the 32mm class 9 PVC pipe has an ID of 38.5mm.
SELF CLEANING PIPEWORK
Air flushing requires a flow velocity of .8 metres per second.
Sediment flushing requires a flow velocity of .6 metres per second.
Nominal 
ID 
Wall 
litres/metre 
litres/min @ 0.8m/sec 
CLASS 9 PVC 

25 
30.5 
1.5 
0.8 
35 
32 
38.5 
1.9 
1.2 
56 
40 
44.1 
2.1 
1.5 
73 
50 
55.2 
2.6 
2.4 
115 
CLASS 12 PVC 

20 
23.65 
1.5 
0.4 
21 
25 
29.75 
1.9 
0.7 
33 
32 
37.45 
2.4 
1.1 
53 
40 
42.75 
2.7 
1.4 
69 
50 
53.65 
3.3 
2.3 
109 
POLYPIPE 

19 
19 
0.3 
14 

25 
25.4 
0.5 
24 
25mm class 12 PVC pipe has 38% more capacity than 25mm poly pipe and 20mm class 12 PVC pipe has 55% more capacity than 19mm poly pipe! 20mm horizontal pipe is rarely used and 40mm pipe is commonly used. Further details can be found on the PIPES & FITTINGS page.
LESS FRICTION LOSS = HIGHER FLOW RATES
FLOW RATES FOR CLASS 12 PVC PRESSURE PIPE
Litres per minute flow rates with 20 metres of friction loss 

HEAD 
20mm 
25mm 
32mm 
40mm 
50mm 
0.25m 
12 
23 
42 
51 
108 
0.50m 
18 
33 
60 
87 
156 
1.0m 
26 
48 
88 
126 
227 
1.5m 
33 
60 
110 
157 
283 
2.0m 
38 
70 
128 
184 
330 
Litres per minute flow rates with 30 metres of friction loss 

HEAD 
20mm 
25mm 
32mm 
40mm 
50mm 
0.25m 
10 
18 
33 
47 
86 
0.50m 
16 
27 
49 
69 
125 
1.0m 
21 
39 
71 
100 
182 
1.5m 
26 
48 
88 
125 
227 
2.0m 
31 
56 
103 
146 
265 
OF INTEREST: Doubling a pipe's internal diameter increases the volume x4 and if the same head pressure is maintained, the flow rate increases roughly x6.
POLY PIPE AND FLOW RATES
A single 19 mm poly pipe vertical drop connected to a 25 mm horizontal poly pipe can also generate impressive flow rates.
A Supadiverta fitted with a single 19 mm poly pipe vertical drop joined to a 13 metre horizontal length of 25 mm poly pipe diverted to a tank’s bottom inlet was found to flow at 25 litres per minute (1,500 litres per hour) with a 1 metre head during flow testing. This is equivalent to a steady rainfall of nearly 43 mm per hour on a 35 sq m roof area harvested by one downpipe.
Substituting the 25 mm horizontal poly pipe with 19 mm poly pipe in the same situation and head pressure still produced a flow rate of 15 litres per minute.
Shorter or larger internal diameter horizontal pipes have higher flow rates.