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Collapsing building © Daniel FriedmanDrain Fixture Units: DFUs

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This article defines & describes Drain Fixture Units or DFUs.

DFUs are a unit of measure of the wastewater volume or loading of a drain waste or vent system. DFUs are used to specify the size of waste piping, vent piping, or related drain devices such as air admittance valves or studor valves.

Here we provide a helpful table giving the number of DFUs for various plumbing fixture types.



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Definition of Plumbing Vent DFU Ratings: Drain Fixture UNITS

Definition of Drain Fixture Unit or Defintion of DFU:

The Uniform Plumbing Code defines Drainage Fixture Unit as follows:

Drainage (dfu). A measure of the probable discharge into the drainage system by various types of plumbing fixtures. The drainage fixture-unit value for a particular fixture depends on its volume rate of drainage discharge, on the time duration of a single drainage operation and on the average time between successive operations. - UPC 2006 

Drain Fixture Unit, or DFU, is a relative measure of the drain wastewater flow or load for various plumbing fixtures. By adding the DFU load rating of all of the individual fixtures on a single drain to be served by a single air admittance valve (AAV), the plumber or designer can select an AAV with sufficient capacity.

As we discuss separately at AIR ADMITTANCE VALVES AAVs, Oatey, an AAV manufacturer, provides the following helpful DFU Load Table:

Drain Fixture Unit (DFU) Table for Common Plumbing Fixtures 1

Plumbing Fixture Type
Drain Fixture Unit (DFU) Load Rating
Comments
Traditional Public Bathroom Group
6
1 toilet at 1.6 gpf, 1 bathtub with shower, 1 sink
Traditional Private Bathroom Group
5
1 toilet at 1.6 gpf, 1 bathtub with shower, 1 sink
Public toilet
4
1 toilet at 1.6 gpf
Private Toilet
3
1 toilet at 1.6 gpf
Washing Machine
3
Presumed Residential
Bathtub with Shower
2
Presumed Residential
Sink with Garbage Grinder (Disposal)
2
Presumed Residential
Dishwasher
2
Presumed Residential
Shower Stall
2
Presumed Residential
Sink
2
Presumed Residential
Drinking Fountain
0.5
 

Notes:

1. Oatey Corporation, "Oatey Sure-Vent® Air Admittance Valves Technical Specifications", Oatey® Corporation, - retrieved 2016/05/08, original source: http://www.oatey.com/doc/aavtrifoldlcs420c101812lr.pdf The company provides AAVs rated at 6, 20, 160, and 500 DFUs.

Watch out: While it is acceptable to oversize a Sure-Vent®; however, an undersized Sure-Vent® (Oatey) or other AAV product will not allow the plumbing system to breathe properly

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Continue reading at DRAIN & SEWER PIPING - home,or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see AIR ADMITTANCE VALVES AAVs

Or see PLUMBING VENT DEFINITIONS, TYPES

Also see these tables of daily and peak water and hot water quantity and usage:

HOT WATER QUANTITY DAILY USE

HOT WATER PEAK DEMAND USE

WATER CONSERVATION MEASURES

WATER QUANTITY USAGE TABLES

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PLUMBING DRAIN FIXTURE UNITS DFUs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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