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?
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Solved by iainsavage. Go to Solution.
I gave the answer here
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.
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.
I can’t look at your model until tomorrow but
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.
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:
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:
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.
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