Fixture units for LEED building

[raptors076]

Hi all,

Most of the methods for water pipe sizing are based on old flow rates. And if you do the calculation according to them, the gpm required is way high compared to high efficiency plumbing fixtures.

1. Can anyone recommend how to put the factors for these fixtures units, especially for LEED buildings.

2. Do we have to use the diversity factor when using the Plumbing fixture unit method in sizing domestic water piping or the diversity factor is already included in the fixture units , please clarify ?

3. Is there any method which do nt use fixture units to size pipe , instead use actually gpm for the fixture?

 

[PEDARRIN2]

You are correct - the literature/sources used for sizing plumbing piping is old, to say the least.

I generally use Hunter's curve and add a factor of 0.6-0.7. You could probably even go 0.1 lower for LEED buildings.

The fixture units (in IPC) are not part of the code, but the appendix - so you are not required to use them - but if you use them and you come up wrong - you have a valid defense. If you don't use them - what defense can you use. They will give you oversized pipe - but not by much. Except for lavatories and showers - the flow rates of fixtures have not changed. Flush valves still flow 25-35 gpm - but the period of flow is less - that is how they get the lower flow for LEED. Now this will affect the flow of the drainage - but not the supply flow - unless the number of lavs/showers vastly overshadows the toilets and urinals. So you would size your piping much the same way.

I would not use fixture gpm as the only basis to size my pipe. I would use the fixture units and keep in mind the required flow rate for a fixture. Say you have 10 water closets in a chase. Using gpm, you would size your pipe for 250 gpm. Using Hunter's, depending on what factor you use, you would size your pipe for 70-100 gpm. The only time I would use gpm to size my pipe is if the required flow for a fixture exceeded the flow that comes out from the fixture units. This typically only happens for small buildings.

 

[raptors076]

The factor you stated I guess is based on your experience.. But did u come across any literature that suggested these factors..

AS far I see pipe sizing does have impact on various factors... oversized pipe may cause problems.. like increased pump size .. I dont knw about the accuracy of sizing them ,, that how accurate can you go in sizing ,, and how much difference would it make if you are off by some percent.

 

[raptors076]

The factor you stated I guess is based on your experience.. But did u come across any literature that suggested these factors..

AS far I see pipe sizing does have impact on various factors... oversized pipe may cause problems.. like increased pump size .. I dont knw about the accuracy of sizing them ,, that how accurate can you go in sizing ,, and how much difference would it make if you are off by some percent.

 

[ChrisConley]

We should definitely be lobbying a group like ASPE to update fixture units for low flow fixtures.

Undersizing pipes won't lead to problems, unless you have a minimum pressure requirement for a fixture. The pressure requirement for flush valves is what drives most of my designs. The problem is that if you have problems, and didn't follow the code, you have a lot of explaining to do.

To clarify, oversized pipe doesn't increase pump size, but high fixture units (converted to GPM) does. The oversized pipe will actually decrease the head on your pump decreasing the size of the pump. If you're talking about a booster application, VFD's make the application work much better in any case.

 

[raptors076]

Chris , I guess I meant to say what you said. But went to far to the oversizing the pipes because of the increased gpm given by the outdated fixture units.


If you guys come across any literature regarind this topic especially for LEED buildins please share it.

I dont know if ASPE have enough funding to conduct a study over this topic and provide us with update information.

Thank you all for the input.

 

[TrippL]

I agree that some fixture units could be adjusted for low flow fixtures such as faucets and showers. However, keep in mind that water closets (tank and FV) and urinals will have the same flow rate when in use. The duration of this flow may be shorter but the flowrate is the same.

Also, fixture units are an estimated value based on diversity and fixture type. In average sized bldg's 1 to 3 stories, adjusting the FU's for the faucets and showers would probably not have much effect on the outcome of the pipe sizes. I can see where in highrise bldg's and larger, heavy plumbing bldg's it could become a factor however remember you are designing the system for the life of the building (relatively) and fixtures get changed out, may not keep up LEED certification and what happens when standard fixtures are put in and the system can't handle it?

The design of any plumbing system has a factor of experienced based feel for it. Just make sure you take into account everything before under-sizing things. An oversized pressure booster system will not hurt anything. Under-sized , high velocity pipes will, Big time.

 

[lma1]

You may wish to look at the new NPCC 2010 (National Plumbing Code of Canada) for those revised numbers on fixture units for water efficient and low flow fixtures.