You may want to revisit your peak demand guestimate. A house with 5 bedrooms and 5.5 bathrooms is much larger than most of us have. If you have a large family and/or will have guests, you could end up running a lot of water-using devices at once and this might result in a household peak demand that exceeds 20 gpm. If you will have irrigated landscaping, and irrigation could happen at the same time as normal household water usage, you should add the peak flow for the largest irrigated zone. If you will have a swimming pool, you might also need to add an additional demand for filling and refilling (I have an automatic filler on my pool and it probably maxes out at about 2 to 3 gpm).
I did a quick Water Service Fixture Unit Method calculation (based on very little data) and came up with an estimated 53.5 WSFUs and a peak household demand of about 30 gpm. Please see the attached spreadsheet.* The Fixture Unit Method is designed to be conservative, so actual peak household demand should be less. However, I prefer to design using the accepted and code-compliant method unless I have good reasons for doing otherwise. All that being said, most of my experience using the Fixture Unit Method has been for large commercial and industrial developments and state and federal prisons. I have only applied this method to residential cases a handful of time. If you have a plumbing engineer, I suggest you have him/her calculate the design peak demand for your particular project.
For this calculation, I assumed the following:
[ul]
[li]Three bathrooms with bathtubs or bath/shower combos, two bathrooms with showers, and one with neither.[/li]
[li]One clothes washer[/li]
[li]One dishwasher[/li]
[li]A maximum of two hose bibbs on at a time (for a house this large, I figure you will have more than the usual four I see around here, but you probably won't have more than one or two on at a time)[/li]
[li]Eight lavatory sinks in the bathrooms (two in the master, two in another, and four in the remaining 3.5)[/li]
[li]One bar sink, one kitchen sink, one sink in the laundry room, and one service sink in the garage or workshop[/li]
[li]Six 1.6 gallon per flush water closets[/li]
[/ul]
* BTW, all of you are welcome to use this spreadsheet as you see fit. I already deleted my company's logo, but you will need to delete my name and initials from headers, footers, and a couple places within the spreadsheet tabs. I first made this spreadsheet while working for myself, so I didn't steel it from my current employer and it's mine to give away. Also, if you find any errors, please let me know.
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