How do I adjust the flow of a plumbing pipe and size it correctly?

[Eng03.pragmaticpe]

I am having trouble sizing pipes for low fixture rate systems with the diversification of fixture units. I want to calculate my pipe sizes with fixture units, but when I do the gpm is too high. In the image below I have set the hose bibb to 2.5CWFU. This causes the pipe to be 15gpm and causes the pipe to be sized to 1". This is fine for high fixture unit systems due to the diversification use, but for low fixture system it incorrectly list the gpm. How do I change it to be a different flowrate for this part of the system without affecting the rest of the system?  

[iainsavage · Accepted solution]

I gave the answer here

https://forums.autodesk.com/t5/revit-mep-forum/modifying-plumbing-pipes/td-p/10037337?us_oa=forums-u...

In the International Plumbing Code which Revit uses, if the calculation is set in the system to use predominantly flush valves on WCs the minimum flowrate is 15 gpm.

If you change the calculation to use flushing tanks you will get a lower minimum flow.

[iainsavage]

Conversion tables attached

[Eng03.pragmaticpe]

Thank you I made a separate pipe type for the small fixture unit system

[Eng03.pragmaticpe]

I have changed it to be a flush tank type, but when I connect the flush valve pipe types to the flush tank pipe types, the total Fixture Unit count disappears and restarts. The flow is also going in the wrong direction. I have attached my model if you would like to review.

[iainsavage]

I can’t look at your model until tomorrow but

• its not the pipe type that matters its the system settings and when you join them together they’ll become one system
• if the flow is going in the wrong direction then something isn’t connected properly or one of your families has the wrong direction on the connector.
• regardless of that Revit follows the IPC method and that says that the minimum flow in a system with flush valves is 15 gpm, no debate.

[Eng03.pragmaticpe]

Sorry for the mislabeling, but I added a different pipe system to have a flush tank system. I routed the pipes so that after the last water closet, it would turn from a flush valve system to a flush tank system.

[iainsavage]

Okay I get what you've done now.

The taps are on system CW1 (predominantly flush tanks) and the WC is on CW4 (predominantly flush valves).

If you separate the two systems the pipe sizing runs fine but when you join one into the other the flow in CW1 becomes disrupted and the sizing fails because some pipes have zero flow.

To be honest I've never tried to join two different types of systems in this way.

I would also question the validity of the probability calculations in the fixture unit method with such an unusual system configuration.

Your system is unusual in that:

• it has six hose taps side by side - how will these be operated? 
• you then have one lone WC connected.

The fixture unit method is intended for larger systems where, because of the total number of fixtures and the way in which humans behave, there is a good likelihood that not all fixtures will be in use at the same time. In very small systems this assumption can break down because if you only have two fixtures say, there is a good chance that both could be used together.

The theory is also based, in the main, on domestic type fittings: WCs, basins, baths etc.

The fact that you need six hose taps side by side would imply that you are thinking they might all get used (or else why not just fit one or two) - so is the fixture unit method valid in this case?

But if you do want to use fixture units with predominantly flush tanks for the hose tap line then so far the best I can suggest is to:

1. put all the fixtures on one system
2. edit the WC family and change the connector to "preset" and assign a fixed flow of 15gpm (or whatever is appropriate for a Zurn flush valve)

Size on this basis. The combined flow is then 32.5 gpm.

That said, maybe someone with more knowledge of the IPC methods could comment - I had never heard of it or used until I started using Revit so I don't claim to understand the basis for the 15gpm (0.95 litres per second) minimum flow in a branch which has zero WCs.

I realise that this doesn't answer your question of how to get two different systems which are joined together to calculate properly - sorry I don't know the answer.