# Forum Home Renovation Plumbing  Flow capacity of a 90/100mmm pipe

## nobloodyidea

Hey 
A potentially embarrassing question.. but here goes. 
I've got a question regarding the difference of the flow capacity of a 90mm pipe and a 100mm pipe. 
I essentially want to calculate if a 100mm pipe could take the maximum water inflow from 2 x 90mm pipes. 
How would I got about calculating this?

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## Bros

Area of a circle will tell you 2 X 90 is much greater than 1 X 100

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## Pulse

Should be fine, presume you are talking about 90mm stormwater from downpipes or similar. These downpipes are never actually full of water. That is why rainheads (the boxes at the top of commercial gutters) are there. They slow the water and allow pressure to build up to increase the flow down the downpipe. 
In short, putting 2x 90mm into 100mm is fine, since DWV grade 100mm is so much sturdier than 90mm stormwater. 
Cheers
Pulse

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## Ronaldo451

I also think you would be pretty safe in most circumstances, but what is the context? 
Most houses have a number of downpipes hooking up to a lesser number of stormwater exits to the street or tanks. It would be pretty unusual to need all the pipes flowing at full capacity at the same time, but, if you are concerned is there scope to put in a backflow 'failsafe'.  
This would allow the water to escape the pipe at a place of your choosing if it does back up rather than into your ceiling cavity, for example, if gutters were involved. This is simply putting in a 'Y' junction in the downpipe with one of the upper 'Y' arms open. 
 As water flows down the pipe and through the Y it cannot go back up the open arm when all is well. If it blocks up or is undercapacity to deal with immediate flow, the water WILL go up and out the open arm rather than continue to backfill the pipe. This is a cheap, quick and simple way to also identify problems immediately if there is a possibilty later on that one of the pipes may become choked with tree roots or whatever. 
Could also be a similar arrangement with a T riser if the pipes were converging horizontally..

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## bricks

> Should be fine, presume you are talking about 90mm stormwater from downpipes or similar. These downpipes are never actually full of water. That is why rainheads (the boxes at the top of commercial gutters) are there. They slow the water and allow pressure to build up to increase the flow down the downpipe. 
> In short, putting 2x 90mm into 100mm is fine, since DWV grade 100mm is so much sturdier than 90mm stormwater. 
> Cheers
> Pulse

  Sorry bloke but what exactly do you base those assumtions on? 
A rain head is installed for one of two reasons  They look speccyThey provide an overflow point at the gutter line in case of downpipe blockage
2 x 90mm pipes can flow alot more than a 100mm pipe will take - ie 90mm storm water will flow around 3.5 litres per second at max - 100mm pvc will flow around 4.5 litres per second. There is no way 100mm can keep up with 2 x 90mm pipes. 
The base of the equation is not acually "can 2 x 90mm be connected to a 100mm pvc" the real question is how much water is going to come off the roof. 
You need to work out your max downfall capacity of your roof. You do this by determining roof area connected to these 2 downpipes, slop of the roof, and where you live. Then see if 100mm can cope with it - chances are you don't actually need 90mm down pipes, but they have been installed as 90mm because it is a cheap standard product.

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## Terrian

> You need to work out your max downfall capacity of your roof. You do this by determining roof area connected to these 2 downpipes, slop of the roof, and where you live. Then see if 100mm can cope with it - chances are you don't actually need 90mm down pipes, but they have been installed as 90mm because it is a cheap standard product.

  the questions to ask are what is the current pipe diameter that connects to the mains stormwater and have the current pipes been able to handle the flow so far.

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## Pulse

> Sorry bloke but what exactly do you base those assumtions on? 
> A rain head is installed for one of two reasons  They look speccyThey provide an overflow point at the gutter line in case of downpipe blockage
> 2 x 90mm pipes can flow alot more than a 100mm pipe will take - ie 90mm storm water will flow around 3.5 litres per second at max - 100mm pvc will flow around 4.5 litres per second. There is no way 100mm can keep up with 2 x 90mm pipes. 
> The base of the equation is not acually "can 2 x 90mm be connected to a 100mm pvc" the real question is how much water is going to come off the roof. 
> You need to work out your max downfall capacity of your roof. You do this by determining roof area connected to these 2 downpipes, slop of the roof, and where you live. Then see if 100mm can cope with it - chances are you don't actually need 90mm down pipes, but they have been installed as 90mm because it is a cheap standard product.

  Bricks, 
I'll quote from page 146 of AS3500.3, which you should have at home. Figure I3 is a design graph for a rainhead. They are designed to do more than just look good and provide overflow. The figure shows that 100mm pipe  can take almost 9L/s with a properly designed rainhead as opposed to 4.5L/s without (which you quoted) 
Obviously a pipe can take an almost unlimited flow as pressure is increased. Your numbers are "rules of thumb" and not answering his question. 
Cheers Pulse ...not Bloke

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## rrobor

Now the area of a circle  I believe is Pii R squared and no one is taking Pii into consideration. So please do so. I'll get the sauce.

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