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# Determining domestic water demand4

## Determining domestic water demand

(OP)
I'm designing an apartment complex with 300 units.
How do you determine your domestic demand to size your watermain? Do you use the fixture counts per the plumbing code or do you use the average daily demand with a peak factor?

### RE: Determining domestic water demand

First, to size the plumbing within the building you will probably be required to use the demand fixture unit method described in the plumbing code. In the U.S. this is most likely to be the Uniform Plumbing Code (UPC).  This method tends to be quite conservative with respect to domestic demands.

BUT, you may have other demands to consider.  For example, you may have irrigation demands, internal fire sprinkler system demands, boiler feedwater demands and possibly others. Each of these requirements may be governed by other Codes and authorities.

For public water mains and hydrants outside the building fire flows are usually the governing factor in main sizing. A typical design criteria for  public main would be Maximum Day Demand plus Required Fireflow at 20 psi residual        ( MDD+FF@20psig )  pressure.

For internal sprinklers local and national Fire Codes will govern and dictate flow rates.

good luck

### RE: Determining domestic water demand

(OP)

We are using two separate lines.....one for domestic and one for fire flow.

If I'm reading your answer correctly I think you're saying the fixture count from the UPC is for the building only and the maximum daily demand is used to size the domestic watermain outside of the building?

In our area we typically use 200 gpd for the average daily demand with a peak factor of 4 for apartments.

### RE: Determining domestic water demand

"In our area we typically use 200 gpd for the average daily demand with a peak factor of 4 for apartments."

This sounds reasonable but if you have time and can get some data from the local water provider you may be able to make a closer estimate.  Some water utilities do have records of actual demand and peaking factors for similar apartments. If available, such records would be an even better predictor of the demand you might expect at your project. Handbook numbers will get you in the ballpark but local records are the equivalent of knowing what the next pitch will be and placing a ground ball single through the infield. Enough already with the baseball metaphors.

good luck

### RE: Determining domestic water demand

2
One of the ways is to use AWWA Manual M22 "Sizing Water Service Lines and Meters".  It works with total fixture counts to get a probable peak demand.  One can adjust for the pressure in the area, unlike using the IAPMO, the plumbing code in my state.  Also one can add in the irrigation demand.  If the complex is to be master metered, it will likely have to be the method and justifications used to size the meter.

The manual is a bit pricey for not AWWA members, but it is worth it.

Good Luck.

### RE: Determining domestic water demand

I work for a company with a plumbing group *and* a civil group.  Keep in mind that the pipe sizes resulting from the plumbing code are often larger than the sizes resulting from peak demand.  This is because the pipes in the building have to be sized much more conservatively than the pipes outside of the building.

The pipes outside of the building should definitely be sized for peak demand, by the method you describe with average flow and a peak factor (the larger the development, the smaller the peak factor).

Please be aware that if you use the plumbing code for pipes outside of the building, they'll likely be too large and it can cause problems.

### RE: Determining domestic water demand

PS - If the AWWA manual is referring to service lines from the building to the road, then that is probably good.

However, you probably don't want to use fixture counts to size the pipe network in the streets, because fixture count estimates are usually too conservative for pipe networks.

### RE: Determining domestic water demand

(OP)
I want to thank everyone for the information. I alos think the developer will thank you as well since his mains will not be a size that could serve all of the southeastern states.

### RE: Determining domestic water demand

i do my reseach in  drinking water treatment plant design,i need some information in intake design & pumps design(high& low lift pumps)

### RE: Determining domestic water demand

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